Cranking Compression Test Numbers

[Hammer107]

I finally broke down and bought a compression tester so I could satisfy my curiousity.

185lbs on the money in both cylinders. They might have varied by at most a lb or two.

I was expecting higher until I looked up my cam specs. If what I understand of camshafts is right I have a fairly late closing intake valve compared to most of the cams I see people running.

What would you guys say this puts my compression ratio at?

Intake 19/47 246 .600” lift

Exhaust 52/24 256 .530” lift


It's a 107 built from a 96 inch block using 10.5 pistons. I don't have the build numbers for deck height etc. Just my cam specs and my cranking compression ratio.

[ejk_dyna]

Well if you are a true 10.6:1 static then you should crank around 196psi cold with your 107. This is at sea level.

Did your builder tell you 10.6:1?

To know for sure you need head cc, deck height, head gasket thickness and bore, and piston dome volume.

[Hammer107]

Where do you get that figure? Does it take cam timing into account?

[Totenkopf]

Hammer, yes it takes into account the intake close. ejk's ccp number assumes sea level. If you assume 420', which is roughly Augusta's elevation, if you were actually at 10.6, your ccp would be 193.5 psi. When you correct to 185 psi, it says your actual static compression is 10.24.

[Hammer107]

What would .040 head gaskets do versus .030 gaskets? We spec'd .030 on the build but the invoice was for .040 gaskets.

It isn't much of an issue as I have a hard time with spark knock as it is but I just want to know I guess.

Where do you find or how do you calculate the static based on CCP?

[Totenkopf]

The head gasket explains most of it then.

What did the chambers cc at? and when the piston was at tdc, how far down the hole was it?

[Hammer107]

I will have to go back and look at our notes. I know doing the calculations it came out to 10.5x and I rounded up to 10.6.

Where do you or how do you find the compression from the cranking pressure?

Also...what affect would the gasket have...approximately?

[Totenkopf]

the 0.010" head gasket change drops it a 1/4 point.

there are a few software choices but online there are some pretty handy ones too.

Here's a couple..........

http://www.rbracing-rsr.com/calculations.htm

http://kb-silvolite.com/calc.php

[Hammer107]

Well..then I should be pretty darn close to our initial calcs......I know it was 10.5x but I don't recall the x.

10.24 plus .25 puts me right in the neighborhood and probably within the accuracy range of the compression gauge as well.

I wonder how much performance increase would be measured with the right head gasket?

[Totenkopf]

I'm sure that cam wants more compression than 10.25. it would help turn on the torque some more. your call for sure but if it were me, I wouldn't be able to let that go. Be sure to check clearances again if you do change it.

btw, your current dynamic compression at 10.25 is 9:1. If you do get it back up to 10.6, the dcr becomes 9.3. Getting the right ignition advance gets more important as you increase the compression.

[Hammer107]

Based on your experience....what is that .25 point worth in TQ and HP? I realize we are throwing darts a little bit here but a best guesstimate?

Reading your posts is getting expensive.

At least the gaskets will be cheap....