Purpose-built competition engines vary in content according to the particular requirements of their intended application. Some are built for very high RPM with a narrow powerband; others are designed for a lower, broader power range with greater emphasis on drivability and endurance qualities. All of them target operational requirements specific to their racing application and are often functionally unsuitable outside their intended performance environment. A drag racing engine wouldn’t last five laps on a challenging road course like Road Atlanta or Laguna Seca, and a sports car engine couldn’t hope to match the high specific output of a 10,000-rpm small-displacement Competition Eliminator engine.
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Bonneville engines require some qualities of both engine types, effectively supporting what amounts to a 5-mile drag race demanding big horsepower to overcome aerodynamic drag and stout internals to endure the long, hard pull. Unlimited sprint car engines run extreme engine speeds with frequent throttling that places severe shock loads on internal parts while supercharged and turbocharged engines each have their own unique requirements that are completely different from a naturally aspirated superspeedway “Cup” engine or sportsman class Saturday night specials.
The focus of this book is primarily directed at naturally aspirated (all motor) engines, but it will be evident along the way that many of the principles also apply to boosted and nitrous applications. In every case, fundamental engine building practices are mandatory, but build content and assembly practices are very much application specific or, in many cases, rules specific depending on the type and level of competition.
Despite broad differences, all competition engines seek optimum manipulation of the properties of fuel and air tocreate maximum volumetric efficiency (VE) and cylinder pressure to drive the pistons and thus the car. Mechanical components are chosen according to established engineering principles and then carefully matched to achieve this goal with the specific requirements of a given application in mind. In many cases these efforts are limited by class rules or operational parameters that dictate pre-defined engine content. Hence it is prudent to list and examine all of the applicable requirements when planning a competition engine build.
Preliminary planning steps help identify potential problem areas and ensure the best possible blend of performance parts to suit the intended application. Operational requirements and sanctioning body limitations require careful deliberation prior to finalizing the parts manifest. This includes critical examination of every potential component and the assistance of home PC computer simulation programs that can help you estimate potentially ideal combinations based on prevailing rules and requirements.
Consider the Limitations
Unlimited engine combinations enjoy the best possible repertoire of race engine theory and high-performance parts. Designer/builders are free to tailor the package to perfectly match known operational requirements without regard to strict boundaries enacted to limit power and speed or to control the expense of a particular racing series.
Unfortunately, many racing venues enforce some level of restriction that suits their particular goals or racing philosophy. That’s not necessarily a bad thing, as it clearly defines fixed goals for you to target. Some builders relish the challenge of wringing every last bit of power from an engine that has been administratively handicapped. Others abhor it. Accordingly, it may be useful to preface a discussion of engine building strategies with an examination of common handicapping methods to review how they might affect a particular build. Some or much of the following may apply to a specific program.
Racing associations often enforce displacement limits to control engine output. These rules are implemented to limit speeds in certain series or to differentiate classes for elapsed time or top speed, as found in drag racing or at Bonneville. Displacement limits are rigorously enforced, but sactioning bodies still present opportunities to optimize specific packages under the prevailing rules in some cases. When displacement is specified, but the bore and stroke combination is left open, builders often gravitate toward the largest possible bore dimension to achieve maximum breathing capability with larger valves and more effective piston area for combustion pressure to apply force against the piston top. This shortens stroke length and generally tends to raise the operational powerband (RPM).
This trend is favored because it aids breathing and reduces piston speed for greater durability. Sometimes builders prefer a longer stroke and a broader powerband more suitable to certain track layouts. This reduces bore size, but the final bore/ stroke combination ultimately seeks the best possible compromise that accommodates the displacement limit while biased as much as possible for breathing efficiency and equivalent piston area. Drag racers lean toward big bore/short stroke combinations, while short circuit, road race, and oval track racers favor more stroke length and a lower powerband to reinforce endurance qualities and torque production for tight corners and shorter straightaways.
