Читать книгу Modern Engine Blueprinting Techniques - Mike Mavrigian - Страница 12
ОглавлениеThis chapter provides information that will aid in understanding essential dimensions and volumes that are considered when planning and executing an engine build.
In general, a long-stroke engine (an engine with a relatively long stroke in relation to bore diameter) revs slower but produces more torque at lower RPM. A short-stroke engine (short stroke in relation to a larger bore diameter) revs higher, and produces peak power at a higher RPM range.
A crankshaft’s stroke dimension is the total stroke of the crankshaft. This is measured from the rod pin’s BDC to TDC positions. When selecting a crankshaft, connecting rod, and piston combination, you use one half of the crankshaft’s published stroke dimension in your decision making. The distance from the centerline of the crank rod pin at TDC, plus rod length, plus piston compression distance is the length that must fit within the block’s available deck height dimension (the distance from the main bore centerline to the block’s cylinder head deck surface).
As an example, if you install a 4.000-inch-stroke crankshaft, the crank and connecting rods have a 6.125-inch length and the pistons have a compression distance of 1.115 inch. The block has been square-decked to a deck height of 9.234 inches and achieves 403.13 ci with 4.005-inch cylinder bores. With that stroke/rod/piston combination, the pistons protrude above the decks by .006 inch, which is more than compensated for by using cylinder head gaskets with a crushed thickness of about .045 inch.
The stroke package must fit within the block, so you must always consider the block’s deck height. LS factory blocks, as an example, are notorious for having unequal deck heights (high/low side-to-side and/or front-to-rear). So before choosing your stroker combination it’s wise to first have the block decks surfaced in order to establish equal deck distance from the crank centerline. You can probably fudge this and assume that the decks are okay, but if you want absolute precision, correct (or at least carefully measure) the block deck height at all four corners (right-front, right-rear, left-front, and left-rear) before spending money on rods and pistons for a stroker combination.
The stroke package must fit within the confines of the block, so you must always consider the block’s deck height. Deck height is the distance from the main bore centerline to the block’s cylinder head deck surface. Most OEM blocks (because of a wider range of manufacturing tolerances) have uneven block decks, especially deck height and deck taper. High-quality aftermarket blocks will commonly provide extra deck height to allow you to cut the decks to exactly suit your piston-to-deck clearance.
Even today’s OEM LS factory blocks, to cite but one example, are notorious for having unequal deck heights (high/low, side to side, and/or front to rear), so it’s wise to first have the block’s decks surfaced in order to establish equal deck distance from the crank centerline before choosing your stroker combination. You can probably fudge this and assume that the decks are okay, but if you want absolute precision, correct (or at least carefully measure) block deck height at all four corners (right front, right rear, left front, left rear) before spending money on rods and pistons for a stroker combination.
Pictured here is a forged 4.000-inch stroker crankshaft from Lunati. This crank was coupled with 6.125-inch-long connecting rods and pistons with a compression distance of 1.115 inches in a factory LS2 block. This block was square-decked to a deck height of 9.234 inches and resulted in 403.13 ci with 4.005-inch cylinder bores. With this stroke/rod/piston/block combination, the pistons protrude above the decks by 0.006 inches. Using cylinder head gaskets with a crushed thickness of approximately 0.045 inches more than compensates for this difference.
When discussing the “length” of a connecting rod, it is not simply the total length. Instead, I am referring to the distance from the centerline of the crankshaft pin bore (the big end) to the centerline of the wrist pin bore (the small end). Precision rod-length specialty tools are designed to precisely measure this. However, if you want to perform a rough check of an existing rod, you can use a long caliper with a dial or a digital caliper with a range greater than the length of the rod.
Using either method, you measure from the bottom of the wrist pin bore (6 o’clock) to the top of the big-end bore (12 o’clock). Record this distance. Then measure the diameter of the wrist pin bore and the diameter of the big-end bore. Record these diameters. And plug the figures into this formula:
Rod Length = A + 1/2B + 1/2C
Performance aftermarket cranks are usually stamped or etched on the face of the front counterweight indicating the crankshaft stroke. In this example, the crank stroke is 4.750 inches.
Where:
| A = | distance from bottom of the wrist pin bore to top of the big-end bore |
| B = | diameter of wrist pin bore |
| C = | diameter of big-end bore |
Although this isn’t the most precise way to measure, it gives you a rough idea of your rod’s center-to-center length, for rod identification purposes. Trying to measure rod length with a ruler by guessing at the two bore diameter centers is a waste of time.
