Читать книгу Practical Engine Airflow - John Baechtel - Страница 8
ОглавлениеEveryone has heard the traditional analogy that an engine is nothing more than a basic air pump, a very sophisticated air pump. In effect, a running engine provides continuously recurring spaces, or power volumes (cylinders), into which air flows due to atmospheric pressure or, in some cases, pressurizing sources such as superchargers and turbochargers. These spaces are essentially empty voids (vacuum) created by descending piston motion. They have negative pressure relative to atmospheric pressure and the atmosphere automatically seeks to fill them through the intake flow paths as each volume is created. The engine is not specifically pumping air, but rather mechanically providing an ongoing series of pressure differentials that encourage air movement through the engine’s inlet flow paths. Air movement, or transfer, is similar to the pumping action; hence, it is referred to as intake pumping.
The intake valve in the cylinder head feeds fresh air to each cylinder for every new combustion event. The size, shape, and configuration of the intake port play a major role in how much air you can feed the engine to increase power.
Every time a piston descends on an intake stroke it creates a cylinder filling and fueling opportunity. This occurs on every other revolution of the crankshaft for each cylinder in the engine. The dynamics of this are extraordinarily complicated on a thermodynamic level and yet simple enough that even when things are pretty far out of whack, the engine still runs and drives comfortably in everyday vehicles. The descending piston creates a void, or space, that atmospheric pressure immediately attempts to fill when the intake valve opens because it is greater than the pressure in the empty cylinder.
The sucking sound you hear at the carburetor is the air rushing in to fill the void. It follows a torturous path through a venturi where it gains speed and mass because fuel is being added. Then it exits the carburetor throttle bores at high speed into a larger staging area, or plenum. The dramatic change in area causes the air to lose velocity quickly, and the local pressure changes. This change presents the first opportunity for the atomized fuel to drop out of suspension.
This factory cutaway of a 1950s Chevrolet 348-ci W-engine shows the inlet path from the carburetor to the cylinder on the driver’s side and a portion of the exhaust path on the passenger’s side. Not much has changed since then. The flow path starts at the air filter and can be traced all the way through the engine to the end of the exhaust pipe (not shown). (Photo Courtesy GM Media Archive)
The next cylinder in the firing order submits a filling request by exposing the empty cylinder via the opening intake valve. The mixture immediately seeks to fill the void in that cylinder by rushing into an intake runner where it picks up velocity and regains some pressure due to the smaller cross-sectional area of the runner. On its way to the intake valve, the mixture may experience a variety of obstacles and area changes that affect its speed and flow characteristics. Curved runners and intake ports that narrow around the pushrod area restrict flow. In a sense, runner taper (see Chapter 4) restricts the flow, but it builds pressure and velocity, which encourages the intake ramming process.
Most street engines and a great many Sportsman racing classes still rely on single-plane 4-barrel intake manifolds to support their induction requirements. Individual intake runners connect to a common central plenum. The intakes are typically outfitted with various-capacity Holley 4-barrel carburetors to suit their high-RPM operating range.
This cutaway view of an Edelbrock dual-plane intake shows the upper and lower plenums and the individual runners that lead from each one. For the most part, the dual-planes are street intake manifolds that build more low-end and mid-range torque than single-plane intakes because they help produce more efficient low-speed flow velocity. In some cases, the dual-plane intake can outperform single-plane intakes throughout the operating range while nearly matching them on the top end.
After negotiating various curves in the manifold, the air may stumble at the gasket interface between the manifold and the cylinder head; this point is rarely an efficient transition unless steps are taken to ensure it. Then the air has to make a relatively sharp turn into the bowl area above the valve where it is interrupted by the valvestem and valveguide. Finally, it has to negotiate its way around the valve and into the cylinder, where it experiences a radical pressure change as it loses velocity. The throat area at the valve is typically the point of greatest restriction, which is why bowl porting is often so beneficial to stock heads.
During each of these phases, the air follows the path of least resistance, primarily influenced by the various shapes, sizes, area changes, obstructions, and surface textures it is exposed to along the way. In a fixed-configuration cylinder head (commercially available) you can take steps to influence the air. This is loosely referred to as porting, and it can make a considerable difference depending on the original layout of a particular cylinder head. Some heads respond better than others, primarily based on the shape and cross-sectional area of the port, configuration (raised or flat), relationship of the valve throat size to the valve size, and other things.
A combination of intake pumping, intake ramming, and wave tuning make up the cylinder filling process. Air rushes in because it is under pressure (atmospheric). The air can achieve considerable flow velocity because the intake path is very small relative to the larger source of air pressure (the atmosphere). This imparts inertia to the air. Fuel molecules rush to fill the void created by the descending piston. Depending on the stroke and the rod length, the piston reaches its maximum velocity somewhere around 75 to 76 degrees after top dead center (TDC). This point corresponds to the maximum velocity of the intake charge moving down the flow path. In a properly sized inlet path, the column of inlet air and fuel achieve enough momentum to continue filling the cylinder even though the piston has reached bottom dead center (BDC) and is beginning to rise.
At this point, the intake valve is still open, but starting to close. Resistance to flow begins to increase, but charge energy briefly overcomes it. This is the intake-ramming phenomenon that is largely controlled by piston motion and the length and cross section of the inlet flow path. It is the most important part of the cylinder filling process because it offers the potential for additional cylinder filling beyond the regular intake pumping process.
But it is only part of a broader seven-cycle process as described many years ago by Patrick Hale in his Engine Pro: The Book, a detailed tech manual that originally accompanied the Engine Pro simulation software he designed. (In 2007 Hale sold the copyright for Engine Pro, his other software programs, and the book to Don Terrell, the founder of speedtalk.com and racingsecrets.com.) The seven-cycle process (also called the horsepower chain) is now broadly recognized and largely adhered to within the performance community.
To reinforce the critical importance of engine airflow, note that top engine simulation programs such as Hale’s original Engine Pro software focus heavily on calculating engine airflow and volumetric efficiency (VE). Sophisticated, modern electronic fuel injection (EFI) systems use similar input from the mass airflow (MAF) sensor to make the proper VE and tuning calculations for optimal performance relative to engine speed and load. EFI is so efficient because it knows the condition of the air mass as it moves through the engine and can provide the proper fueling calculations for maximum efficiency. It also monitors the air leaving the engine to help it determine the proper air/fuel ratio and the efficiency of the combustion event.
Internal Combustion Fundamentals
The basic requirements of internal combustion (IC) engines are complex, particularly from the chemical and thermodynamic standpoints. From a less complicated perspective, we all understand the physical factors that characterize the process. Simply stated, the well-known breathe, squeeze, pop, and sneeze make the magic based on the available air/fuel supply and a throttling device to manage engine speed.
