Читать книгу Ford Mustang 1964 1/2 - 1973 - Frank Bohanan - Страница 7
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CAMSHAFTS, CYLINDER HEADS AND VALVETRAIN
Any internal combustion engine requires the properly timed and correct amounts of three basic things to function: air, fuel, and spark/ignition. Relative to air, the combination of the camshaft, valvetrain, and cylinder heads have the greatest influence over its potential for flow through the engine. Of course, the intake and exhaust systems play a role as well, but their contribution is secondary (see Chapter 3). All early-production Mustangs came equipped with overhead-valve pushrod engines so this chapter covers upgrades applicable to these engines only.
I cannot make specific camshaft/valvetrain recommendations because of the many variables. I can, however, state that a roller hydraulic camshaft almost certainly is the best option for most of the vehicles covered in this book. In the case of a daily driver, it’s unlikely a new camshaft or most other valvetrain modifications are cost effective.
The streetable track-day car almost always gets a new camshaft along with other valvetrain modifications. The cylinder heads, camshaft, and valvetrain are interdependent so they need to be upgraded as a system; they provide a new level of performance. Most vehicles are first modified by upgrading the more readily accessible, bolt-on components such as the intake, exhaust, carburetor, etc.
When all of this has been done the choice is then between leaving the stock engine alone (other than maybe just installing some higher-ratio roller rocker arms), installing a power adder (nitrous, supercharger, etc.), or taking the engine apart to replace the cam, valvetrain, and/or heads. Installing a new combination requires a much higher level of commitment to performance and requires more skill and more knowledge, though not necessarily more cost. You need to determine your budget and performance goals.
You can get similar performance going either route for a similar setup, though some power adders, such as nitrous, are more intermittent in their usability. An “all-engine” solution provides increased performance under all conditions, whenever you want it, but often reaches a point where the engine is too peaky in nature. A mild to moderate performance engine typically relies on bolt-on parts and/or a power adder, with perhaps the only “internal” engine change being the use of higher-ratio roller rocker arms. Once you decide to go beyond moderate performance, power adder or not, you inevitably need to make changes to the internals to safely, reliably, and fully achieve the higher power/performance levels you’re after.
Modifications to the stock valvetrain are necessary when going to higher RPM and they can provide additional performance. The Comp Cams Pro Magnum steel roller rockers shown here are not only stronger to handle the higher valvespring loads associated with a performance camshaft but they also reduce friction and wear compared to stock parts. They free up horsepower and provide much longer life. It’s always a good idea to squirt on some oil before installing the valve covers.
Flat-tappet camshafts have no place in a modern street/strip/track engine and solid roller camshafts are suitable only for pure race cars. Today’s advanced roller hydraulic cams are the ideal choice for most high-performance street engines. They offer the advantages of roller design and internal hydraulics allow them to self-adjust. You get the best of all worlds. You can run very aggressive camshaft profiles with minimal friction and periodic adjustments are not required.
Modern hydraulic roller camshafts essentially provide racing technology for the street but do so without the need for the continuous maintenance of solid rollers. The latter still have their place with the highest-revving and/or highest-output engines used mostly in drag racing. Hydraulic roller cams are now generating very high flow rates, power, and performance while remaining totally streetable.
Many hydraulic roller camshafts are available for Ford small- and big-block engines. A wide range of manufacturers offer so many combinations for various applications and setups that it’s impossible to cover all the relevant combinations. I assume you will use a hydraulic roller camshaft because very few vehicles need a solid roller. Simply put, more lift is better as long as the valves don’t contact the pistons. More valve lift is essentially free performance because the engine breathes better with no real consequence in terms of driveability, idle quality, etc.
Older camshafts needed more overlap and duration to provide higher lifts and flow. Advances in roller hydraulic camshaft ramp design make it possible to achieve pretty high lift without the need to also have excessive overlap and duration. In general, you should go for as much lift as you can while still ensuring an adequate margin of clearance so the valves never hit the pistons. This must be verified. Any reputable engine/machine shop knows to check this and a crate engine manufacturer will surely protect against it to prevent returns due to engine damage.