Once the desired powerband has been defined, PC engine simulation software (such as Performance Trends Engine Analyzer Pro or Motion Software’s Dynomation 5) can often provide valuable direction in choosing the optimum bore/stroke ratio and corresponding connecting rod length. These are important considerations when planning a series-specific engine. It may be somewhat instructive to know what your competitors are doing, but don’t assume that it is necessarily best for your own effort.
Through careful evaluation and computer simulation, you may identify a superior combination that takes greater advantage of other factors such as car weight, rear axle and transmission ratios, shift point RPM drop, tire size, and other contributing factors that affect the total vehicle package. Within a fixed displacement window, you have to identify the best possible bore and stroke combination for the way that the car will be driven. Whenever the rules stipulate these sorts of limitations, new opportunities often emerge.
Aluminum blocks are accepted in many types of racing, but there are several factors to consider whenever you have a choice. Aluminum blocks are pretty comfortable in a 500- to 800-hp environment and some racers run them at substantially higher power levels. Still, many builders prefer an iron block, especially for applications that are supercharged or nitrous assisted. They feel that iron race blocks are more dimensionally stable in those high-stress environments. Block stability promotes superior ring seal and less friction, which many feel is an acceptable trade-off for any weight penalty caused by the heavier block.
Aluminum blocks have improved dramatically over the past decade or more and the current liners are very good. If you have the option of running an aluminum block, consider the weight savings versus the application carefully to determine how well it serves your needs. If weight is a critical factor, aluminum becomes attractive, but alloy blocks are a good bit more expensive. Evaluate how hard your combination is going to stress the block with RPM and high cylinder pressures and proceed from there. And don’t forget to consider thermal characteristics as they apply to cooling and heat retention in the combustion space (see Chapter 3 for more details).
Some sportsman racing series limit cylinder bore size and in some cases even the spacing. These are typically cost measures designed to curtail the use of expensive cylinder blocks with revised bore spacing that permits larger bores while retaining desirable cylinder wall thickness and stability. Sprint Cup engines are a good example. They are displacement limited to 358 ci and a maximum bore of 4.185 inches. Cup engines previously operated with a bore spacing of 4.400 inches, but NASCAR allowed a bore spacing increase to 4.500 inches to accommodate larger bores, bigger valves, and revised valve geometry. All this is in an attempt to level the playing field among various brands. If the cylinder bore is not specified, you must choose a bore dimension that best suits the particular application as defined by airflow and combustion chamber requirements, compression ratio, flame travel, and other factors, including a specific stroke length that also accommodates your operational requirements.
Compression ratio is an effective means of limiting power in some series and also curbs cost. It typically influences piston and cylinder head selection where a particular cylinder head may also be specified. When cylinder head and chamber size are dictated, piston configuration, deck height, and gasket thickness must be juggled to chase the compression ratio requirement. Short tracks frequently enforce a 9:1 rule while NASCAR engines are limited to 12:1. Unlimited drag racing and Bonneville engines often exceed 14:1 while stock class drag racers are limited to the original factory compression ratio of their particular vehicle. Compression ratio limits generally dictate flat-top pistons, which encourage efficient combustion while maintaining desirable quench to promote charge turbulence and maintain mixture quality.
Hypereutectic pistons are often specified, although forgings are permitted in some series. There is certainly less bang for the buck without higher compression ratios, but given specific parameters, experienced engine builders adjust contributing components to best suit any fixed compression ratio, particularly with an eye toward increasing the effective compression ratio via camshaft timing and effective inlet tuning to improve VE.
Some lower-level sportsman organizations require stock cranks and rods. These are cost measures frequently accompanied by camshaft, cylinder head, and induction limits. Power is generally limited in these applications so durability issues are infrequent. In claimer racing and many drag racing classes, cast cranks, and stock rods are successfully employed without distress.
Weight limitations for the crankshaft are also common, something on the order of a 50-pound minimum in most cases. This dramatically reduces the cost of racing in these classes, but that doesn’t mean you can’t consider blueprinting the assemblies for optimum balance in the desired RPM range and making discreet modifications to reduce crankcase windage.