A basic understanding of the process requires that you recognize the following core contributors to the engine power equation:
• Airflow
• Fuel supply
• Flow paths
• Compression
• Ignition source
• Throttling device
• Containment device (cylinders)
Among these key factors airflow is the most difficult to manage. Thanks to modern performance components it is relatively easy to feed the engine enough fuel. And compression is easy to achieve with the advanced sealing characteristics of modern piston ring technology. Lighting it off is also easy with high-tech digital ignition systems while various carburetor and throttle body systems easily manage throttling concerns. Although complex thermodynamic and chemical processes govern the efficiency of all this, you don’t necessarily require too keen a grasp of the deeper science to understand airflow through the engine and the various elements that tend to resist air motion and subsequent cylinder filling.
At this point, you are not yet concerned with air/fuel mixture quality, but simply the overall definition and efficiency of the flow path from the atmosphere above the air cleaner to the atmosphere behind the tailpipe. Pressure and velocity changes that occur along the entire flow path play a pivotal role in governing engine output. There are many ways to influence and alter an engine’s air movement and the various forms of resistance that dictate its efficiency.
The carburetor is the traditional self-compensating fueling device that mixes air and fuel in the proper proportion and feeds the mix to the engine via the intake flow path, which consists of the intake runners and the intake ports.
High-performance electronic fuel-injected applications typically incorporate a large single throttle body or a four-hole unit that passes only air because the fuel injectors introduce the fuel.
Electronic fuel injectors come in various sizes to accommodate engine displacement and horsepower ratings. High-performance systems usually have the injectors in the intake runners.
The intake port is the flow path that directs the air/fuel mixture into the engine. It is the primary influence on engine performance.
The exhaust port is always smaller so that the high cylinder pressure helps evacuate the cylinder after the combustion event.
Valve size and placement relative to the bore size, particularly the throat-diameter-to-valve-diameter ratio, determine the effectiveness of the port and its ability to turn the air into the cylinder with the smoothest possible flow.
The combustion space incorporates the combustion chamber, piston top (at TDC), intake and exhaust valves, spark plug, and a fuel injector if the engine incorporates direct injection.
The carburetor (or throttle body) is also the throttling device that regulates engine speed and power output via butterfly valves that vary air delivery to the engine. A throttle linkage connected to the gas pedal operates the butterflies.
Patrick Hale’s horsepower chain introduced three additional cycles to the traditional four-cycle engine model. They include intake ramming from the charge inertia effect, exhaust blowdown to account for the initial high-pressure exhaust evacuation, and the valve overlap period as a significant cycle affecting the intake and exhaust relationship. (Illustration Courtesy Scott Lozano)
Our task as engine builders is remarkably similar. We want to understand the air mass condition and the various influences that act on it so we can manipulate it to improve efficiency and power output in the power range most useful to our application.
Air moving through a running engine experiences a dramatic series of pressure changes before it finally exits the tailpipe and returns to atmospheric pressure. The seven cycles, or processes, identified by Hale define these pressure changes and how they combine to produce torque and horsepower. If you follow the air pump analogy and also think of the engine as an air processor, you can more accurately understand the major steps used to create power:
1. Intake pumping
2. Intake ramming
3. Compression
4. Fuel burning and expansion (power stroke)
5. Exhaust blowdown
6. Exhaust pumping
7. Valve overlap
The traditional four cycles are 1, 3, 4, and 6 on this list. These are what you have always had to work with, but as Hale points out, the major gains in engine output come from working with the three additional cycles that exert enormous influence on the overall process.
You must also consider the negative pumping effects that accompany these processes, including the cumulative consequences of friction, mixture compression, and airflow resistance (more commonly referred to as pumping losses). Resistance to the motion of the rotating assembly and the free movement of air through the engine are also primary culprits. The air does not specifically require pumping except in the case of supercharged applications designed to boost and improve the normal characteristics of atmospheric cylinder filling, or natural aspiration. Instead, it reacts to pressure changes to fill the cylinders.
One of the most important factors of the seven cycles is the close interrelationship among them. Each cycle represents a specific process inexorably linked and influenced by the cycle before it and the one following it. In Hale’s words, “The output from one process defines the input for the next.” They are inseparable. Each process affects the next in an unbroken circle or, as Hale calls them, links in the horsepower chain. It takes two revolutions of the crankshaft (720 degrees) of rotation to complete the seven processes for each cylinder. And then it begins again. Each process must be fully optimized to ensure maximum performance from the engine.
A fault, or less than optimal performance, from each process affects every subsequent cycle and degrades the power process. Hence the inputs and outputs and what you do with them within each process define how well your engine performs within its operating environment.
As Hale indicates, each of the processes adheres to a different set of physics. You can only manipulate their performance by changing the shapes, sizes, and various interactions of the components that make up the overall engine.
For example, commercial exhaust headers are by necessity a compromise based on a broad range of engine sizes and requirements. Header size and length are largely determined by what fits a specific engine and chassis combination. It’s up to the engine builder to calculate and select the correct sizes. And to be honest, any full racing effort uses custom-built headers specifically tailored to that particular engine’s requirements and operational characteristics. If the wrong headers are used many links of the chain become compromised and less than optimal performance occurs.
If residual exhaust gases remain in the combustion chamber through some failure of the exhaust blowdown or exhaust pumping process, they contaminate the fresh intake charge and seriously degrade the power potential. The contaminated charge then affects the entire process with a resultant power loss. That’s why each process must recognize and complement the subsequent process to ensure optimal performance.
The individual seven cycles control the movement of air through the engine and ultimately influence the whole character of an engine’s performance potential. It is very important to visualize their effect on the high-speed air column as it moves through the engine. These cycles influence the airflow resulting from pressure changes that lead to superior power output.
Intake Pumping
The intake pumping process begins when the exhaust valve closes (EVC). This event initiates during the valve overlap period and slightly after TDC. At this point the intake valve has also opened (IVO). The intake valve is accelerating toward its full-open position. The piston is descending at some given rate dictated by the stroke and rod length, typically faster with shorter rods and slower with longer rods. In either case, this exposes cylinder volume to the intake port at some particular rate and offers a filling opportunity. The highest demand (or draw) typically occurs about 75 to 76 degrees after TDC, where the piston achieves its highest velocity (speed), thus creating the lowest pressure in the cylinder.
High-velocity air in the intake ports gains inertia to help ram-fill the cylinder above and beyond that achievable by normal pressure recovery. This creates the ramming effect that Hale calls “intake ramming.” (Photo Courtesy Smithberg Racing)
In Hale’s description, the descending piston (center) creates a depression (or low pressure) above the piston that tugs on the intake charge the hardest, somewhere in the neighborhood of 76 degrees after TDC. At this point air is rushing to fill the cylinder at maximum velocity. At BDC (left) the successful ramming process is still packing the cylinder to a density that exceeds the cylinder’s physical capacity, and pressure begins to rise prior to any actual compression activity. When the exhaust valve opens the cylinder is still under high pressure and the initial blowdown is very rapid. Following that, the piston pushes the remaining charge out of the cylinder as it once again rises to TDC (right).