For most street-driven vehicles valve lift in the area of about .550 inch is plenty for some pretty high-power/high-performance levels. The optimum figure, of course, depends on the power level desired, the intended use, the other components being used, and the camshaft timing events (including how it was installed, i.e., advanced, retarded, or straight up). Keep in mind that valve lift (versus cam lobe lift) is affected by the rocker arm ratio used. On an otherwise stock engine going to a higher rocker ratio increases valve lift to improve flow but usually not enough to cause a problem with clearance.
However, it’s best to start with a standard (usually 1.6:1) rocker arm ratio for a new cam. Then, if still more power is desired and it is safely possible to do so, you can easily change to higher-ratio (1.7:1 or so) rockers on the intake and/or exhaust valves to see if that helps. The rocker swap alone doesn’t normally require that you remove the manifold and heads, etc.; just taking off the valve covers should do it. The reduced friction of the roller-style rockers frees up some otherwise wasted energy while also providing longer life due to less wear.
Changing from a 1:1 ratio to a 1.7:1 ratio, for example, increases lift at the valve from .550 to .585 inch, though in some rare cases it may also be necessary to change the pushrod length if the higher lift at the valve causes interference problems between the rocker arm and the valve.
Ford Racing, Edelbrock, Crane Cams, and others offer roller timing sets for the small-block Windsors and 385-series big-blocks. This Crane Cams roller-style timing set needs to be used in any high-performance Ford engine build. The double-roller chain is stronger and more durable than OEM-style chains. The sprockets of this timing set are also made from 4140 billet steel that’s been nitrided for extra strength and hardness.
The type of hydraulic roller lifters you use depends on whether you’re using a newer or older OEM block or an aftermarket block. The newer (5.0L, for example) OEM blocks and most aftermarket blocks have provisions to use OEM-style hydraulic roller lifters. If that’s the case, regular OEM lifters (or compatible high-performance/race versions) are used for moderate- to high-power engines. The limitation is usually either RPM or the type of block being used. Stock-type blocks can usually be modified to accept the OEM system. Two holes are drilled and tapped in the valley area to accept the bolts for the lifter retaining plate (the “spider”). This setup works reasonably well up to about 6,500 rpm with a cam of .550 inch or less lift.
If you have a block that doesn’t make provisions for these lifters all is not lost. Crane Cams and others offer retrofit hydraulic roller lifters that look just like the solid roller lifters of old, except they have hydraulic internals. These drop into an older-style block and are even simpler to install than the newer, OEM-style roller hydraulic setup because these don’t require a spider or “dog bone” lifter guides. They also have precision-fit plunger assemblies and other features that provide increased RPM potential.
Crane’s Retrofit Series, for example, can also be used to replace OEM-type hydraulic rollers for potentially higher RPM because they are more stable at high revs. Sufficiently strong, hardened (to prevent wear from contact with the guideplates), compatible pushrods need to be specified by length after the rest of the valvetrain has been assembled. When only production components are used a standard-length pushrod can be used. However, even when combining components from the same aftermarket manufacturer it is still necessary to determine the correct pushrod length for your particular engine before installing them. This is due to the unpredictable combination of components and tolerances involved plus the need to ensure the rocker arm geometry is correct to avoid interference.
All major valvetrain manufacturers offer roller rocker arms. Roller rockers reduce friction over conventional rocker arms and you often see an increase in horsepower. Ideally, the rocker tip/roller should be slightly off-center at rest, pass over the center of the valve tip as the valve opens, and reach a point about the same distance from the center (on the other side) at full lift. You also want to ensure there’s no interference between the rocker arm and any of the other components throughout the full range of its motion.
