The Stovebolt Site Forums The Shop Area Engine Shop Compression Values

Hey GUys,
What would be an acceptable compression value for a 51, 235? I'm getting between 7-90. The eng does not smoke. Also, any special methods you use to take it. I did it cold. 40 degrees.

Glenn,

Glenn,

Are you indicating the cylinders range between 7 psi and 90 psi?


Stuart

If thats cylinders ranging from 7psi to 90 psi you have definately got a problem. Time to tear it down & see whats goin on.

even if it's 70 - 90, that's too much variation, which is more important than the numbers, tho 90 is too low, and 70 is way too low

Bill

Generally speaking, you want no more than 10% variation between cylinders. When you run your compression test, see if squirting engine oil into the spark plug holes raise the compression to within 10% of each other, and higher, generally. If it makes a significant difference, then ring seal is compromised and you need a rebuild. If not, then perhaps your problem is the head needing a good rebuild. That should point you in the right direction.

Glenn,

I think Bill realized the typo...70/90! My specs show 150 psi for this engine, plus or minus 20 psi. There is much controversy surrounding compression tests, their value, the procedure to get real results, and the results themselves.

A typical approach (I think) would be carburetor blocked WOT, several squirts of oil in each cylinder and crank away.

You have a old truck with an old engine (I assume), so you shouldn't expect new readings. In the usual scenario, the test is used to find a problem. Variation between cylinder pressures, as Bill mentioned, is an indication of a problem.

Taking a reading dry, then taking another after injecting oil into the cylinder can help separate ring wear problems from valve problems.

You have another post indicating your pushrods might be bending as they work...this could effect your readings for sure.

I guess the bottom line is the compression test can give you a indication of the wear of the engine (rings). You already know it's old, so this isn't hot news. What the compression test can do better is help you diagnose and pinpoint cylinder/head/valve problems.


Stuart

Thx Guys,
It is funny how you can proof read something and still miss errors. The compression readings were 70-90 psi. I also suspect the method used to take them. I objected but was over ruled by age. I'm only 55. The first 3 plugs were pulled and the readings were taken. Then the second 3 were pulled and the reading taken with all 6 plugs out. Didn't make sense to me. No oil was added. I'll do it again myself pulling only that cylinders plug and using alittle oil. Is it a good idea to warm up the engine first? Thx.

Glenn,

Glenn,

I think whoever told you to pull ALL the plugs as you do this operation is the big prize winner. Are you hunting for a glitch or just assessing the old girls health?

Stuart

the engine should be warmed up before hand so the valves train/seals are oiled and there's a normal oil film on the cylinders - should be done w/ the choke full out or carb blocked - test with ALL plugs out, not in, so you get max cranking speed - don't add any oil to the cylinders until you check each, then add oil the minute before you retest any low ones - as said, good would be 120 +/- 10%, all 6 fairly even around 90-100 is ok, but if any are 20 or more less than the rest you got problems

Bill

It's a good idea to use the blow-gun and blow the crud from around the plugs before you pull 'em,to keep it from falling into the cylinders,but I'm sure you already know that...
Speed