Most sanctioning body cylinder head rules generally strive to limit cost. Many ruling bodies enforce an iron cylinder head rule with further restrictions on valve size and material, valvespring pressure, combustion chamber size, intake port volume, spark plug placement, and other contributing factors. Some rules permit bowl porting while others forbid everything including multi-angle valve jobs and back-cut valves. Classes that allow aluminum racing heads are usually unrestricted unless they involve original factory cylinder heads in a stock drag racing class.
Some racing classes limit spring diameter and configuration, retainer material, valve shape, and other factors. A careful evaluation of allowable cylinder head configurations and hardware usually extends to camshaft selection and further reflection on intake manifold and exhaust system compatibility. Choices focus on achieving maximum possible VE within the stated boundaries. Savvy builders also key in on component choices and attending port volumes and valve sizes to take best advantage of transmission and rear axle gearing, tire size, and specific RPM requirements for the tracks the engine will visit most often.
Intake manifold and carburetor restrictions are prevalent in many types of racing. They are primarily intended to limit airflow and RPM potential. In some cases more than one choice is offered and the final selection is based on which configuration generates the best VE and torque-tuning potential. That’s why, where rules permit, a twin-carb, high-RPM tunnel ram is chosen over a single 4-barrel for a drag racing application, but you’re not likely to see a tunnel ram intake on a road racing car.
Some classes dictate the use of a dual-plane intake, which often extends to the use of a stock cast-iron manifold. When the intake manifold is specified, all you can do is identify the manifold’s characteristics and tailor your package accordingly. (Chapter 9 discusses how to map manifold characteristics on a flow bench to obtain a ballpark view of individual port strengths and weaknesses.) Once you have a clear picture of the manifold’s efficiency you can evaluate potential steps to ensure its contribution to maximum performance.
Depending on other restrictions these may include rocker ratio or cam timing adjustments to individual cylinders based on individual runner flow dynamics. Or it may be addressed by manipulation of header dimensions to complement and possibly broaden the torque range dictated by the intake manifold’s fixed dimensions. If allowed, carb spacers may support better mixture quality and, in the case of dual-plane intakes, staggering jetting from side to side may also provide some improvement, particularly as it relates to the lean side of the engine. Many circle-track classes also require a 2-barrel carburetor of a specified size with no modifications allowed, although repositioning of the carburetor location on the intake manifold is sometimes permitted.
Camshafts and Valve Gear
Camshafts are one of the most common restrictions. Most often a rule limitation restricts the use of roller camshafts, specifying only flat-tappet camshafts, frequently with an additional stipulation of stock lifter diameter. Some classes further stipulate hydraulic lifters only or a valve lift rule, which is typically .500 inch. Other classes factor cost by limiting racers to stock or hypereutectic pistons and other restrictions that may include OEM blocks only, iron heads, stock exhaust manifolds or spec headers, and other requirements more closely associated with “claimer”-type engine regulations.
In addition to cam type and lifter restrictions, camshaft rules may also stipulate a minimum engine-idle vacuum to control camshaft profiles. Other internal component restrictions may include stock crankshaft and connecting rods and most certainly, wet sump oiling with a stock pump and perhaps a racing-style oil pan. These engines are pretty inexpensive, but a thorough understanding of the basic limitations and potentially favorable component relationships may illuminate previously unconsidered paths to power. Think torque management and positioning within the applicable powerband.
Exhaust systems are usually less restricted. Most often they involve a muffler requirement, stock exhaust manifolds, or possibly a spec header. Builders often consider mixed rocker arm selections optimized to provide appropriate exhaust event timing to aid cylinder blow-down if allowed.
Spec mufflers may be required and testing with backpressure readings can help pinpoint the most beneficial exhaust pipe cross-sectional area, effective primary pipe lengths, and collector sizes if headers are permitted. Modeling this on a PC simulator like PipeMax (see Chapter 14) can pinpoint the best overall dimensions. Simulators are front-loaded with all the mathematical fundamentals of engine performance. They can’t always predict absolute power and torque, but they can illuminate trends according to the physics and that amounts to a pretty good road map for serious engine builders.
