With a 10:1 compression ratio I get 205 psi when performing a compression test. I normally expect to see something in the 140 to 160 range on stock engines, but mine is modified and uses a performance camshaft, although it’s a mild one as I wanted to get max performance, but retain a smooth idle.

I wanted to know if there is a correlation between compression test pressure and static compression ratio, and this took a while to research.

Basically, there is, but one needs to find the dynamic compression ratio first. Unable to sort out how to actually compute the DCR, I used an on-line calculator as per this link:

http://www.wallaceracing.com/dynamic-cr.php

I used what I knew about my engine and camshaft and calculated the DCR as 6.81:1

I thought the next step was using Boyle’s Law of gasses, but I was wrong. Apparently for an engine cylinder, one needs to use the adiabatic expansion formula. Don‘t ask me to explain it, but apparently a pressure change is not proportional to volume change if there is no exchange of heat. I don‘t remember much from my course in thermodynamics back in 1979, but I do remember an old stationary engineer who gave me his definition of thermodynamics: “The thermo is when you drop your thermos on the floor, and the dynamics is when you bend over to pick it up!”

I digress… so back to the adiabatic expansion formula:

P2/P1 = (V1/V2)^1.41

Substituting V1/V2 with 6.81, I get 14.95 and;

(Comp Pressure + 14.7) / 14.7 = 14.95

The Comp Pressure works out to 205.11 psi which compares nicely to my measured pressure of 205 psi.

I’m not sure I’m satisfied with all this until I can understand it more, yet I’m pleased to find that (it seems) SCR can be mathematically massaged, along with engine and cam specs, to find the DCR and therefore predict a compression gauge maximum reading.

Anybody know enough about thermodynamics to confirm or deny all this? ]]>