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Originally Posted by GreyGoose006
to reply to curtis, i get what you are saying.
if i understand correctly, there is an optimal pressure that an engine will have to start ignition.
say that pressure is 175 psi for example.
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Not quite what I was talking about... I was talking about PEAK cylinder pressures... as in, after ignition. For best operation, the expanding flame front should be at its peak pressure at about 20-23 degrees ATDC. Increasing the compression or changing the octane doesn't appreciably alter flame front speed, so that is why ignition advance is most efficient at the same place regardless of other states of tune.
your clarified question can't really be answered. An engine like a flathead Ford couldn't run on 85 octane at even 6.5:1 without detonating, but I can run 85 in my LT1 with 10.5:1. Way too many variables to predict.
But... making some assumptions, like let's assume its a traditional small block chevy with smogger heads. That would fit your #2 scenario of 8.5:1 compression. Optimal spark lead on most SBCs is probably 38* total. So, if you increase compression to 9.5 and retard ignition accordingly, you are out of tune. Your flame front's peak cylinder pressure is occuring after it can do its best work on the crank. Same goes for the 10.5:1 example. Its best ignition lead is still about 38* but you have to retard it even further.
Now lets say you drop the compression to 7.5:1. Now you can (in fact need to) advance the timing to compensate for the extra distance and lower pressure before ignition so that you can still get your peak pressure after ignition in the right place on the crank.
Compression doesn't change flame front speed.
Octane doesn't change flame front speed.
Flame front speed is the largest factor in when cylinder pressures peak, so since flame front speed is relatively fixed, ignition timing is the biggest key. The moral of the story is, build your engine in whatever compression you need for your cam, set the timing for peak power (probably the same for any given engine family), and then run the octane you need to keep detonation away. Anything else is a crutch and will operate with less power and efficiency than if you were running the right combo of parameters.
If that weren't the case, then there would be no need for high octane gas. We could just all run 10.5:1 on 85 octane by just retarding the ignition, but that's not the case.