Basic Motorcycle Engine Fundamentals - Compression, Bore, and Stroke

Creating Synergy

Unlike mechanical compression ratio, corrected compression ratio takes into account the point at which the intake valve closes after the piston passes BDC on the intake stroke. The later the valve closes, the lower the corrected ratio. Corrected compression should be matched to cam specs and fuel octane for optimized engine performance.

Blueprinting is the term used for the careful selection, custom fitting, and meticulous assembly of engine, drivetrain, or chassis components to very precise high-performance specifications. All components of a blueprinted engine are checked for proper dimensions, concentricity, perpendicularity, and fit. When deemed necessary, proprietary modifications to components are made. Finally, the engine is mocked up and checked as a rotating assembly.

Pistons are available in different dome shapes to fit a variety of combustion-chamber designs and compression ratios. Be sure to match the piston dome to your combustion-chamber design and intended compression ratio.

In last month's installment, we explained several critical engine dimensions, along with key terminology that will help you avoid making costly mistakes when buying or building your engine. This month we will look at a few additional relationships that are important for achieving a "happy engine" and will touch upon the importance of the bore/stroke relationship.

Mechanical compression ratio (also known as static ratio) is a mathematically calculated number derived from the relationship of combustion-chamber volume and cylinder volume. Mechanical compression ratio is determined by adding net combustion-chamber volume, which includes head-gasket volume, to cylinder volume. The sum is then divided by net combustion-chamber volume. A mechanical compression ratio of 9:1 means that the intake charge is compressed down to one-ninth of its original size. Within certain limits, the higher the ratio, the greater the combustion heat and cylinder pressure, resulting in increased efficiency, power, and fuel economy.

Compression ratios for Harley-Davidson overhead-valve V-Twin engines typically range between 8:1 and 16.5:1, with 9:1 to 10.5:1 the optimum for most pump-gas engines. Keep in mind that increasing displacement without increasing the combustion area will increase the mechanical compression ratio.

Corrected compression ratio is similar to mechanical compression, with the exception that the corrected ratio takes into account the point at which the intake valve closes when determining cylinder volume. Knowing the corrected compression ratio is important for building a "happy" engine, especially if it will be run on pump gas. The corrected compression ratio will always be less than the mechanical ratio because the corrected ratio is calculated using a smaller cylinder volume (since the piston is past bottom dead center and moving upward when the intake valve closes). This results in a smaller cylinder volume being used for the calculation, thus a smaller ratio. The corrected compression ratio is important for optimizing the mechanical compression ratio to cam specifications and gasoline octane.

By now it should be clear that selecting the wrong combination of parts or inaccurate machining can cause major problems when building an engine. A slight error or accumulation of errors can easily cause piston deck height to be too high, so the piston hits the cylinder head, or too low, resulting in excessive squish clearance and lower-than-anticipated compression. When buying new or used parts or even a complete engine, never take for granted the dimensions of any of the specifications previously discussed, which brings us to another misunderstood term: blueprinting.

"Blueprinting" an engine can mean various things to various people. An engine can be assembled to either stock specifications or high-performance specifications, which is also referred to as blueprinted specs. Stock specifications have a very wide tolerance range due to production-line manufacturing variances. And, as you can imagine, these specifications are not optimized for best performance.

On the other hand, in regard to performance specifications, blueprinting is the term used for the careful selection, custom-fitting, and meticulous assembly of engine, drivetrain, or chassis components to very precise high-performance specifications. The specifications are generally collected from engine or parts manufacturers, previous experience, and intended application, and possibly are limited by a sanctioning racing organization. All components of a blueprinted engine are checked for proper dimensions, concentricity, perpendicularity, and fit. And, when deemed necessary, proprietary modifications to components are made. Finally, the engine is mocked up.

"Mockup" means that the engine is assembled, rotated, and all clearances checked. The combustion chamber is usually cc'd to verify the mechanical compression ratio, and adjustments to components are made as necessary. Finally, the engine is disassembled and meticulously cleaned for final assembly.

Blueprinting also includes checking everything. For example, never install a new part directly out of the box. Do not assume that a new part has the correct dimensions, is the same size as the last one you bought, or is labeled correctly. And always check and double-check everything and measure everything twice to maintain a high level of quality control.

Additionally, blueprinting involves maintaining accurate and complete records on component sizes, engine dimensions, and clearances. Create a build sheet for every engine with all its critical dimensions and specifications well documented. The advantages of blueprinting are that you can run the maximum ignition advance, most compression, perfect jetting, and best cam timing, knowing that you did your best to ensure uniformity in each cylinder. The rewards are less vibration, greater durability, increased component life, and optimized power.

An ancillary phase of blueprinting is the careful inspection, measurement, and documentation of critical engine dimensions and clearances during engine teardown. Careful examination of the engine, including piston and bore conditions, bearing wear patterns, valvetrain clearances and condition, valve lash (adjustment), and valvespring pressures-just to name a few-will provide vital information for accurately evaluating the durability and longevity of critical engine components and their operating conditions.

