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

Power Tips
Displacement is "king" for building torque because the easiest and most effective method for increasing torque is to increase engine size. For high torque, a 95ci engine is better than an 88, and a 124ci engine is better than a 95. Size does matter. Although building a low-end torque engine can be easier on engine parts, it's also harder on drivetrain components. Moreover, too much torque can overpower the rear tire and chassis, making a bike hard to launch. Most V-twin street riders are craving more torque down low, but what they forget is that engines have gotten so big, 120- to 140ci or even larger, that they often overpower the available traction. In these cases, the bike's launch can be improved by trading off some low-end torque for high-end horsepower. This is done by moving the torque curve higher up in the rpm band by installing free-flowing induction and exhaust systems, better breathing heads and performance cam(s).

While displacement is "king," remember that similar size engines can have widely varying power levels due to differences in airflow capabilities. All other things being equal, a big engine will make more torque down low than a smaller engine, but it will not necessarily make more horsepower up top. To make more horsepower on the top end, a big engine must be supported with free-breathing cylinder heads, carbs or throttle bodies, cam(s), and exhaust systems to feed the larger displacement at high rpm. For example, installing big-bore cylinders on a bone-stock Twin Cam 88 or 96ci engine will improve low-end torque, but the engine will run out of air at a low rpm (lower than a stock engine) and not develop optimized horsepower for its size. This is because the induction and exhaust systems cannot flow enough air for the big engine at high rpm. Accordingly, volumetric efficiency falls off early in the rpm band, and the torque curve drops like a lead balloon. Since cylinder filling is poor at high rpm, little torque is made up top, which also means that little horsepower is made up top.

Despite your engine combination, always strive to build a balanced engine with airflow matched to the engine's displacement and critical rpm range. Although this requires matching engine displacement with the proper heads, cam(s), carb or throttle body, and exhaust system for the application, which can be costly, it will also result in the broadest, flattest and highest torque curve. This will result in optimized acceleration and the greatest suitability for street riding. Generally, using smaller valves, smaller ports, smaller carburetors and less cam timing will improve low-end torque at the expense of top-end horsepower.

Another key factor for maximizing torque and horsepower is mechanical compression ratio. Most street engines are pump-gas limited, which means that the quality of the gas will dictate the maximum compression and cam timing that can be used without encountering performance problems. In order to optimize torque and throttle response, always maximize the engine's compression ratio to the quality of the fuel, and then match the cam(s) to the compression, engine displacement and rpm range. Depending on several variables, pump-gas engines are normally limited to between 10:1 and 10.5:1 mechanical compression ratio. Move the engine's compression ratio into that range, then build and tune the engine to that delta. If you don't, you are leaving power on the table.