Today’s quality aftermarket connecting rod manufacturers (such as Scat, Lunati, Oliver, GRP, Crower, Callies, and more) produce rods at extremely tight tolerances for center-to-center length. It is very uncommon to find a set of rods that are not precisely at the specified length and all the rods in a set are not matched.
Although you can rely on performance rod dimensions in general, when blueprinting, you don’t want to assume anything. Measure each rod for length, small-end diameter, and big-end diameter. Knowing exactly what you have is better than guessing or assuming. If you’re using OEM rods, you must check all dimensions due to the greater potential for tolerance deviations.
Rod length has a direct relationship to engine performance characteristics. Granted, the rod length is part of the TDC dimension (from centerline of the crank rod journal at TDC to piston dome location relative to the block deck), but the rod length can be selected in combination with crank stroke and piston compression height in order to tailor the engine for certain performance characteristics. A shorter rod is slower at the BDC range, but faster at the TDC range. A longer rod is faster at the BDC range but slower at the TDC range.
Here’s an explanation from Stahl Headers: “With a longer rod, the intake stroke draws harder on the cylinder head from 90-degrees after top dead center (ATDC) to BDC. On the compression stroke, the piston travels faster from BDC to 90-degrees before top dead center (BTDC) with a longer rod; but travels slower from 90-degrees BTDC to TDC, which may change the ignition timing requirement. It is possible that a longer rod could have more cylinder pressure at 30-degree ATDC but less crankpin force at 70-degrees ATDC.”
Rod dimensions: A) center-to-center length; B) bend and twist; C) rod width; D) rod offset; E) big end bore; F) small end bore; and G) bearing tang location.
Check each piston location at TDC, relative to the block deck (after the block has been decked). If there is any deviation from bore to bore, you may be able to swap rods (if there is any CTC length deviation in your rod set) in order to equalize all piston deck heights.
On the power stroke, the piston is farther down the bore for any given rod/crank pin angle. At any crank angle from 20- to 75-degrees ATDC, less force is exerted on the crank pin with a longer rod. However, the piston is higher in the bore for any given crank angle from 90-degrees BTDC to 90-degrees ATDC, so cylinder pressure could be higher. A longer rod spends less time from 90-degrees BTDC to BDC, which allows less time for exhaust to escape on the power stroke and forces out more exhaust from BDC to 90-degrees BTDC. If the exhaust port is not efficient, a longer rod helps produce peak power.
Connecting rod length always refers to the distance from the center of the wrist pin bore (small end) to the center of the crank journal bore (big end). (Illustration Courtesy Lunati)
In order to place the piston at or near the block deck on TDC, the rod and crank stroke combinations can include a shorter stroke crank with a longer rod or a longer stroke crank with a shorter rod.
Long Rods
A longer connecting rod provides a longer dwell time at the TDC range. This helps to extend the compression state by keeping the combustion chamber volume small, which is good for mid- to upper-RPM torque. A longer rod reduces the rod angle, which helps to reduce friction. Also, with a longer rod, you can run a shorter piston compression height (that means a lighter piston), which helps to gain RPM.
However, longer rods are less efficient at promoting volumetric efficiency at low engine speeds. The piston moves from TDC (downward) at a reduced rate, gaining its maximum speed at a later point of crank rotation. Longer duration camshaft profiles tend to reduce cylinder pressure during the closing period of the intake cycle. Longer intake manifold runners with slightly smaller port volumes may be needed. Longer rods also pose more of a clearance issue (camshaft, bottom of cylinders, and pan rails).
Short Rods
Shorter rods provide higher intake and exhaust speeds at lower engine RPM, which improves low-end torque (and promotes higher vacuum). Shorter rods increase piston speed as it travels from TDC on the power stroke, which increases chamber volume. This delays the point of maximum cylinder pressure, which is a good match for forced induction (supercharger, turbo, and nitrous injection). Shorter rods also allow more radical camshaft timing. However, since a shorter rod increases piston travel from TDC, at high RPM the piston can run away from the flame front faster, which can decrease total cylinder pressure.
Most high-quality aftermarket rods have a high degree of precision machining, with center-to-center lengths dead on or within 0.0005 inch. Even so, always measure each rod, each crank rod journal stroke, and each piston to verify dimensions and to avoid a stack-up of tolerances. For example, if one rod is 0.0005 inch shorter than the other rods, dedicate that rod to the crank throw that is 0.0005 inch longer, or with a piston that has a compression height that’s 0.0005 inch taller. If there are any deviations in tolerances, measuring each component will allow you to mix and match to optimize equalization of all cylinders.