The piston descends and the intake valve opens farther, the flow rate (velocity × area) increases until the valve reaches maximum lift at about 108 degrees after TDC. This is the intake pumping process, or the rapid transfer of the air/fuel mixture into the cylinder in pumping fashion. It ends when the piston reaches BDC at the bottom of the stroke and begins to reverse direction.
Thus begins the opening cycle of the traditional four-stroke internal combustion process. At this point, the intake valve is still open.
As Hale said, the output of each process affects the input of the next process. In this case excessive camshaft overlap (during the valve overlap process) can allow residual exhaust gases from the still open exhaust valve to contaminate the fresh incoming charge from the intake valve. This means that the exhaust blowdown and exhaust pumping cycle has inadequately evacuated the spent cylinder gases or that valve overlap needs to be lessened to accommodate the inadequate scavenging effect.
It is exceedingly difficult to manage the pressure changes: when they begin, when they end, and what happens as they change. In the presence of fixed piston motion, you can only alter valve timing or intake/exhaust flow characteristics to control each of the seven cycles.
An ideal intake pumping process begins at EVC and ends when the piston briefly stops at BDC. Assuming a 100-percent fresh-intake charge, no contamination can occur once the exhaust valve closes. Under ideal conditions, the intake pumping process ends with the piston at BDC, and the same 100-percent fresh charge occupies the specific swept volume of the cylinder at atmospheric pressure and temperature, give or take some heat contributed by the hot cylinder.
This cylinder filling scenario achieves a VE of more than 100 percent because the clearance volume (chamber) is also filled with 100-percent undiluted charge. The simplest description is an 11:1 compression ratio where the clearance volume is 10 percent of the swept volume. The ideal result is a VE of 110 percent.
Unfortunately, some conditions resist our efforts to fill the cylinder adequately, and these are the problems we address as engine builders. A few of the engine features that affect the intake pumping process include:
• Intake port flow capacity
• Carburetor and intake manifold runner steady-flow characteristics
• Degree of influence of restrictions
• Maximum piston velocity and camshaft timing
• Charge contamination during the overlap period
Intake port flow capacity must be measured on a flow bench at a high pressure drop and at the mid to high valve lifts that you intend to run, which might be established by modeling or consultation with your cam designer.
Carburetor and intake manifold runner steady-flow characteristics are best determined by the average plenum (manifold) pressure during intake pumping, although that’s difficult to accomplish on the front end. Many builders often extrapolate by flowing the port with the manifold and carburetor attached to determine a more realistic approximation of the flow characteristics. They’re simply determining the existing steady-state flow capacity and characteristics of the available flow path.
Finally, the maximum piston velocity and crank angle can be calculated via RPM and rod length–to-stroke ratio so it can be related to camshaft timing. This is done for you in most modeling programs.
With the goal of providing 100-percent VE at BDC plus the clearance volume VE, Hale’s work describes VE as a strong predictor of engine speed at both peak torque and peak horsepower. The pumping process is not so much a function of the flow path cross section, but rather a result of the overall intake steady-flow capacity.
Flow-bench measurements are rightly viewed as trend indicators. A flow bench operating at 28 inches of water cannot emulate the same characteristics of a rapidly descending piston, which can easily induce a pressure drop more than double that of the flow-bench capacity. Under more dynamic conditions rapid piston motion results in a very strong tug or “yank” on the intake charge that the flow bench cannot replicate.
Intake Ramming
This cycle accounts for the considerable momentum (inertia) that the intake charge has accumulated during the initial pumping process. As the piston reverses direction at BDC, the intake valve is closing. But the momentum generated by a properly configured flow path continues to ram mixture past the valve and into the cylinder, even as the cylinder volume begins to decrease and cylinder pressure begins to increase.
Capturing more air at speed via velocity ramming is the primary intent of most air scoops. The racy look is an attractive co-benefit. And, like air cleaners, scoops have evolved into some pretty bizarre shapes in an attempt to slow and direct the air to build even inlet pressure.
A storm is brewing between the air scoop and the header tips on this supercharged dragster engine. Air enters the engine at atmospheric pressure along with the ramming effect at speed. The supercharger compresses it and feeds it through the intake manifold and cylinder head ports to the cylinders where the magic happens. Supercharged drag engines generate a lot of exhaust volume and zoomie headers are the most effective way of passing it through without restriction.
Depending on the flow path characteristics and the strength of the intake column flow volume, it continues filling the cylinder well past BDC. At some point, equilibrium is achieved as rising cylinder pressure finally overcomes the intake charge momentum. Proper valve timing is essential here because the intake-ramming event ends when the intake valve closes (IVC), approximately 60 degrees after BDC.
Intake ramming is an essential component of true performance engines. You might call it VE augmentation. Recall that VE is the ratio of the actual trapped mass in the cylinder at the end of the intake valve event (and the ramming process) to the actual mass of the swept volume of the cylinder at ambient temperature and pressure. The ramming event accounts for VE percentages that often exceed 110 in racing engines. In very high-end engines the additional filling contribution of the ramming event can approach 18 percent with effective wave tuning offering another 2 percent of additional filling capacity. (See page 58 for more information about wave tuning.)
Again, the previous cycle can affect the current cycle. For example, extra-large ports and valves can initially help the intake pumping process, but they kill the subsequent ramming process because they can’t build enough velocity to accomplish and sustain it. Port energy simply remains too weak except at very high engine speeds, which may be well out of your particular engine’s operational range. It’s why oval-port Chevy big-blocks often outperform big-blocks with larger square-port heads.
Influences on Intake Ramming
Engine features that play an important role in the intake ramming process include:
• Precise camshaft timing of IVC and RPM to prevent intake system reversion
• Overall intake runner length
• Intake runner cross section
• Total intake runner volume
• Intake system pressure waves (arriving at the intake valve before valve closing)
• Overall plenum volume
• Intake port flow capacity (as measured at lower pressure drops and low to moderate valve lifts)
Street supercharging has become a mainstream means of providing additional airflow to performance engines. Matt and Debbie Hay’s award-winning 1988 Pro-Street Thunderbird sets the bar high with its radical fuel-injected twin-blower setup feeding a 351-ci small-block Ford V-8.
Engines with individual runner induction systems are tuned for a very specific operating range such as that found at Indianapolis. When Chevrolet returned to open-wheel Indy racing in 2002, it brought a 3.5-liter naturally aspirated methanol-burning engine that won 14 of 15 events and captured all three major titles: the driver’s championship, manufacturer’s championship, and team championship. (Photo Courtesy GM Media Archive)
Wave tuning can further aid the cylinder filling process. These finite amplitude pressure waves should be timed to arrive at the valve before BDC and again just before IVC to lend their energy to ramming even more intake charge past the valve to increase VE.
