[Dave]

Originally Posted by Rsv1000R

It isn't the torque, it's the powerband, After taking rpm and torque at the rear wheel into account there shouldn't be any difference, 100 hp is 100 hp. Now maybe a street tuned twin will have a wider powerband that a peaky 600.
Hp is torque over a period of time, gearing will change torque, but not hp. So by the time you change the gearing of a 10,000 rpm twin to match a 15,000 I4, you lose 33% of your torque.

As to why a twin won't make the same hp is valve area and rpm. The bigger valves of a twin take up more space across the chamber, and the valve area doesn't increase as fast as displacement does, so you end up with less valve comparably, and the mass of the valve is a key limit to rpm and cam profile and spring life. Smaller valves have less mass and can rev higher. Hp is all about torque over time, higher rpm if you can maintain torque makes more hp.

F1 engines and now MotoGP engine show a similar development paths. More smaller cylinders that rev higher, with exotic methods to close the valves.

power pulses as well. More cylinders can do more work more often.