Comp Cams, Dart, and others offer stud-mounted rocker arm hardened guideplates to help improve valvetrain stability. These from Dart Machine are used to keep the pushrods properly located relative to the rocker arms. The plates restrict the lateral movement of the similarly hardened rocker arms. These Dart plates are unique in that they’re adjustable to allow precise and independent centering of each pushrod under its respective rocker arm.
When using non-OEM/direct replacement components, verify that the pushrod length is correct to avoid problems. This is especially true with an aftermarket block and/or a block or heads that were machined (such as for a change in deck height). Use an adjustable checking pushrod (shown) to determine the correct length/geometry. Vary the length of the pushrod to achieve the correct placement of the rocker arm tips over the valve tips while closed with the proper lifter preload adjustment.
The camshaft and its maximum lift has the greatest affect on choice of springs. To some extent, the mass of all the components between the camshaft and the valve tip also comes into play but this is more of a concern to racers running at extremely high RPM than it is for those with street cars.
Beehive springs provide better valve control at a reduced weight when compared to standard springs. In addition, these springs are less susceptible to harmful harmonics. The oval-shaped wire allows a shorter spring height for a given pressure rating while the tapered profile and smaller retainer reduce mass to allow higher RPM use. This type of spring is commonly used on production engines and race engines. (Photo Courtesy Crane Cams)
The correct springs provide just enough seat pressure and open pressure at maximum lift (using the correct installed height), without any spring bind. Excessive spring pressures not only cause unnecessary stress on the valvetrain components but also hurt performance and increase the likelihood of component failure. You can use a single, dual, or beehive spring but it needs to be suited to the head and the particular engine setup. It’s best to follow the recommendation of the camshaft supplier or, if the springs come with the heads, verify their compatibility.
Assembled cylinder heads typically come with springs and hardware optimized for a particular use in terms of rev range, spring pressures, and so forth. They work well with the majority of camshafts aimed at the same purpose but it’s always best to verify that with the camshaft manufacturer beforehand. There’s no need to use exotic spring hardware unless you’re going to really high revs.
Street-driven vehicles rarely need correctly rated conventional dual springs along with steel retainers and locks unless you plan on going to very high RPM and/or use a very aggressive camshaft. Dual springs provide some extra safety and redundancy. If one of the coils fails, it can prevent the valve from dropping into the chamber. Aluminum or titanium retainers are only suitable (justifiable) for race use, as are special keeper/lock designs. (Photo Courtesy Crane Cams)
For the vast majority of engines covered here a good-quality steel retainer is fine for a rev range of about 6,500 rpm or less plus they last indefinitely. Lightweight titanium retainers with special valve locks may be required at the super-high RPM of race engines. The same goes for lash caps, spring locators, and stud girdles. Hardened-steel spring cups should, however, always be used on aluminum heads to prevent the springs from wearing away the surfaces they rest on.
The basic choice for valvestem seals is between so-called umbrella seals (usually what’s used in production) and more-sophisticated Teflon/PC seals (commonly used in racing). Umbrella seals work well for most of these engines as long as they are made from Viton or a similar high-temperature-rated material, which works better than rubber and lasts longer, especially in a higher-output engine. Teflon seals are even more effective at keeping oil away from the valvestem, possibly too much so in street use.
Depending on the type of valveguides, not enough oil may get between the valve and the guide with PC seals, thus potentially causing premature wear. Installing PC seals requires the cylinder heads to be machined around the valveguides so they fit. This is an extra expense that may not be justified on a street performance car, yet it may be more feasible for a streetable track-day car, which sees less street use and where the valveguides can more likely be modified to ensure the appropriate amount of oil reaches them under all conditions. It really comes down to a tradeoff between maximum sealing and maximum valveguide life. Other factors such as spring pressure and maximum RPM also influence the choice.