Pressure wave timing is largely a function of inlet tract length. Rapidly moving positive and negative pulses traveling back and forth between the valve and the inlet entry carry energy that can be harnessed to push more charge into the cylinder if timed correctly. Opening and closing valves generate a reflected pulse as does a large area change such as found at the inlet entry. When a pulse encounters an area change or closed valve, it reflects back in the opposite direction and changes its energy value, or tense.
For example, a negative pulse encountering a closed valve reflects a positive pressure pulse because of the stacking effect of the air against a closed valve. When that pulse travels back to the inlet entry, it loses energy because of the area change and reflects a weaker negative pulse traveling back to the valve.
The pulses swap phase, if you will, because sometimes they encounter a closed valve and reflect positively while others encounter an open valve and reflect negatively. So the inlet can reflect a positive pulse that can help fill the cylinder. These are very high-speed pulsations, but their timing can be calculated and manipulated according to the length of the flow path. Because they move at supersonic speed you generally have to take advantage of every third or fourth reflection to suit the packaging constraints of the inlet paths.
Inlet path length is typically calculated to take advantage of the third reflected wave for best power. The second wave works well for fuel-injected OEM applications, and most single 4-barrel race engines work best with the fourth wave. Builders no longer need to spend time calculating these lengths because it is done for them in a very affordable (less than 50 bucks) modeling program offered by Larry Meaux called PipeMax. Simply input your engine values and choose the recommended length from the calculated values. More on this later in Chapter 4.
It is important to time the IVC point correctly. We used to refer to it as late intake closing for the purpose of continued filling, but it was never adequately explained that the additional filling occurs because of intake charge inertia, or ramming. Because the piston is now rising and starting to initiate a pressure buildup, you must close the intake valve at the point where the pressure rise in the cylinder equals the pressure of the intake flow to ensure maximum ramming. This is the opposite end of the combined intake events (pumping and ramming).
At the beginning of the filling process, you have the potential for contamination because the intake valve opens before the exhaust valve closes (overlap). At the end of the filling process, you encounter a stalling effect to the flow if the valve is left open too long. Although minimal compression takes place in the early part of the compression stroke, it eventually reaches a point where it overcomes the strength of the incoming charge. This is the point at which you need to close the valve and count your blessings.
The intake ramming process provides the VE increase above 100 percent plus clearance volume. In very refined applications such as a Pro Stock engine, it can approach 130 percent. The momentum effect of the ramming process and the pressure waves that assist it are engine speed dependent and thus most effective at certain engine speeds. Chapter 4 includes a discussion of how the momentum effects of the fast-moving inlet air column affect the flow direction, mixture quality, and pressure recovery characteristics of any particular inlet and combustion chamber combination; this means they must be contemplated as a total system.
Compression
The compression cycle begins at IVC. Prior to this, the piston has already begun to rise, but the intake valve is still open, and there is no appreciable pressure change until the inertia of the intake charge is overcome. Even then it is minimal, but in some cases it’s enough to dam up the intake process causing reversion, or the stacking up of the intake column to the point where a cloud of fuel vapor forms above the carburetor entry.
The actual compression event begins after the piston is well on its way up the bore. And it ends before the piston reaches TDC because the spark plug fires at about 35 to 30 degrees before TDC, ending the compression process and initiating the next cycle: the fuel burning and expansion process. Depending on the particular geometry of the slider crank relationship the piston may still be anywhere from 1/8 to 1/4 inch down the bore when the compression process (stroke) ends.
The bowl area below the valve serves as a conditioning space designed to help the flow transition as efficiently as possible around the entire circumference of the valve head. Valve throat diameter and the chamber wall and roof characteristics immediately around the valveseat largely determine the port’s flow efficiency.
A Hemi head is less susceptible to shrouding because most of the valve circumference is not blocked by adjacent chambers walls as it is in a wedge-type chamber.
Here, the efficiency of the combustion chamber and fast-burn characteristics can affect pumping losses. The plug fires before TDC to give the combustion kernel time to grow and begin building pressure. The earlier the plug fires (more timing, as in 42 degrees before TDC instead of 35 degrees before TDC), the more pressure builds ahead of TDC. Early pressure rise resists piston motion and incurs a pumping loss to overcome the resistance. Modern fast-burn chambers require much less initial spark timing and thus reduce this parasitic characteristic.
The failure of the previous two processes to adequately fill the cylinder most often contributes to the faulty link that can occur in the compression process. Mixture density diminishes, and there is less trapped mass to compress before the firing sequence. It’s a vicious circle. If any player on the team stumbles it defeats the whole process and power suffers accordingly.
The compression process is shorter than the actual physical stroke of the crankshaft. That’s because it doesn’t begin until IVC, and the piston is already part way up the cylinder. It ends when the plug fires, although the piston has not yet reached TDC. As an example, you might say that the compression portion of a 3-inch stroke only involves 2.2 inches of piston travel. These are arbitrary numbers, but you get the idea.
The ideal compression process gains no heat from the cylinder walls and fully stratifies the fuel vapor charge across the entire combustion chamber to achieve optimal combustion. The maximum unfired cylinder pressure (to prevent detonation or pre-ignition) for the fuel type and air/fuel ratio is achieved by piston motion just prior to the plug firing.
A higher static compression ratio translates to higher cylinder pressures depending on the IVC point. You can manipulate the rod-to-stroke ratio to alter piston speed and piston position at IVC. Plus, heat transfer can be partially controlled with thermal coatings, and you can select and modify piston domes and chambers to more favorable configurations to enhance the burn.
Compression Event Influences
Although ideal conditions are difficult to achieve, you can work with the following features to optimize the compression event:
• Static compression ratio
• IVC (determines the effective compression ratio)
• Rod/stroke ratio
• Initial temperature and pressure of the trapped air/fuel mass
• Heat transfer from the piston top, cylinder walls, and combustion chamber
• Piston dome and combustion chamber shapes and characteristics
Custom intakes with tapered runners are used to build pressure and velocity in the inlet flow path. This supports the intake ramming cycle, which relies on charge inertia to continue filling the cylinder. The radiused inlets encourage smooth airflow into each runner.
Fuel Burning and Expansion Cycle
The expansion cycle is the money cycle, as they say, although many argue that the ramming cycle is the most important. The combustion cycle is wholly dependent on the success of the preceding cycles. Its contribution is also influenced by ignition quality and consistency, fuel quality and mixture consistency, chamber efficiency, engine load, cooling, and other factors that combine to influence the quality of the burn and the power derived from it.
This cycle begins with the spark-induced ignition of the fuel mixture and subsequent burn and expansion of the gases. It is not an explosion, but pressure and temperature build rapidly, and the expanding gases push the piston downward. Combustion pressure multiplied by total piston area results in thousands of pounds of pressure exerted on the piston. Depending on the type of engine, pressure normally peaks about 12 degrees after TDC.