As long as the requirement is clearly identified in the rules, savvy engine builders always find ways to optimize within those rules. If a spec header is involved and you’re not comfortable with discreetly modifying pipe dimensions, consider the possibility of different cam timing in cylinders whose intake and exhaust timing peaks differ due to unequal port or pipe cross-section or length. Like other components, any restriction on the exhaust system should be carefully evaluated for its effect on torque production and positioning and how it affects other parts of the engine package. There are usually ways to refine and assemble a more robust package even within restrictive rules.
Many series call for sealed spec motors. Spec engines have their internals specified with no substitutions or modifications permitted. They are assembled and sealed by an independent source to prevent tampering. Racers purchase a complete engine package ready to run. In addition to setting a power limit, these engines stabilize cost because the supplier can build them in volume with fixed or predictable expense. They are surprisingly powerful and typically very close in power production from one engine to another.
Some classes still impose a limit on ignition systems and require the use of a stock-type high-energy-ignition (HEI) distributor, for example, or a stock coil. Approved aftermarket distributors are accepted in some classes, but may be limited to those that do not require a separate ignition amplifier box. These restrictions are narrowly aimed at cost containment and in that regard they are probably a good thing for many sportsman racers.
Fuels and Fuel Systems
Many organizations require a spec fuel or pump gas and may limit the type of fuel pump and fuel line size. Even these restrictions leave room for interpretation if you consider the burn characteristics of the fuel. This may cause you to re-evaluate cam timing or the ignition curve and even things like float levels, jetting, or air bleeds based on the specific gravity of the fuel, octane tolerance, and burn rate. If you have a spec fuel, request a complete breakdown of its chemical properties to include heat and energy values, burn rate, specific gravity, evaporation characteristics, and anything else you can study.
If you understand the burn characteristics of the fuel you can configure the cylinder head package and fuel strategy to extract maximum available energy. Further consideration should also be given to the size and location of the fuel supply, fuel system components and flow rates, and anticipated dynamic g-loading that may affect fuel flow to the carburetor and even individual fuel molecules suspended in the runners. Fuel is where the power is so it’s imperative to learn everything you can about what it can do for you.
Power and performance are sometimes restricted with an arbitrary engine speed limit. This forces you to fashion a powerband that best suits operational requirements within the allowable range of engine speed. You are forced to target torque application according to tire size and gearing as they relate to track surface and length and how tightly the corners restrict vehicle momentum.
This limitation often accompanies a spec engine, which makes the racer’s job easier. But non-spec engines may be further limited by compression ratio, airflow, or mechanical specifics such as a dual-plane intake manifold or 2-barrel carburetor. These applications are often trickier to design than unlimited engines. Thoughtful contemplation of all the factors and time spent with a PC engine simulator may help pinpoint selections and adjustments you can control to build the most effective package.
From the standpoint of specific goals it’s usually instructive to list your operational requirements in terms of what the engine is expected to accomplish and how it is supposed to do it. Some parameters to consider include maximum and minimum engine speed and the dynamic operating range of the application. It’s often helpful to examine similar engine packages including other brands with a critical eye.
Identify the things they are doing well and things that don’t seem right. A broad range of factors affects the choices, but not everything is applicable to your application. Evaluate how much time the car spends in any particular gear and how that relates to torque and power positioning in the overall RPM range of the engine. Gearing and tire requirements, track type, and length influence the dynamic operating range including such things as RPM drop on shifts and projected engine loading in endurance or high-RPM, high-cycle environments. Each of these may have its own unique cooling and oiling quirks too.
Drag racing has a fixed-length track with varying track conditions including traction and atmospheric variables. A road race car encounters completely different track and environmental conditions at almost every race. These elements may prompt you to evaluate the value of torque versus high-speed power and the RPM where you want the torque and power to apply.
Compiling all the critical factors affecting a particular application helps develop a mature package that is better suited to your needs. Make a list including as much as you can learn about competitors’ combinations and what the rules permit. Evaluate individual elements on their own merit and their contribution to the car’s performance.
