Great writeup on Dynamic Compression

Ivoryk3ys78 said
Hey YR What are the cam specs you are running with the 11:1CR?

My dad's 383 is iron headed with 915 heads. They were heavily shaved and ported. It is all cced out and is a true 11:1. He has an Ultradyne solid roller with 254@050 intake and 110 LSA. Our best octane here is 92. I've never heard his engine ping. I know he runs right at 38. The engine does not run hot.

Wallace racing has a Cranking cylinder pressure calculator. You also get an estimate if you use the dynamic compression calculator there. I don't know how accurate those estimates are.

Cranking Pressure Calculator

It depends on what actual timing you put in them. You don't put in .050" timing, and actual true seat timing is too long. For a typical hydraulic, you would use advertised for a company that uses a .006" lift for advertised. Solids are more of a guess (for me).... most advertised are at .015" lift and if the lash is greater than that then your intake closing will be earlier, so you have to use something less than advertised. (Edit to add: I see YR has said this same thing above.) Steeper ramps.... increase intake closing number. So somewhat of a guessing game, but one that you can make use of with some thinking, knowledge of what it actually represents, and maybe seasoned by experience. Used right, the Wallace calculator nails cranking compression very closely.

But DCR is useful for looking at the mid to low RPM situation IMHO, which is where I use it as I build engine for wide RPM use. At least the article addresses the low vs high RPM intake operation. At the low RPM's the intake is on its own to pull air in, and now you are dealing w much lower VE's and the cylinder pressure pressure increase you get by pushing DCR higher is part of what you do to increase torque in those lower RPM ranges. I don't need to know VE's to make useful decisions.

BTW, the term effective compression ratio, as the article uses it, has been in use for at least 50 years in hot rod circles. I can remember reading that term, and understanding it meaning, in the 1973 hot rod rags LOL. Having data on VE's was quite a dream at that time!

And be aware that the term 'effective compression ratio' used in the Wallace calculator is DCR modifed by boost pressure and altitude effects. It is useful, and I look at it, but it is not effective CR in the same exact sense in which this article speaks.