The bulk of the work occurs here and trails off over the next 100 degrees or so of crankshaft rotation. Pressure decays as the piston descends, until the exhaust valve opens (EVO) around 120 degrees after TDC. You get what you get at this point, and there is little you can do to influence it except provide complementary fuel to control detonation and regulate ignition timing to optimize the burn characteristics of the combustion chamber. Remember that the piston top forms the floor of the combustion space, and its shape and rate of approach can also influence combustion characteristics to some degree.
Exhaust Blowdown
The discharge process can be divided into two separate events characterized by different physics and thermodynamic processes. The first is exhaust blowdown, which initiates at EVO. Cylinder pressure is still relatively high, and some of the gases may still be burning. In some cases, a combination of poor cam timing and late ignition may even cause burning to continue into the header pipe. For the most part, high cylinder pressure attempts to exit the cylinder as soon as the valve cracks open. Exiting gases briefly achieve supersonic flow until the valve opens farther.
The blowdown cycle is the primary source of exhaust noise. The cylinder blows down rapidly because of the high-pressure gases escaping past the exhaust valve. Most of the exhaust exits the cylinder via its own high-pressure energy, and the event concludes when the piston reverses direction at approximately BDC. Depending on the timing, it’s possible that the cylinder pressure could still be higher than atmospheric, and the piston is moving very slowly, so it’s not precisely at BDC in every case.
Exhaust Pumping
This is the second discharge function, which begins when the piston begins to rise on what is traditionally called the exhaust stroke. Most of the cylinder has already blown down, but as the exhaust valve opens farther the rising piston pumps out residual gases. The valve reaches maximum lift at about 70 degrees after BDC and the piston achieves maximum velocity around 105 degrees after BDC.
On the exhaust stroke, the piston is chasing the exhaust valve, which is trying to close before the rising piston catches it. The piston pumps the remaining gases out of the cylinder, completing the exhaust pumping process when the intake valve just begins to open (IVO) as the piston approaches TDC.
The exhaust blowdown is a high-pressure self-induced evacuation of the cylinder. The exhaust pumping cycle is the forced expulsion of the remaining low-pressure gases by the rising piston (the mirror image of intake pumping).
Exhaust Pumping Influences
Here are some of the factors that affect the exhaust pumping cycle:
• Rod/stroke ratio (influences piston speed and acceleration)
• Exhaust-port steady-flow capacity (at low to moderate depression and moderate to high valve lift)
• Exhaust system pressure-wave tuning
• Maximum piston speed
• Cylinder displacement
• Bore area–to–exhaust throat area ratio
• Initial pressure and temperature
Important goals to accomplish during the exhaust-pumping event include minimizing the pumping effort and the residual mass of spent exhaust gases remaining in the cylinder at IVO. It’s also important to reduce the cylinder pressure at IVO to less than intake port pressure to prevent spent gases from flowing back up the induction path and contaminating the next charge.
Valve Overlap
For a brief period around TDC, both valves are open at the same time, and various things can occur depending on the strength of the inlet flow and the remaining exhaust pressure. This event is called the valve overlap period. It begins just after IVO, and slightly before TDC. Valve overlap doesn’t hit the piston because it is still up in the chamber at low lift. The exhaust valve is open but almost closed, and the piston does a drive-by of both valves at TDC. The overlap event ends when the exhaust valve finally closes and the intake pumping process is in full swing.
A proper overlap event ensures that all spent gases are expelled from the cylinder. The ideal condition is to have the exhaust scavenging process, assisted by pressure wave tuning, provide a slight tug on the intake charge just before EVC. The overlap event is necessary, complicated, and even quite valuable when properly timed.
Conclusion
As described by Hale, the seven cycles, or processes, are very much interrelated and overlap each other with their input and output influences. The illustration on page 12 is especially useful in visualizing the ongoing cyclical process and how each event influences the next. It is useful to pay close attention to the overlapping portions of each cycle relative to the various valve events and piston positions on the inner circle of the diagram.
For example, the compression cycle begins opposite the IVC point and ends at TDC. The fuel burning and expansion cycle overlaps compression to slightly before TDC to indicate the plug firing at whatever timing is set. Study these relationships closely to gain a solid perspective of how all these events interact. Then ponder the actual pressure changes and thermodynamic processes occurring within the chain every step of the way at mind-bending speed.
Engine power comes from the chemical oxidation of the fuel that the engine burns. Burning more fuel provides the potential to increase power if the essential requirements of internal combustion are served adequately. Burning fuel requires an oxidizer to support combustion. In a stroke of extraordinarily good fortune, all IC engines enjoy a remarkably convenient and unlimited supply of oxidizer in the form of the earth’s atmosphere. The 21-percent oxygen content that keeps us all breathing easy also makes engines run by providing the oxidizing component that sustains the combustion process. Hence engine airflow is effectively the controlling factor of high-performance engine output.
Overlap Influences
The following either directly or indirectly affect the overlap period to varying degrees:
• Configuration and influence of the combustion space
• Rod/stroke ratio (influences piston speed and acceleration)
• Exhaust system pressure-wave activity
• Initial pressure and temperature in the cylinder and the intake and exhaust ports
• Inlet system pressure-wave activity
• Instantaneous plenum pressure
• Intake and exhaust port flow capacity (at low-pressure ratios and low valve lifts on the flow bench)
The goal of optimizing engine airflow is to produce performance and competition engines that deliver maximum torque and horsepower in an operational range most suited to the vehicle’s specific application. This is characterized by high VE and effective management of engine airflow within the RPM range that constitutes the desired operational power band. Maximum torque across an application-specific range of engine speed is the objective. Torque is the twisting force representing the potential to perform work. Engine torque is the force potential, or turning moment, applied to the crankshaft flange, or flywheel, when combustion pressure is transferred to the crank throws via the connecting rods. When the flywheel turns, torque is measured by the resistance to rotation. Once the flywheel is turning, torque applies over a period of time, and horsepower is calculated via this formula:
HP = torque × RPM ÷ 5,252
Where: 5,252 = mathematical constant
Torque is the measure of an engine’s ability to perform work. It is characterized by high volumetric and combustion efficiency. Horsepower is the rate at which the work is performed. Torque accelerates the mass of a race car while horsepower, a derivative of torque, supports vehicle motion (speed) by maintaining the application of torque over time.
Engine builders strive to produce torque rapidly over a pre-determined range of engine speed (RPM) chosen to support the engine’s final application. It is referred to as “transient torque,” or the rate at which a loaded engine can accelerate through a specified range of engine speed. The greater the transient torque, the faster the engine is able to accelerate the vehicle under load.
All engines generate a variable torque curve that peaks at some point in the RPM range as determined by dimensional characteristics of the flow paths. This peak represents the most efficient point in the engine’s operating range and closely mimics the VE curve. The point of highest VE creates the torque peak. Various tuning and engine configuration techniques enable you to adjust the position of the torque peak to the most favorable spot in the power band and to reshape the curve around it for maximum performance benefit.
