Compression ratio and octane


Registered User
So I was reading through some discussions on piston compression ratios and fuel octane and I was curious as to what the members here thought.

We all know that lower octane fuels are more prone to pre detonation than higher octanes.

However I was reading that high octane fuels in an engine with a low compression ratio is a waste of time If anything the lower octane would help with combustion efficiency. Higher compression means higher octane is needed, lower compression lower octane is perfectly adequate.

Now I've always been under the impression the higher octane the fuel the better if your car can make use of it, however the lower compression low octane does make sense, in theory. These discussions were on Facebook so I've taken it with a pinch of salt lol.

What's people's opinion's on this?

Alex C

Well-Known Member
VCDS Map User
Not a qualified opinion but as fas as i understand it;

high octane fuel is used for its knock resistance

high temperature and high cylinder pressure, be it through high compression ratio pistons or high boost levels promote knock

The ECU on our engines use knock sensors to adjust ignition timing: if knock is detected timing is retarded to counter it (timing pull); subsequently power is reduced slightly

Using a high octane fuel reduces the likelyhood/onset of knock allowing the engine to run at optimal ignition timing for longer


Registered User
So if in theory were running low compression and low boost, lower octane fuel would be fine. But if we were running low compression high boost, higher octane is required?

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depends on many other things. reduced CR is often done to allow for higher boost pressures, as the overall limit of the motor is going to be its cylinder pressure and det limit.. again several other factors influence this also.

those building motors and deliberately dropping the CR, and then choosing a low octane fuel............ are..... well, wasting a lot of time and effort if so.