I'm considering building an all-motor H22. One issue I've never seen explained is how to calculate what compression, coupled with cam overlap, will work with a given gasoline octane.

For example, here in California, we have 91 octane. If I build an 11:1 engine with, say, Crower 2 cams, it'll probably be fine (no knock issues.) But what if I got to 12:1, running the same cams, or even Crower 3s? The additional overlap will lower the actual compression, but how do you know where to start?

 

http://www.empirenet.com/pkelley2/DynamicCR.html

 

I got flamed for posting that link.

 

Probably because you do not have a 67 Chevy, when that info ment something, or breaking in your engine with non-detergent oil, back in the day.

 

Used to have a '69 fleetside, a '78 Nova, and I plan on building a '56 Bel Air two door.

 

Well, i think dynamic compression would change with inlet charge density, which takes into account a myriad of factors. Hence the term, dynamic.

 

Okay, thanks for the info, that's straightforward enough.

Of course, this leads to the next question, how do engine builders know what max dynamic CR can be used with a given octane? I'm guessing, here at least, it's more of just seeing what other people have done, and doing the same.

 

Well, in my opinion, DCR isnt really a factor in octane requirement. Static compression is more the concern for octane requirement (we are talking variable valve timing engines correct?) most *knock/ping* occurs under heavy load, low RPM conditions, away from where peak power is usually developed.