Pressure-Volume Diagram
Pressure-volume (PV) diagrams seem a little kooky to those not used to working with them, but they reveal a lot about the engine’s airflow characteristics, and they help pinpoint abnormalities that need correcting. A PV diagram is a visual representation of the fluctuating pressure and volume changes in a running engine. These are the same pressure changes you seek to influence in your efforts to alter the engine’s airflow characteristics to suit a particular application or purpose.
A PV diagram illustrates the same information displayed by a pressure crank angle diagram. A PV diagram consists of two primary loops that isolate work performed from work consumed, or wasted, in the process. Each loop is annotated by a plus sign (+) for positive work and a minus sign (–) for lost, or negative, work.
The area in the lower loop represents the pumping losses of the intake and exhaust strokes. It is commonly referred to as the pumping loop because it illustrates the intake and exhaust pumping cycles. The upper loop depicts work produced by compression, combustion, and the power stroke. Take note that the lower loop (–) follows a counterclockwise direction and the upper loop (+) follows a clockwise direction.
This drawing is numbered to indicate the various points where the cycles begin and end. As you follow the sequence, bear in mind the cylinder pressure and piston displacement, as indicated on the X and Y axes at each point along the way.
The action begins at the same point as on the pressure crank angle drawing: point 1, IVO. Between point 1 and point 2 is the intake stroke. The pressure drops below atmospheric through this part, as indicated by the sag in the loop. As the loop rises from its lowest point, it indicates “pressure recovery” in the cylinder as the incoming charge exits the valve and fills the cylinder.
From point 2 to point 3 is typically the compression stroke, but the pressure spike you see between point 2 and the other loop indicates the intake ramming cycle, or inertia charging, as the charge velocity continues to fill the cylinder. A fatter loop at this point indicates rising VE due to ramming.
At point 3, you can observe the cylinder pressure increasing and the charge volume decreasing due to compression. When ignition occurs at point 3, cylinder pressure quickly spikes to peak based on charge density and combustion efficiency. It then begins to decrease as it pushes the piston down the bore. The volume remains relatively constant at the pressure peak and then increases as the burning gases expand down the cylinder bore against the piston top.
Points 4 through 6 represent the power stroke. Point 5 is the pressure peak that occurs approximately 10 to 12 degrees after TDC.
The piston reaches BDC at point 7 but the area from point 6 to point 7 represents the exhaust blowdown cycle where high cylinder pressure at EVO rapidly expels the bulk of the spent gases.
From point 7 to where the loops cross again is the exhaust pumping cycle. The crossover point represents the overlap period when the higher pressure determines what happens while both valves are open. If the exhaust pressure is still greater than the intake pressure, it tends to push residual exhaust back up the intake flow path. If the intake pressure is too great, some of it rushes through and out the exhaust before the door shuts. In a perfectly matched system, the last remaining residual exhaust exits the cylinder and invites the new intake charge to follow, which it does until the valve closes and cylinder filling begins again.
As engine speed increases, the PV loops tend to move upward on the pressure scale; very slightly on the pumping loop, but considerably on the exhaust loop. This is caused by insufficient time to blow down the cylinder because exhaust cycles become shorter as engine speed increases. Pumping work increases due to higher residual cylinder pressures, and horsepower begins to fade.
The PV diagram looks different in a supercharged application where positive pressure in the inlet tract eliminates the sag in pressure between points 1 and 2, and the whole loop tends to rise and become fatter because of the constant increase in inlet pressure.
All of this can also be related to crank angle in a pressure crank angle diagram.
The pressure volume diagram illustrates the pressure and volume changes that occur within the cylinder and combustion space during a typical cycle. The loop follows a clockwise direction as the air-fuel charge moves into and out of the cylinder and processes into power. The horizontal line on the lower loop shows how the pressure is negative as the piston descends while the valve opens. As the valve closes, pressure begins to rise during compression until the ignition point at which it spikes; expansion pushes the piston down the bore.
Power is governed by the overall efficiency of the specific component mix. It’s relatively easy to supply enough fuel, but it is considerably more difficult to maximize airflow without the aid of a power adder. For any given collection of parts, an engine achieves a torque peak influenced predominantly by intake and exhaust tuning relative to its size (displacement) and engine speed. Stroke length, piston speed, and the valvetrain dictate the overall operating range while the intake cross section and ramming set the torque peak RPM. Within these parameters the engine’s displacement (particularly with large bore and short stroke), intake flow capacity, and intake valve diameter provide the greatest influence on peak torque.
Taken to the extreme for a barely legal street car, Street Nationals champion Tim Arkebauer’s PSI blown 1969 Camaro exhibits all the symptoms of street overkill in a radical Pro Street ride that’s also capable of getting down the track at a pretty formidable clip. Although a bit impractical for the regular street driver, this setup moves all the air you could ask for at any throttle position.
The engine generates a torque peak at its highest volumetric efficiency. Below this peak, torque trails VE because of insufficient intake velocity. Above the torque peak, torque falls off due to insufficient time to fill the cylinders.
Through attentive manipulation of these and contributing component hardware, the torque curve can be shaped and positioned to suit the engine’s final application. The trick lies in properly matching the math, and the component combination, to not just produce a torque peak at a desired RPM point, but also to fatten the curve below and above the peak by increasing VE. Hence, the major importance of the intake ramming process and supporting wave tuning is to pack the cylinders as full as possible throughout the effective RPM range.
This is the principal focus of all competent engine builders (designers), and it begins with the pursuit of VE relative to the engine’s static air capacity. The air mass component depends largely on available air density and the VE that a specific component mix is capable of generating. It is primarily governed by inlet and exhaust flow-path dynamics, combustion chamber efficiency, valve timing, and elements of the bottom end and valvetrain that dictate final RPM capability.
The shape of the torque curve closely mimics the VE curve at peak torque. This is the point of maximum engine efficiency, and it typically reflects the lowest wide-open-throttle (WOT) brake specific fuel consumption (BSFC) numbers. Below the torque peak, torque trails the VE curve due to reduced combustion efficiency caused by inadequate intake flow velocity (ramming), air/fuel separation issues, and poor mixture quality. Above the torque peak, torque and VE decline due to insufficient time for cylinder filling caused by increasing engine speed (RPM).
Fortunately methods are available to address VE inefficiencies on either side of the torque peak and to inflate the overall torque curve. This refers to the “area under the curve” and seeks to expand the torque curve in all directions. Successful efforts to increase torque automatically improve horsepower. More important, a broader torque curve often produces greater acceleration even with a slight reduction in peak torque because it applies more torque over a broader range.
