scuf23

Got the readings done:

Right side: 115,120,125,120
Left side: 110,115,110,115

The guy at the garage told me that above 100 it is ok

Since I have not much knowledge on engines, would like to get your feedback ... I mean I know it is lower than the original specsm but it doesn't seem too bad either, does it?

Daze

above 100 is good but almost the same in all 8 cyl is more important. A good motor has no more than 15% difference between cyl. Your furthest 2 are only 12% apart so you are good to go.

urban_cowboy

Daze is right about being consistant. If the are the same, that means the engine is either not wearing or wearing evenly. The amount of compression is going to depend on your compression ratio and cam. The compression in my 351 Cleveland is 225psi but that is 11:1 CR.

I case you care...
Compression = (Compression Ratio minus one) to the 1.2 power multiplied by Atmospheric Pressure

Given Atmospheric Pressure at sea level = 14.7 psi, we have the following:

So 11.0:1 is 235psi
10.0:1 is205psi
9.5:1 is 191psi
9.0:1 is 187psi
8.0:1 is 152psi

You can test the cylinders, then look atweatherunderground.com to see what the atmospheric pressure is in your area that day. You may need to convert from inches mercuryto psi.

110psi is going to be like 6:1 at sea level...seams kinda low. Maybe my formula does not translate well or maybe you are in a lower pressure area of the world. Texas is basically sea level and the formula works pretty good for the engines I know the compression ratio for.

htwheelz67

I'm not sure about that formula.........theres alot of factors in what your cranking compression will be related to static compression. On the same engine, with say 10-1 compression, one with a big high overlap cam may be only 165 psi but with a small rv type cam with low overlap it could be 200, cam position (retarded/advanced) will affect cranking compression as well. Theres no way you can tell what your compression is just by cranking compression.

67mustang302

+1

A blower cam will show a lower cranking compression as well on account of the later intake closing. That formula can help to get a ballpark idea though. With all the variables to take into account it's hard to be accurate. If it's an old engine with a bunch of miles that wouldn't be too bad. The older engines had mediocre compression/cam setups and the technology lends itself to reduced sealing over time.

kalli

was confused about that as well.

if I convert 11:1=235 into my metrics (bar) then this rather sounds like a Diesel engine.

At the same time what most people forget is that a compression test is useless when the throttle is not fully open.
Same for engine temperature. engine needs to be at operationg temperature.

A question on the side. I have a compression checker (cheap) that has a rubber foot to hold against the spark plug hole.
Are they any good readings off them ? as a mechanic I usually only worked with the screw-in ones and I have nothing to compare so I'm unsure ..

Thanks for any ideas

Kalli

urban_cowboy

235 is not unsual. I have a 70 Cleveland with 11:1 4V closed chamber heads that runs that on all cylinders with a RV cam. As I said before, the cam will affect the accuracy of the corrilation, but it is a general guideline.

kalli

forget my question. the thread inspired me, so I went out for lunch and got myself a compression test kit.

Can't wait to test and see results ;-) hopefully good ....

Kalli

Daze

+1!! ALSO don't forget for a correct reading all 8 spark plugs need to be removed.

urban_cowboy

I did not know that. You know,you learn something everyday! I knew about the oil can trick to check for worn rings and knew to test with WOT, but I did not know to remove all plugs.