Say, for example, other racers are running headers that are too large for the RPM and exhaust volume being generated in the optimum torque range. If header size is not restricted you may improve performance by running an optimum header size based on torque peak requirements. That’s only one example, but advanced engine building requires that you evaluate the contribution and compatibility of every single component, including doing the math that governs how these parts interact and perform (see my book Performance Automotive Engine Math for applicable formulas).
As previously stated, numerous factors affect the operational requirements of the engine. The effective operating range incorporates minimum and maximum anticipated engine speed, torque peak and power peak positioning, and functional operating conditions. That might include high engine speeds, sprint or endurance conditions, high cyclic loading, anticipated torque requirements for maximum loading, gearing and shift points, and the RPM drop on each shift.
The importance of torque curve shaping and positioning relative to gear selection in the transmission and final drive is discussed later in the book. This is of course reflective of track type and length, projected racing conditions, and shift frequency. To accommodate these concerns it is critical to match induction and exhaust components for torque optimization. When you really examine it, there are so many interrelated factors affecting competition engine design it can seem overwhelming. That’s where careful planning makes the winning difference.
Once you have defined your operational requirements and objectives you can make preliminary judgments about basic engine architecture and the essential concerns that govern all competition engine builds. The following essentials affect parts selection: machine work, prep work, and assembly practices. It’s best to make a list up front and revise it often as you develop and groom your package. Work in pencil as you will undoubtedly make frequent revisions.
An alternative is to put the information into a spread sheet so you can change and update it as required. Many of these entries give pause to review and alter previous entries as you cultivate the best possible combination for your racing application. Accordingly, many of the PC engine simulation programs help you do a better job of pre-planning and testing engine ideas. (See sidebar, “Engine Simulation Programs” on page 16).
Unlocking Hidden Power
On the assumption that all cylinders fire consistently (see Chapter 13) I now direct our attention to details that foster uneven power production in any grouping of cylinders called an engine. For the purpose of discussion I also assume a V-8 engine, which remains the predominant powerplant used in most domestic racing series. Upon further reflection what you really have is eight individual engines (cylinders) linked by a common crankshaft and cylinder case (block assembly). These engines are all racing toward the finish line in formation.
Your job as an engine builder is to promote a dead heat where every cylinder is a winner despite whatever performance shortcomings individual cylinders may exhibit. None of these engines are perfectly identical and once they are linked as a group, numerous factors conspire to upset the balance of power from one engine (individual cylinder) to the next. In almost every case, some cylinders are hauling ass while others lag behind. Savvy engine builders concentrate on modifications to bring those stragglers up to speed.
More specifically a single cylinder can be tuned for maximum power relative to a given displacement and VE. Since all engines (single cylinder or more) perform best when VE is optimized you can mathematically select an engine speed range where you want maximum torque and horsepower to occur. VE is a function of the total flow path through the engine and thoughtful optimization of flow path components (i.e., shapes and dimensions) promotes maximum output. Proper sizing of inlet and exhaust passages relative to piston position and valve timing are fundamental to these efforts.
For any given displacement there exists an ideal set of flow path dimensions (cross-sectional area and length) that produce maximum torque relative to a particular (desired) engine speed. These dimensions are known to control the ideal mean flow velocity (MFV) that accompanies peak torque production. Adjustments to these dimensions allow you to shape and position (RPM range or powerband) the torque curve for best performance in any given application. It is known to the discipline that an ideal MFV of 240 to 260 ft/sec accompanies peak torque.
Engineers have long recognized that this optimum flow path velocity is affected by cylinder displacement, compression ratio, and valve timing relative to piston position, which is controlled by stroke and rod length (rod/stroke ratio). Mean flow velocity is largely governed by flow path cross section on the inlet and exhaust flow paths. A larger cross section requires more engine speed (RPM) to achieve MFV and position a torque peak. A smaller cross section shifts the torque peak to a lower engine speed. Decades ago, widely noted engine authority Jim McFarland defined a mathematical equation for calculating the ideal cross-sectional area to achieve MFV at any given engine speed.
