Big Horsepower Or Gobs Of Torque | Horsepower VS. Torque | Tech

No Replacement For Displacement
Essentially, an engine's torque is determined by the percentage of cylinder fill (volumetric efficiency) and cylinder displacement. The greater the cylinder fill, the greater the torque will be. To increase power, it is important to improve the engine's ability to breathe. Peak torque is reached when the engine runs out of air or loses its ability to breathe better. And that is the point of maximum cylinder fill. An engine will continue to make more horsepower even when torque is falling as long as rpm increases faster than torque falls. If maximum torque is the point of maximum cylinder fill, then maximum horsepower is the point where torque begins falling off faster than rpm increases.

While every engine produces torque, an engine that produces peak torque at low rpm is typically referred to as a "torque" engine, while one that produces peak torque at high rpm is considered a "horsepower" engine. The most effective way to increase torque is to increase displacement. For this reason, increasing displacement on a stock Twin Cam engine by installing big-bore cylinders offers a lot of bang for the buck, even though the additional displacement will cause the engine to run out of breath at a lower rpm. Once engine displacement is increased, airflow improvements can be made by installing a free flowing exhaust system, higher-flowing head, larger carburetor or throttle body, bigger cam(s) and less restrictive air cleaner.

Changing the bore/stroke combination for a given size engine displacement can change the engine's power curve. As an example, a short-stroke combination will typically raise the rpm at which peak torque occurs, which will improve top-end power potential at the expense of bottom-end torque. For a low-rpm street engine that does not rev more than 5,000 or 5,500 rpm, a longer stroke is usually more beneficial than a larger bore because increased stroke can make more torque down low, which is where street engines spend most of their time. However, stroking a V-twin can be more costly than installing large-bore cylinders, so it often makes more economical sense to just install big cylinders and be done with it for mild or moderate hop ups.

When high rpm operation is a priority (about 6,500 rpm or higher), a large-bore/short-stroke combo offers the best power potential because it will unshroud the valves and allow installation of larger valves for improved breathing. But this design also requires high-quality heads and valvetrain components to support the engine's high-rpm airflow requirements, which usually results in a costly engine and overkill for most low-rpm street engines. As an aside, most aftermarket V-twin crate engines are big-bore moderate- or short-stroke combinations for durability reasons and not necessarily higher performance. A shorter stroke results in less crank leverage, therefore reduced thrust loading against cylinder walls and less bearing loads. A short stroke also results in less piston speeds for reduced wear and longer piston skirts for improved cylinder sealing. As a result, aftermarket manufacturers have less warranty concerns.