Bore and Stroke GeometryIn part, the engine's bore/stroke geometry determines whether you end up with a torque- or horsepower-oriented engine. Other factors that influence power characteristics include camshaft timing, along with intake and exhaust tract design. As displacement increases, it's important to weigh the decreasing benefits of added displacement against the expanding liabilities of expense, durability, and maintenance.

Torque and horsepower are terms loosely used to describe an engine based on the engine's power characteristics. Although all engines make torque, in general a "torque" engine is one that produces higher torque at lower rpm, while a "horsepower" engine makes large amounts of torque at higher rpm. A long-stroke engine tends to be torque-oriented, while a large-bore, shorter-stroke engine usually falls into the horsepower category. Another way of looking at it is that a long-stroke torque engine has greater rod leverage but produces fewer bangs per minute. On the other hand, a shorter-stroke horsepower engine has less rod leverage yet produces more bangs per minute.

For an equivalent displacement, a big-bore short-stroke horsepower-type engine requires higher rpm to reach its power potential than a long-stroke engine. Although a big-bore short-stroke engine requires more emphasis on proper valvetrain design, piston, ring, and cylinder wear are usually reduced. The reduction in wear is attributed to greater piston skirt length for improved stability in the bore, reduced rod angularity, and lower piston speeds. On the other hand, a long-stroke engine doesn't need to be revved as high to reach maximum power, and valvetrain design is usually less critical due to lower rpm. But because of greater rod angularity, reduced piston skirt length, and higher piston speeds, piston, ring, and cylinde-bore wear are often increased with a stroker.

In recent years, Harley engine builders have followed a trend of adding displacement by either increasing bore size or combining a moderate stroke increase along with a larger bore. One reason for the trend toward a large bore is to keep piston speed at a reasonable level for increased durability. A second reason concerns airflow. A large bore allows larger valves and helps reduce bore shrouding of flow around the valves. As a result, potential airflow is greater at high rpm, provided the cylinder head, camshaft, induction, and exhaust are optimized. But the engine will have to be revved higher to take advantage of the potentially greater airflow. And if the rider is not willing to rev the engine to the required rpm, the extra airflow will not help.

For example, if a street-ridden 106ci stroker engine is compared to a 107ci big-bore motor, a reasonable rpm limit for the stroker would be about 6,500 rpm. In contrast, roughly 7,000 rpm is reasonable for the big-bore engine. If the rider is extremely conservative and limits engine speed to 5,500 rpm (which many riders do), a long-stroke motor should be considered because it typically has higher low- and midrange torque. Conversely, if the rider is willing to rev the engine to 6,500-7,000 rpm or higher, the big-bore engine offers a potential horsepower advantage because it theoretically has higher mechanical efficiency and potentially greater airflow.

Often the terms "square" engine, "over square," and "under square" are mentioned. A square engine is one where the bore diameter and stroke length have identical dimensions. An over-square engine has a larger bore than stroke, while an under-square engine has a longer stroke than bore.

To make a large amount of power, the engine must be designed to burn the most air/fuel mixture in a given amount of time. To achieve this requires the greatest amount of cylinder filling and the most bangs (rpm) per minute. If designed and assembled correctly, a big-bore short-stroke engine (square or slightly over-square design) has a potential advantage over a small-bore long-stroke engine (under-square design) in accomplishing this. But there are reasons why some large-bore short-stroke street engines fall short of their performance objective.

First, most street induction systems cannot supply the required airflow at high rpm. Second, the valvetrain is not capable of supporting the high rpm required to achieve big horsepower numbers. A third limiting factor is an inefficient combustion chamber, but this problem is relevant to both big-bore and stroker engines that are not assembled to close tolerances. However, a high-flowing induction system and bulletproof high-rpm valvetrain usually require a lot of money and effort to build. Therefore, it is often easier and less costly to build an engine with a slightly smaller bore and longer stroke, since it will require less rpm to achieve maximum power. In the end, the engine builder's budget and philosophy on how to achieve power will often determine the bore/stroke geometry.

Engine bore/stroke geometry can also be dictated by the number of transmission gears. For example, some AHDRA racing classes are restricted to a high-gear-only transmissions. Since only one transmission gear (high gear) is allowed, most racers build a slow-revving, high-torque engine by using a longer stroke. In contrast, other classes are allowed greater displacement and a five-speed automatic transmission. Since five gears are available, most racers build a high-rpm horsepower-type engine by using a large bore and relatively short stroke. Additionally, there are several racers specializing in hot street bikes. Some of these racers prefer to use only three gears in the quarter-mile, so they use a long 5-inch stroke to build a high-torque engine. Other racers use all five gears when racing. In this case, they shorten the stroke for higher rpm and horsepower, but less torque at the bottom of the rpm band. The important point to remember is that each racer tailored the engine's bore/stroke geometry to other elements of the total racing combination-in this case, the transmission gearing.

As you can see, there are many factors to consider when designing and building an engine. And having a fundamental understanding of the relationships among critical engine dimensions can help you achieve a "happy" engine.