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Octane and compression ratio do not go hand in hand. The head design plays a big part as can cylinder size.
The head design can play a part in it due to volumetric efficiency.
During most of the rev range you are getting less than 100% cylinder filling due to valve overlap (the intake valve may be closing while the piston is still descending or the exhaust valve is still open when the intake opens) and other factors. You may well have a mechanical compression ratio of 13:1 say but if you only have 80% of the cylinder filled with mixture you are not compressing it anywhere close to detonation. On the other hand, you could be running a 12:1 ratio and still experience detonation at certain points in the rev range if the engine is breathing exceptionally well due to cam and head design where you might see 110% volumetric efficiency - that's stuffing 275cc of mixture into a 250cc cylinder.
A very poorly designed engine could perhaps run 14:1 on standard pump gas with no detonation at all.
During most of the rev range you are getting less than 100% cylinder filling due to valve overlap (the intake valve may be closing while the piston is still descending or the exhaust valve is still open when the intake opens) and other factors. You may well have a mechanical compression ratio of 13:1 say but if you only have 80% of the cylinder filled with mixture you are not compressing it anywhere close to detonation. On the other hand, you could be running a 12:1 ratio and still experience detonation at certain points in the rev range if the engine is breathing exceptionally well due to cam and head design where you might see 110% volumetric efficiency - that's stuffing 275cc of mixture into a 250cc cylinder.
A very poorly designed engine could perhaps run 14:1 on standard pump gas with no detonation at all.
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