The diameter of the piston converts the PSI in the combustion chamber to pounds force on the piston and thus connecting rod. So a larger piston tends to produce more torque which because of the shorter stroke, can be reved higher. Horsepower is a calculated value involving torque and speed, for this RPM. Now HP in and of itself does not accelerate your car from a street light or stop sign. TORQUE does that! HP is more being done faster, so HP is what creates trap SPEED or 200MPH NASCAR lap speeds. These two values are intertwined by virtue of torque at an RPM is HP. A F1 engine creates fenominal power in a small and light package that is formidable in a car of about 1,100 pounds. From stopped they do not accelerate well, thus the 10,000RPM to just get moving out of the pits without stalling. That 800HP engine is totally usless in a 3,500 pound street car. They are all HP with a lack of torque.