Should very high valvespring pressures be needed an additional means of increasing valvetrain stability and reducing unwanted deflection is to use a rocker arm stud girdle. It ties the rocker arm studs together so they reinforce each other. This requires longer adjustment nuts and taller valve covers. (Photo Courtesy Trick Flow Specialties)
Umbrella seals control oil well in many cases, especially when they are made of a superior and more heat-resistant material such as Viton. They just slip on with no machining required and are very inexpensive. They are suitable for a daily driver and probably most high-performance street cars. Teflon/PC seals (not shown) generally provide better oil control, but may allow too little oil to reach the guides. The guides are often machined with spiral grooves to help retain enough oil on the valvestem to prevent excess wear. (Photo Courtesy Comp Cams)
It’s common practice when upgrading the camshaft and valvetrain to also upgrade the cylinder heads, at least to some degree. By changing the cam, you inevitably need to increase the flow through the heads to get the most benefit from the cam and other upgrades. To achieve optimum performance the engine and its various components need to be treated as a system. All the various parts are matched to achieve the best balance of performance, driveability, and durability. This aspect is neglected far too often by those who just throw parts together without evaluating their compatibility. It’s almost always a question of balance between the desired outcome and the characteristics of the components. On the high end of the power and RPM scale things can surely get a lot more extreme than I address here.
Head Gaskets
You must ensure the head gasket bore size is correct (especially on a stroker engine) and that the gasket thickness is what you want in terms of durability and compression ratio. Standard “blue” composite, Teflon-coated gaskets are fine for most naturally aspirated engines. When power and RPM become very high and/or a power adder is entered into the mix, it’s best to step up to a multi-layer steel (MLS) gasket set (such as those from Cometic). MLS gaskets don’t require grooves to be machined into the head’s deck surface but they do have certain requirements for the surface finish of the block and the heads.
To better utilize the superior sealing qualities created by MLS gaskets and the thicker deck surfaces of AFR, Dart, and other aftermarket heads it’s also best to use ARP head studs instead of bolts. They’re made from a vastly superior steel alloy that’s much stronger and stretches much less even when the highest cylinder pressures are encountered. Studs are also inherently superior to bolts because they help distribute the clamping load more evenly over the deck surface and within the block, and this greatly reduces head gasket failures.
MLS gaskets, such as these from Cometic, are a huge improvement over older gaskets. Steel’s natural strength and resistance to heat is far more durable than other materials. When multiple thin layers of the proper type of spring steel are stacked they have a greater ability to maintain a seal under extremely high combustion pressures. They naturally flex to maintain a seal, yet return to the nominal position when the load is removed.
A bolt twists and stretches as it’s installed, which can lead to less-accurate torque readings. A stud only stretches as the nut is tightened so the torque reading is more accurate. This improves durability and sealing plus it also helps reduce the distortion of the cylinder walls. That improves sealing and performance while also reducing oil consumption. The proper lubrication and torque figures must still be used but studs are just better at clamping things together. They also can speed up engine rebuilds at the track.
Cylinder Head Comparison
Because the daily driver doesn’t justify a cylinder head upgrade for the expected power level and budget the following is a discussion of a street performance car and a streetable track-day car only. And because there can be some overlap between these vehicle types based on power level they each have a different head. The Dart head has a 347 stroker and the AFR head has a 427 stroker. Both are aluminum so each saves about 30 to 40 pounds per head compared to iron. Generally, they also allow up to about .5 more compression ratio to be used because aluminum heads get rid of combustion heat more efficiently. (Nowdays the only reasons to stay with iron heads are for originality or to lower cost.)
Example One: Dart Machine
Dart offers the Pro1 series of cylinder heads in several configurations. The 170- and 195-cc heads (62-cc combustion chambers) are best suited to moderately to highly modified street cars. The 195-cc version provides enough flow to reach the limitations of the stock engine block and other components. The 195 head is better suited to a more heavily modified (bolt-ons with a more aggressive cam plus more displacement/CR and/or a power adder) street car. The 170-cc version is best suited to a mildly to moderately modified (some bolt-ons and a fairly mild cam) smaller engine and/or when low- and mid-range torque are more of a priority than top-end power.
