Quote Originally Posted by NLMoschitta View Post
Holy cow, that is some awesome info there, thanks for taking the time to write that out!

Why do you recommend a compression ratio of no less then 9:1 and 10:1 with iron and aluminum heads (respectively)?

One thing that is a little fuzzy... A higher compression engine requires a higher octane fuel? And with out that higher octance expect detonation?

And... With high performance engines, the higher compression ratios result in the fuel and air being compressed real tight, and when they combust throwing the piston back down the cylinder harder resulting in more pep?

Thanks
Nick
Nick, it's not a matter of recommending no less than 9:1 or 10:1, it's a matter of recommending NO MORE than that with a daily driver pump gas motor. Like I said, there are always exceptions to that rule. You can jockey the whole mess around by extending the intake closing point, tightening up the squish to 0.035", polishing the combustion chamber and piston crown to eliminate hot spots that could cause pre-ignition and optimizing the spark timing to prevent detonation. There are many iron-headed motors out there working every day at 11.0:1 static compression ratio, but everything must be optimized to make it work on pump gas. Then, the first time you pump in a load of bad gas, you're in big trouble. Better to build the iron head motor at 9.0:1 or a little higher and leave yourself a cushion to allow for variances down the road.

Yes and yes on the fuzzy thing.

Yes on the pep thing. Sounds like you have a pretty good understanding of how it all works.

As an aside, you can run a little higher static compression ratio with aluminum heads because of the higher heat rejection of aluminum over iron. The whole idea of higher compression is to keep the heads cool and you can do that better with aluminum. Do some research into the LT1 motor that Chevrolet produced. They used a reverse flow cooling system so that the heads got the coolest water that was available in the system before any of the other engine components. Traditionally, the heads and manifold are the last to get water and by the time it gets to the heads, it's already hot.