Thread: Making Combustion Chamber Bigger

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Originally Posted by FAYLUR

Thanks guys. So there are many variables,both externally and internally that come into play here when talking dynamic compression ratio and combustion pressures. Sounds like an area that just one spec out of a dozen or so could be out and cost an engine.
This is a tough one to find alot of info. on (I'm trying to learn) because of evidently, those many variables. As one motor with 9:1 SCR and 200+ cranking pressure may require a certain high octane, another motor built the exact way except with a different cam,may have 9:1 SCR and 180- cranking pressure because of it's intake valve closing point being higher than the first motors,and run fine on a lower octane,yet be faster?
I was hopeing for a 'rule of thumb' figure that would link Dynamic compression ratio with a certain or 'required' octane but it looks like an "inexact" science.
Mr. Ericson and Mr. McCarthy have quite a bit of experience in this I would suppose, would you guys,or anyone else chime in?

Like you said it is an "inexact" science.
Many years ago when we built "hobby stock","stock car engines" we use to try and cheat by installing a cam that was cut on a tighter l/s.
This would allow us to run a little higher comp. ratio and not get caught.
There are so many differnent variables.
Even something as simple as "spark plug reach" will affect it.
There is no " magic number".
If you can get "dyno time" this is the only true way to find this out I.M.O.