These heads were developed using Dart’s “wet flow” technology, which provides higher flow numbers and better atomization of the air/fuel mixture. It considers the differences between designing a head with a more representative air/liquid mixture and just using dry air. This better simulates how an actual air/fuel mixture behaves as it flows through the head and can significantly affect port shape in pursuit of the best real-world flow and performance. Airflow and fuel flow are measured to help optimize each head not only in terms of maximum flow but also in terms of how well a fixed air/fuel ratio is maintained across the full flow range.
This comparison of an OEM 5.0L head (bottom) and an aluminum Dart Pro1 head (top) reveals many differences. Besides being about 30 to 40 pounds lighter the Dart head also has a revised, more-efficient, and faster-burning combustion chamber design (while still retaining the stock volume, 62 cc in this example). Valve sizes are bigger for better flow and both valveseats have been precisely machined for better flow and durability. The port shapes and volumes (195-cc intake, 65-cc exhaust) have been optimized for their intended power level. The exhaust ports have been raised slightly (.125 inch) to aid flow, yet these heads still can accept virtually all stock components or their aftermarket equivalents. They also have other design enhancements such as a thicker deck and superior materials.
The change in combustion chamber design of many aftermarket cylinder heads can require the use of some different components. This Dart Pro1 head requires the use of a spark plug with a longer reach (left) than the plug type used with the stock head (right). If the stock-reach plugs were used the result would likely be misfire because the plug gap is sunk so far into the head. (Photo Courtesy Dart Machinery)
Besides the difference in intake port volume (that’s what the numbers 170 and 195 refer to), the higher-flowing 195 head also has a bigger intake valve (2.02 versus 1.94 inches). These heads retain the standard valve angles and port spacing so all OEM-compatible bolt-ons should fit. They’re made from a virgin 355-T61 alloy for greater strength and less risk of casting problems. They benefit from heart-shaped combustion chambers that increase combustion efficiency and burn rate. They also reduce the potential for dangerous preignition or detonation.
The exhaust runners have been raised .135 inch to improve flow through the 1.60-inch valve. The exhaust valveseats have been hardened and radiused to improve flow. Similarly, the intake valves rest in multi-angle valve-seats to improve flow into the cylinders. All of the valves are located by long-lasting manganese-bronze valveguides.
Dart Pro1 CNC heads further improve on the standard Pro1 heads by using precise CNC machining to ensure each port and chamber is just like the others (dimensionally) and thus should flow nearly the same, which is very desirable. The Pro1 CNC heads are available in 210- and 225-cc port volumes and are obviously meant for high power levels and RPM in heavily modified engines with aftermarket engine blocks and more aggressive cams.
The slightly raised exhaust ports of the Dart Pro1 heads shouldn’t cause any problems with the fit of headers. These heads (as well as the CNC versions) come with dual-exhaust bolt patterns to allow the use of headers with wider bolt spacing. This reduces the restriction coming out of the port because the header tubes are not pinched to provide bolt clearance.
When building a high-performance Windsor small-block, the AFR Renegade is an option. These heads have large 2.08-inch intake valves and standard CNC machining. These 205-cc heads flow more than the previous version of 225-cc heads yet are still suitable for street use because they don’t sacrifice mid-range flow or velocity to get the higher maximum flow numbers. This greater “area under the curve” allows AFR to claim their heads generally outflow similar heads of most competitors while their full-CNC heads cost only a small amount more than the as-cast heads of competitors.
AFR also offers a 220-cc Renegade head meant for large displacements and even higher RPM ranges. While the 220-cc versions may be a bit marginal for street use the 205s are ideal because of their high flow even at low lifts and revs.
Example Two: Airflow Research
The AFR 205-cc Renegade heads are a different animal with a different purpose. They are still a street-worthy OEM-style bolt-on head that accepts all of the components meant for a stock head but they’re meant for significantly higher power levels. The interesting thing about these 205-cc AFR heads is that this generation outflows the previous-generation 225-cc heads. The technology employed by AFR somewhat separates flow from port size in that a smaller port volume can be used to maintain better low-speed/part-throttle performance without losing too much flow at the top end. AFR strives for more “area under the curve” by maintaining relatively higher flow at lower valve lifts as well as just at peak lift.