Flow is discussed in Chapters 9 and 14. For now just recognize the existence of an optimum flow velocity and the critical sizing of flow path dimensions to adjust the location (RPM) of the torque peak within the operational powerband. Optimize these elements in each individual cylinder and you achieve maximum torque production with the ability to adjust its position in the RPM range for maximum efficiency.
Now link that cylinder to another cylinder, or worse yet, seven other cylinders, and you encounter the potential for mass chaos in the system. Inlet flow paths (in most cases) are now linked via a common source (the plenum) and the exhaust paths are linked (in most cases) by collector. While convenient for the purpose of packaging and fuel distribution this arrangement fosters influential dynamic relationships between cylinders to the extent that unequal power production can occur even when components have been properly matched to accommodate appropriate wave tuning.
Powerful wave dynamics and pressure changes communicate between cylinders via the inlet and exhaust flow paths. The consequence is often inconsistent fuel delivery and/or the promotion of poor mixture quality, which leads to inefficient combustion and unequal power delivery governed by the specific dynamics influencing each cylinder. This is often characterized by varying air/fuel ratios between cylinders, unequal exhaust gas temperatures (EGTs), poor combustion efficiency, and the potential for detonation in one or more cylinders.
A single cylinder can be tuned for maximum efficiency by optimizing intake and exhaust flow paths according to displacement and engine speed and by providing complementary ignition and valve event timing. Now contemplate how to tune all eight cylinders for maximum and equal output even as some individual cylinders exert undue influence on their neighbors via pressure “cross talk” within the plenum.
In effect individual cylinders must be tuned separately, which also presents an opportunity to tune specific cylinders for different torque peaks that can effectively broaden an engine’s net torque production. In this context, it means that individual cylinders may incorporate differing components such as cam lobe timing, rocker ratios, inlet and exhaust dimensions, and other component variations that become useful tuning elements in concert with normal jetting and timing adjustments.
How Cylinders Influence Each Other
Multiple cylinders connected via a common crankshaft and inlet and exhaust systems are influenced by assorted dynamics transferred through the flow paths and mechanical connections that link them together. Crankshaft radial deflection caused by cylinders connected via common crank throws can affect piston motion in companion cylinders. This can lead to incorrect ignition timing in the affected cylinders or even in cylinders at the far end of the crankshaft due to net torsional crank deflection.
Valve event timing is similarly affected by camshaft twist due to high valvespring pressures, which contribute their own net loss to the torque equation. The resulting inconsistent valve events may transfer harmful pressure waves to other cylinders via the common intake plenum. These disturbances can interfere with the flow dynamics of adjacent ports in the manifold resulting in inconsistent flow rates and continuous variations in air/fuel ratio from cylinder to cylinder. This can also be affected by residual exhaust gas contamination that may transfer between cylinders during camshaft overlap periods.
Steps to counteract or alleviate these problems are required once they are recognized. Thus it becomes useful to examine the most common problem areas and suggested basic remedies. Each of the primary contributors to VE can be adjusted individually and/or collectively to improve efficiency and overall net torque. These include intake manifold dimensions, cylinder heads, camshaft, rocker arms, exhaust headers, and ignition timing, all of which may be affected by dimensional or mechanical inconsistencies contributed by deflection or distortion in the reciprocating assembly.
For further insight into planning and developing a competitive package see Chapter 2 on governing torque and horsepower and how they interact. You hear a lot about hitting on the right combination, but it really shouldn’t be a hit-or-miss proposition. Every component in a race engine must be selected and massaged for optimum compatibility with other parts and the net contribution they all make to producing maximum torque and horsepower in the most useful range for the final application.
You must think critically about each engine component both individually and collectively. In the end, the most successful engines are highly evolved according to very specific component matching and operational requirements that include counteracting or limiting the effects of unwanted mechanical and/or pressure wave influences that upset the balance of power within an engine. The fundamentals of gas exchange control the process, but you can influence the action in many different ways to ensure that each and every individual cylinder contributes appropriately to net torque and power production in the specific engine speed range most suitable for your particular application.
Written by John Baechtel and Posted with Permission of CarTechBooks