Calculating CR

[Jbird]

Am I having a senior moment or is it possible to calculate compression ratio from the numbers you get when doing a compression test?

I did a compression test on the BBC I'm putting in my new old truck before I pulled it out of my old old truck. The compression test showed all cylinders at 155-160 lbs. Wouldn't those numbers mean this engine has a CR of a bit more than 10:1. <155 divided by 14.7=10.54>?? I'm pretty sure I'm mistaken because the truck never pinged even on the cheapest gas in the middle of summer. Enlightenment desired.
  Jbird

[enjenjo]

In a word, no. Cam duration, and overlap have a big effect, as does cranking speed. One of the reasons that the difference between cylinders is more important than the absolute numbers. I do remember a Buick I once had that was a for sure 9 to one motor, that would pump numbers in the range of 185 regularly.

[kb426]

The only method that I trust is to fill the assembled engine at tdc and do the math. This requires filling the top ring groove with grease so there is no leakage, a 150 cc buret, and the spark plug hole has to be the highest point so there is no air trapped.

[Crosley.In.AZ]

measuring the cylinder head and related parts is the correct method.

as posted there are too many varibles for using the compression reading only. Some high compression motors witha large camshaft will have a moderate psi reading.

On the flip side a low end / RV type camshaft can build higher operating pressures that would cause a moderate compression motor to 'ping'

Most racers look at leak down tests when checking the health of a motor

[Jbird]

Oh yeah, that little bump stick thingy that opens and closes the valves.
 I knew that, but after eleven hours of yankin crap apart and those compression test results bouncing around my addled brain, I had to ask.
That's what I get for letting my brain do the thinking. I'm gonna put a lumpier cam in it before I put it in my new old truck, so I'm gonna do another comp test just to how much difference the cam made.
  Jbird