Larger port-volume heads are still the preferred choice for maximum output, mostly competition-oriented vehicles where general street use and lower RPM performance are not priorities. The two things that make the greatest contribution to this capability are the larger, 2.08-inch 21-4N lightweight intake valves (exhaust is 1.60 inches and is made from superior 2132 stainless steel) and the full use of CNC porting on every head.
The larger intakes allow for more flow through the increased “curtain area” as the valve is opened. The use of CNC machining for the ports and combustion chambers improves flow by producing more-complex shapes and a smoother surface finish than an as-cast head. It also does so the same way every time, thus ensuring greater consistency from cylinder to cylinder and head to head. This allows for optimum tuning because the fuel and spark requirements for each cylinder are much more similar, eliminating the need to tune for the worst-case cylinder. Even the best hand porting cannot duplicate this level of consistency or accuracy on a port-to-port, head-to-head basis.
As expected, the AFR head shares much of the technology seen in the Dart head: It was developed using wet flow technology, the deck is extra thick (.750 inch), the exhaust ports have been raised slightly (.125 inch), the intake valves get a multi-angle valve job, and the exhaust valves are radiused. In keeping with its higher flow and RPM goals the AFR head also has lightweight valves with 8-mm stems to allow better flow (by creating more flow area) and potentially higher RPM use with less risk of damaging valve float. The lighter valves allow for the use of lighter springs, retainers, and so forth, which can help reduce the reciprocating mass of the valvetrain. This improves valve control so the camshaft profile reduces float and allows more RPM.
The completely CNC’d combustion chambers on the AFR Renegade heads make a very significant contribution to performance. The consistency between chambers is a critical asset; 58- and 72-cc versions are available. The heart-shaped chambers improve combustion speed and efficiency. Also, the AFR chambers have a smoother finish to aid in-cylinder motion and minimize the potential for sharp edges, which can cause hot spots that lead to detonation. The valves have been significantly unshrouded to aid flow in and out of the cylinders. Note the funnel effect around the exhaust valves.
Precision CNC machining can create very complex and efficient port shapes. They also can be almost identically reproduced over and over again, on the same head or any number of heads. This greatly improves the performance potential when both heads are so close in specification.
The springs are a small-diameter dual design made from chrome silicon vanadium with nickel that’s given a special heat-treating process. AFR claims these springs perform as well or better than even “beehive” springs up to about 7,000 rpm, plus they provide the additional benefit of redundancy in the event of failure.
For the most extreme applications, AFR offers even stronger, higher-rate, larger-diameter springs that can be used up to .710-inch lift. These are of a similar material and construction plus they undergo a two-stage shot-peening process to further increase their strength and durability.
AFR’s heads all come with more-durable and more-reliable bead-lock keepers (which spread the spring load more evenly and are less likely to fail). They also have special hardened-steel spring seats with integral locators (to protect the head surface and keep the springs properly centered around the valveguides and their high-quality Viton valvestem seals). AFR’s clean-sheet-design heads are free from the restrictions of the OEM head design and are fine for all but the most radical engines/vehicles intended to be driven on the street, however rare that may be.
Valve Covers
To top off your new heads you want to use a stronger cast-aluminum valve cover set and silicon-metal gaskets instead of the stamped-steel and cork OEM-style components. The cast covers not only look much better but they are more rigid to reduce the chance of leaks.
The perfect complement to cast covers is composite gaskets, which have a metal core covered with silicon gasket material. The metal cores usually have built-in stops to prevent overtightening of the fasteners while the silicon is usually molded with robust knife-edged sealing surfaces, which can be used over and over because no gasket sealant is needed. The covers also come off very easily.
For engines with a 5.0L EFI-style intake manifold that extends over the head a lower-profile valve cover is generally needed for manifold clearance. Check internal clearance to the cover too.