In a nutshell, run a shorter rod for the street or whenever low-end torque is the priority and run a longer rod where you want peak torque to occur higher in the engine RPM band.
Keep in mind that some rod, crank stroke, and piston CD combinations don’t work or are impractical due to clearance constraints or unavailable piston compression heights.
Rod ratio refers to the relationship of the rod length to the crankshaft stroke. Theoretically, a higher rod ratio produces more torque at peak RPM, and a lower rod ratio produces more torque at lower RPM.
Depending on the type of engine being built, there is a target range for rod ratio. So, a higher rod ratio for racing and a lower rod ratio for street performance seem to make sense.
Here’s the formula for calculating rod ratio.
Rod Ratio = rod length ÷ crank stroke
For example, you have a rod length of 5.700 inches with a 3.000-inch stroke. Using the formula:
Rod Ratio = 5.700 ÷ 3.000
1.90:1
Computer Software
Computer software doesn’t build an engine. Machining and assembly skills are still required for the build itself. However, software related to engine building can serve as a tremendous aid in your quest to design and, in some cases, test a virtual engine build combination. These programs allow you to experiment with various engine component and dimensional combinations. That gives you theoretical performance insight into horsepower and torque via different bore and stroke combinations, cam profiles, cylinder head and intake flow, rocker arm ratios, and more.
Most programs include handy calculations that allow you to plug in various data and obtain quick answers, so you don’t have to perform the math on your own. This type of computer program allows you to play “what-if” games by trying different combinations of components.
For pro engine shops that utilize CNC equipment, highly sophisticated programs are available for digitizing, designing, and machining individual components. This type of program (Mastercam, etc.) applies only to design and machining processes and are highly technical in nature. They are not discussed here.
The following manufacturer list includes programs suited to the needs of the enthusiast. It does not, however, include “games” designed to let you play drag racer, oval track racer, or road racer. I’ve limited the findings strictly to those that apply specifically to the process of engine component selection, engine math, modifications, and dyno simulation. They each have different capabilities. The information here should save you time and aid in your selection of various component and dimensional choices.
Comp Cams
The Desktop Dyno 5 software program is reportedly designed to apply to any four-cycle engine, ranging from four-cylinder to twelve-cylinder applications. It has an interface capability that provides a series of DirectClick menus. This permits the selection of specific components and the ability to enter parts with custom specifications.
A series of built-in calculators includes a CamMath Quick-Calculator, Induction-Flow calculator, and an Air Flow Pressure-Drop calculator. Test combinations are illustrated with detailed graphs that display projected horsepower, torque, volumetric efficiency, and engine pressures. The program’s automated testing tool provides additional support as you attempt to determine optimum component combinations. Another useful feature is a combustion chamber modeling program. Windows 7 compatible.
ProRacing Sim
The DynoSim5 is a Windows-compatible program designed to try out your engine build in a virtual state. According to the manufacturer, this software package allows you to experiment with simulations of a variety of build platforms, including forced induction (turbocharging, supercharging, and nitrous injection).
Additional features include ignition curve modeling and rocker arm ratio calculation. The DynoSim5 is also designed for program updating online. A CamDisk8 supplement provides data applications for more than 6,000 camshafts. A specific cam profile can be selected and added to your virtual engine build for testing and simulated dyno results.
The Dynomation-5 Wave Action Engine Simulation is an advanced engine simulation software. It illustrates live pressure waves and airflow through the cylinders and engine passages. In addition to providing horsepower data for a given combination, the wave action program provides a 3D cutaway engine view that illustrates airflow, intake and exhaust port pressures, and velocity according to the cranskshaft angle.
The DesktopDyno5 offers a host of simulation features. (Photo Courtesy ProRacingSim.com)
This program provides an aid in analyzing intake runner length, volume, and shape in conjunction with cylinder head flow and variations in camshaft profile. Windows 7 compatible.
Challenger Engine Software
Challenger offers three programs suitable for engine design and analysis: Virtual Engine Dyno Professional, Dynamic Compression Ratio Calculator and Camshaft Selection Utility, and Engine Builder 3D. They are only available by purchasing website downloads. All are compatible with Windows 95, 98, 2000, NT, XP, or Vista.
A sample screen display from Virtual Engine Dyno Professional.
