What are your Compression Test Results?

[Argess]

Before I re-built my engine, I had a bit over 195 psi with 9.5:1 static CR.

Now I have 220 psi. Seems quite high to me. I did have a tiny skim taken off the block deck and cyl heads, but sholdn't make very much difference.

I would be interested to know what numbers you have gotten during a Comp Cranking test.

Thank-you.

[patrickt]

Originally Posted by Argess

Now I have 220 psi. Seems quite high to me. I did have a tiny skim taken off the block deck and cyl heads, but sholdn't make very much difference.

220PSI is not a big deal. I've had "warm" (not "hot") street engines that were higher than that. Consistency throughout the cylinders is what matters.

[Argess]

Yes, I was just wondering about the actual number. I found one fo those on-line dynamic CR calculators and it only came up with 177 psi. Although how it figures it out with the only cam input being the intake closing angle, I don't know.

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

[patrickt]

Originally Posted by Argess

Yes, I was just wondering about the actual number. I found one fo those on-line dynamic CR calculators and it only came up with 177 psi. Although how it figures it out with the only cam input being the intake closing angle, I don't know.

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

A purely absolute 100% stock untouched mild 428, according to my shop books, can have up to 210psi. I have never put a compression gauge on the FE in my Cobra. I did have an Olds with pop-up pistons and a ratio of about 10.6 to 1 that had high 230's on the gauge. To be honest, I can't remember the last time I read a "my cranking compression numbers are too high" thread. Usually it's the other way around.

[Hotfingrs]

I've got an 11:1 CR engine and my cranking pressures are a hair over 280 lbs. All because of the cam. Different cam would give you different numbers.

[Argess]

OK....that's reassuring info. Subsequent to the original post, I did check my 66 shop manual and found a max of 200 for 9.5:1 engines and a max of 210 for 10.(somehting):1 engines. So I guess I'm fine. Thought I might have screwed up my cam installation by a tooth, or something, although that proabbly would have reduced the pressure regardless of advanced or retarded.

I guess I can credit the higher readings with the hand-fitted Sealed-Power rings vs the Hastings which I used to use. The slight CR increase from skimming the block deck and heads probably didn't do a whole lot.