If the ideal mix of engine components targets an engine speed range most beneficial to the application, superior vehicle performance is ensured across the board. Complementing these performance gains with correctly matched gearing and tire combinations ultimately leads to faster cars and better racing through the effective production and utilization of torque. This works effectively even for engines operating well above the torque peak because the upper end of the torque curve expands accordingly, thus contributing more horsepower to the car’s performance.
The cornerstone of power building is volumetric efficiency. The more air an engine can process, the greater its power potential. VE is determined according to the engine’s static air capacity, or displacement. A displacement of 400 ci represents 100-percent air capacity for an engine of that size. At any given engine speed, a percentage of that volume is being processed into torque depending on a host of variables that conspire to limit airflow. Without these pesky restrictions, atmospheric pressure can easily fill the cylinders completely (100 percent) every two crankshaft revolutions.
In practice this is difficult to achieve because airflow is restricted by a throttling device (carburetor, throttle body, or other), imperfect intake manifolds, intake ports, valves, and all the attending flow restrictions and pressure dynamics present in a running engine. Hence VE in a production engine rarely exceeds 80 to 85 percent. As previously mentioned, VE is reduced below the torque peak due mainly to insufficient airflow and poor mixture quality. And when operating at low RPM the piston pushes some of the charge back out after BDC so no ramming occurs.
Above the torque peak, VE is limited by inadequate time to fill the cylinder due to increasing RPM (typically two effects apply past peak VE, one for flow stagnation and one for the loss of intake/exhaust wave tuning). One of the engine builder’s primary goals is to exceed the static air capacity of the engine and optimize combustion efficiency once fuel is introduced to the process. Savvy engine builders skillfully manipulate the component composition to accomplish this, broadening the torque curve and positioning it to best suit the intended application.
Hard-core racing applications such as NHRA Pro Stock still rely on highly specialized tunnel ram intake manifolds topped with twin Holley Dominator 4-barrels. Despite the precision drivability of electronic fuel injection (EFI), this combination actually makes identical or better power when finely tuned within its particular operating range.
Air filters are an important induction component because they are the first airflow restriction encountered by incoming air. Over time, racers and hot rodders paid greater attention to air filters and their utility for directing air into the carburetors or throttle bodies. This integral filter top is designed to provide greater flow capacity and is said to help straighten and smooth the airflow.
Highly efficient for their day, mid-1960s Corvette fuel injection systems were thinly disguised tunnel rams with sealed tops and side-entry throttle bodies. It was quite a tidy setup with a bit of ram tuning. Air capacity was more than adequate on this small-displacement 375-hp 327-ci small-block. (Photo Courtesy GM Media Archive)
All engines generate their torque signature based on displacement, engine speed, VE, flow-path dynamics, and not surprisingly, specific architecture (I-4, I-6, V-6, V-8, V-10, V-12, etc.), each of which applies different attributes to cylinder filling, mean net torque, and overall engine smoothness. Every combination generates a torque peak, or “sweet spot,” where its operational dynamics achieve maximum VE.
With competition engines, this often exceeds 100-percent VE, sometimes by a considerable margin. At 100 percent a cylinder contains a volume equal to the same space at atmospheric pressure above the inlet.
The volume within the cylinder also contains a fuel mixture that reduces the amount of air (and oxygen content) by the specified air/fuel ratio. So the two volumes are similar but different. More correctly you might argue that 100-percent VE means that the cylinder has achieved pressure equilibrium with the atmosphere. When a cylinder exceeds 100-percent it effectively becomes naturally supercharged. This occurs by the ramming effect of proper inlet sizing and valve timing to mildly pressurize the cylinder at IVC.
By specifying components to meet VE requirements builders target intake ports, dimensional qualities of intake manifolds and exhaust headers, carburetor size, rod-to-stroke ratios, valve timing, and static compression ratio. The specific component matrix is adjusted to suit the application’s operational requirements.
Oval track and road racing engines typically call for a component mix that produces a broad torque curve over a wide range of RPM. This affords the engine builder an opportunity to tune the intake and exhaust systems separately to effectively broaden the power band. Conversely, drag racing applications seek a higher and narrower power band in which intake and exhaust tuning are more closely aligned. Identifying and targeting the required power band is one of the engine builder’s first steps.
Although VE and engine speed are closely aligned, it is critical to target VE modifications to the desired engine speed. If a drag racing engine leaves the starting line at 7,000 rpm and cycles to 9,000 rpm through the gears, its VE at 5,000 rpm is largely irrelevant. And, of course, an engine delivering power between 4,500 and 7,200 rpm needs a broader tuning efficiency because of its parts combination. Hence airflow management within the targeted engine speed range becomes a central challenge in matching or exceeding an engine’s potential VE capacity.
Trapped Mass and VE
When all is said and done, all you’ve got to work with is the effectively trapped mass you are able to contain and compress within the combustion space (clearance volume) at the point of ignition. That is the effective VE. Larry Meaux uses the following formula in his broadly popular PipeMax Header Design software program. It incorporates parameters that you may fail to consider:
Trapped VE = measured CFM – ring-blowby CFM – CFM lost during overlap
In a properly configured engine, the ring blowby and CFM (cubic feet per minute) lost to overlap should be minimal, but must be considered for accurate modeling. And thus they are integral to modeling programs such as Meaux’s PipeMax, Hale’s Engine Pro, and most other high-quality simulation programs. For the purpose of airflow improvement, you are largely limited to static or steady-state flow measurements of individual flow path components or some combination thereof (where, for example, you might flow a head port with the manifold and carburetor attached).
Many street-strip types of high-performance engines still do not achieve 100-percent VE largely because they are a mixture of economic and manufacturing compromises. Cylinder heads are designed to have broad application on various engine sizes, and the camshafts are typically catalog grinds with the same prerequisites. The intake manifolds and headers cannot be properly sized for optimal performance, and the carburetors are often improperly sized. So these engines, with higher performance than most, may still only achieve a VE somewhere in the 90- to 96-percent range.
Your efforts to improve engine airflow and thus VE center on removing those compromises and substituting optimal flow efficiency across the engine; not just in the intake path, but also in the exhaust path, and most important, past the valves. Unless restricted by rules, you are free to employ any means possible, including some pretty formidable technologies that make racing engines breathe better than ever.
Combustion efficiency is typically indicated by BSFC. This expresses fuel usage in pounds per horsepower per hour. BSFC is frequently misunderstood. Many people mistakenly believe that it is an indicator of rich or lean fuel mixtures, but it actually is a measure of efficiency that indicates how well the engine uses the fuel it burns. More specifically, it is the rate in pounds of fuel per horsepower per hour that a given engine consumes to make power. Most engines have a range of optimal efficiency, and BSFC defines that range.
As you may have already surmised, the term “brake” is the first word because BSFC is usually measured with the engine running on a dyno. BSFC figures are typically quoted for WOT conditions, but it is also a measurable quantity that relates to fuel economy at part-throttle operation. In the performance world, it is used to judge the efficiency contribution of various engine combinations and to predict certain requirements such as fuel-injector flow rate.
