What do these compression #s mean?

[aigel]

3.2 with about 130k miles:

cyl# - compression (psi)

1 - 160
2 - 160
3 - 165

4 - 180
5 - 180
6 - 180

I realize that there is 11% difference between the highest and lowest reading. So, that's close to the 10% people wish for. But what's up with the difference between banks? What does that mean? Where does or could that come from? Is that bad?

Thanks,

George

[Early_S_Man]

George,

The allowable 10% deviation is usually from the mean (or average) ... so you are fine, since your mean is 170.8 psi!

[Joe Bob]

They look OK....have you done a leak down?

[aigel]

Thanks guys.

Mike, there is no leak down data. This is a car that a pal of mine is buying. I think it will not be necessary to do a leak down since these numbers check out.

Still, I am curious where the 20 psi difference between the banks can come from???

George

[kstylianos]

George,

Has the engine been out and apart for a rebuild?

Could possibly have .25mm shims on one side and .50mm shims on the other. One banks heads could have been flycut a little more than the others. Different cams in each bank. There are many variables that could affect compression between banks. I agree with you that a 20 psi difference between banks is something to worry/investigate. Could also be as simple as the valves are a little too tight. If so how long have they been that way?

Do some homework before diving into this one.

[aigel]

Charlie:

I was talking to my pal about it, and my most likely theory was that it may have been flycut on one side (e.g. due to broken studs) and not on the other. The records are not complete but the car is inexpensive enough to the point where it does not really matter, even if we'd have to go in in a couple of years to freshen things up.

I doubt very much that the factory did anything like this. I also was thinking that the drivers side has the converter and more heat next to it. Is it at all possible that things on that bank were hotter when the comp. numbers were taken? Probably not likely to affect it down to 20psi.

Thanks!

Cheers, George

[kstylianos]

George,

Just some quick calculations here for comparison, but assuming a stock compression ratio of 9.2:1, flycutting the heads about .25mm will bump up the static CR up to ~9.5:1. Don't know what the actual PSI change would be, but would guess it would be quite obvious.

If some heads studs broke one one side and damage to the head sealing surface was sustained, then this is a possible theory. An additional .25mm or .50mm shim should have been used on that side to compensate for head milling.

Have your friend dig for more background if possible.

BTW: Don't think that the cat would have any affect on the compression readings. Do a cold test to see

[aigel]

The way things look now, we'll have all the time in the world to measure whatever we want. The car is spoken for and will be picked up soon. Let's hope the engine is great, because I'd be the one to rebuild it, if anything goes.

Cheers, George