Static compression is no indication of dynamic compression ratio. The valve duration & overlap, please intake air velocity, determine the dynamic compression ratio. Which is typically around 10-11/1 for a bike running on low octane unleaded fuel. For one running on higher octane then 11.5-12.5/1 is a useful working range, above that you need Very high octane race fuel or delayed ignition to avoid knock.
To explain dynamic compression image a 12/1 static compression engine, if you took the cams & spark plug out then set the cylinder to BDC & inserted the spark plug when you turned he engine to TDC you'd have 12/1 compression, on air !
But if you turned a stock engine over slowly on the induction stroke at BDC you would have the intake valve open, say for 70degrres ABDC (original ex250-H1) so for nearly half the compression stroke the intake valve is open, if we say 1/3 the piston travel, then at slow revs the dynamic compression ration will be around 8.5/1 which is the reason that there is no bottom end !
As rpms in crease & the intake air velocity increases, this delayed closing of the inlet valve allows the air which is now ravelling in through the throttle bodily to force its way into the cylinder & as air is compressible its own mass keeps it combining in. The trick is to close the valve at just the right time for the rpm you want peak torque to occur. ICE engines can reach 110% intake efficiency, meaning there is 1.1 bar of pressure in the cylinder when the valve closes.
This is why you will find all static high compression engines have quite long valve durations & usually a good sized overlap ! The original x250-h1 had 12/1 compression, but 280 degree duration cams, putting dynamic compression around 11/1 which is a good number for standard unleaded fuel.
The timing of the valve closing events determine at what RPM the engine will be most efficient. For a road bike peak efficiency at peak revs is not needed, as they rarely spend much time there, especially as the displacement increases, they become to fast or unrideable. But in smaller engines that can't create enough power to worry you when riding it, then the higher in the rpm your reach peak efficiency the more peak power you will make & they spend more time there ! Peak HP normally follows peak torque by 1500-2000rpm regardless of the bike engine size.
The original ex250 had peak torque around 10,800 rpm, with peak power around 12,600 rpm, with 1400rpm over rev, which is a flexible zone that allows you to manipulate speed in all but the top gears when in that rpm range. This is useful on a road bike but would mean peak torque is to low on a race or LSR engine.
YMMV
