can anybody give me the technical reason why a motorcycle engine can rev to 17500 rpm, but car engines only rev to 7000 rpm ? and would it be possible to make a car engine to get the revs this high?

 

Someone can expand on things, but basically bike engines are small and lightweight, with very short strokes. The cam timing is optimised of these high rpms, so below 5 or 6k, depending on engine, they are useless - especially for a heavy car where torque is required. F1 engines now rev to 20k based on the same principles.

 

As per Rick.

They have very short lightweight pistons, short stroke and relatively big bore. They also have very light flywheels

At the top end, they generally don't use hydraulic lifters (to make sure they can open fully in such a short period). Some even postively pull the valves closed instead of purely relying on spring to close them.

It's cool engineering!

Neil.

 

Oh, my CBR600 was about 14:1 CR too!

Neil.

 

As above really, there are two fundamental reasons:

1: The components are lightweight enough to physically manage to rev that high without pulling a conrod in half

2: The state of tune
due to the short stroke, large valve area, big port size, and "wild" cam profile, everything is optimised so that the usable torque curve occurs a lot higher up in the rev range

 

chip said it on another post - it's mostly to do with stroke length.

the longer the stroke, the further the piston has to travel for a given engine speed. obviously at the ends of each stroke, the piston has to slow down and then accelerate in the opposite direction again, and the further the piston has to travel in each stroke, the faster it has to go to cover that distance in the same time so the acceleration has to be much greater.

it's the acceleration that puts components under high loads and stresses, rather than the speed.