A particular cylinder head may make more power with less fuel, and that’s an indicator of higher efficiency, most likely because of improved cylinder filling and a more efficient combustion chamber that extracts more energy from a given fuel mass. Guidelines for evaluating BSFC are well established and are frequently used to predict engine performance. One-half pound of fuel per horsepower per hour (0.50 BSFC) is the default norm for most calculations, but you can adjust this for competition engines.
Herein lies part of the problem if you think of BSFC numbers as indicators of mixture ratios. At 0.37 BSFC, a Pro Stock engine may be thought to be running too lean when, in fact, it is operating at the highest level of efficiency. In contrast, supercharged engines run richer mixtures to complement boost pressure and discourage detonation. They run richer; not because they are inefficient, but to complement specific combustion characteristics inherent to boosted applications, not the least of which is charge cooling and the need for more fuel to augment the greater volume of air being supplied by the supercharging device.
When evaluating BSFC numbers, lower is almost always better (even when supercharged); 0.60 is still more efficient than 0.65, as long as the combination supports safe combustion without detonation or overheating. Any engine still needs to run at the air/fuel ratio that produces the best power. That’s usually about 13:1 in naturally aspirated engines and 11.6 to 12:1 in supercharged applications. One engine with poor efficiency may generate a BSFC of 0.55 while another may run at 0.40, and yet both may have the same air/fuel ratio. You can’t just run the engine lean and expect to get a low BSFC number.
Remember that there is no magic BSFC number that guarantees max horsepower. However, there is a BSFC number that your particular engine generates when performing at its best. The lower the number (within reason), the more efficient your engine is at converting fuel into power. Tune for maximum torque and let the BSFC indicate how efficiently you generate that torque. For example, at an indicated BSFC of 0.50, the engine burns 0.5 pound of fuel per horsepower per hour (lb/hr). If the engine makes 500 hp, that’s 250 lbs/hr. If you’re building a racing engine, your BSFC should be way better; something on the order of 0.38 to 0.42.
These figures are for gasoline only; they differ considerably for methanol applications. With methanol the engine uses much more fuel, something on the order of 1.0 to 1.3 BSFC at the minimum and in some cases as high as 1.7 or more depending on the application. It’s very much dependent on the application and the efficiency of the particular combustion chamber. In drag racing, most applications are supercharged so the BSFC trends higher in, say, an injected car or an injected circle-track car. Short-track cars typically run around 1.0; sometimes even less.
Read your plugs and tune accordingly. Whatever BSFC your engine generates is what you get when you have your best tune onboard. Don’t worry about it for track purposes. And if you don’t think it’s good enough go back to the drawing board and figure out what part of your combination is causing the inefficiency. Determine where that falls in the horsepower chain and take appropriate steps to remedy it.
Remember that BSFC is affected tremendously by the burn characteristics of a particular combustion chamber and the filling and emptying abilities of the flow components. You can chase that free lunch all over the engine, but the horsepower gods have dictated that you have to pay for it somewhere. That’s where optimizing each of the processes in Hale’s horsepower chain becomes so important. Optimization and efficiency go hand in hand.
The induction and exhaust systems are the primary contributors to the total engine airflow equation. The camshaft plays the role of traffic cop, determining when and where air flows and the specific timing of air movement during the power production process. The induction system is the most important, but exhaust system efficiency plays a critical role in the efficiency of overall air movement, and the camshaft still pretty much runs the show.
BSFC Comparisons
Here are a couple of handy equations you can use to calculate BSFC for comparison purposes:
BSFC = lbs/hour of fuel consumed ÷ uncorrected HP
BSFC = (BSAC ÷ A/F ratio)
Where:
BSAC = brake specific air consumption
A/F = air/fuel ratio (typically at peak torque)
When checking dyno results, BSFC is always calculated from the uncorrected, or raw, power figures. If you divide the fuel usage by corrected power numbers, the calculation will be incorrect, and it will throw you off.
Larry Meaux at Meaux Racing Heads provided the following handy equation for making ballpark estimates of fuel consumption when planning a fuel system:
Fuel Consumed = ci × rpm × 0.0001
Where:
0.0001 = mathematical constant
For example, a 565-ci engine running at 7,500 rpm would consume 423.75 lbs/hr (565 × 7,500 × 0.0001).
The induction system comprises everything upstream of the intake valve including the valve, intake port, intake manifold including plenum and distribution runners, and any associated spacers that may be employed. It also incorporates the air and fuel metering device (carburetor or throttle body), air filter, and air scoop or in some cases various types of cold-air packages that redirect cooler air to the induction system via underhood passages and strategically located air inlet openings. In the absence of a supercharging device air moves through the engine based on the creation of pressure differentials or voids (empty cylinders) into which it naturally flows (naturally aspirated) because of atmospheric pressure.
When contemplating engine systems that contribute to maximum VE we typically think of only the induction system. But as Hale’s horsepower chain of processes demonstrates, every link in the chain contributes to or influences the overall VE equation. The induction system is the most familiar player. But it goes nowhere without the exhaust system and all the subtle and less recognized contributions made by the correct combination of short-block components to gain the optimal rod-stroke ratio, effective ring seal, and proper valve and ignition timing. To be sure, most of the important VE actions occur in the cylinder heads where the air/fuel charge is introduced, processed, and discharged with the short-block providing convenient means of transferring the power generated into a rotational force at the flywheel.
In this book I spend a great deal of time on cylinder heads and intake manifolds, but not to the exclusion of any other factors in the horsepower chain. The system has three basic elements: induction system, exhaust system, and short-block. Within the induction system you have air scoops, air cleaners, carburetors, throttle bodies, carb spacers, intake manifolds, and cylinder heads with intake and exhaust valves and the all-important combustion space. The exhaust system also incorporates the cylinder heads and valves plus the headers, collectors, and in some cases full exhaust systems with mufflers or shaped discharge orifices such as collectors. The short-block contributes to VE by providing cylinders or displacement volume. The rotating assembly influences airflow via the rod-stroke ratio and its ongoing rotation. The reciprocating activity provides piston motion to achieve cylinder filling, compression, power transfer, and exhaust pumping.
These elements are inseparable and although the physics and thermodynamics that govern them are extraordinarily complex you only have to grasp a portion of it to gain a working knowledge of how you can influence and manipulate engine airflow to suit your power demands. To that end, I define and examine the engine airflow paths and discuss the appropriate processes that need to occur in each of them. But first I discuss some of the basic properties of air and fuel and how they affect your decisions about how you mix and process them into power.
Despite all of the modern technology and power adders, a single 4-barrel carburetor with conservatively sized intake passages in a light car still makes a remarkably fast hot rod.
Although turbocharging is quickly becoming the go-to power adder, traditional supercharging is still the darling of the hot rod set.
