What is a good compression?

I just did a compression test on a (very) cold motor of 1982 Mondial 8 (basically 2v 308 motor).

Got average 147, stddev 8, min 137, max 159.

Also just pulled motor for several reasons including EFI and Supercharger.

Is that a good compression or do I have to do rings? 32k miles and in general very good shape.

Thoughts?


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I think your compression is good. However, if you are going to supercharge the engine, I highly recommend replacing the pistons and rings with units suitable for the job. Depending on how much boost you plan to run, there are a few other modifications you should make as well. If properly done, you can have a reliable and powerful engine
Good luck with your project.

 

I wouldn't be exited about putting a blower on a 2 valve injected car with the poorly designed ring package Ferrari used in them no matter the compression results. It was more about poor oil control than compression. Put a blower on it and you will likely just have an 8 cylinder oil pump.

 

What do the valves, faces, seats, etc look like at present?

 

When it comes to compression numbers, it’s important to have your numbers close as possible
I’m not sure 20 between cylinders is a good thing.

 

It's a 14% spread from high to low. If the engine is already out, I'd be doing other tests and looking for cause.

 

Haven't opened up yet....just got it out ...will do leak test very soon ...to change rings however is a lot more work than valves right? Do I have to separate transmission from main block? I guess loosen rods to get pistons out too?


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It's all relative. Once the engine is on a stand, I don't find changing rings to be much more or less hassle than dealing with valves. My personal issue in replacing the rings (if needed) would be the parts cost, not the procedure.

To remove the pistons - yeah, you have to separate the rod bearings. I don't know if this engine has a tendency to have create ridges at the top of the bore, so check for that if/when you remove the pistons. If it's present, you'll might need to shave the ridges down before you pull the pistons out. (But again, I'm not an expert on rebuilding this particular engine.)

 

It was done on a stone cold motor - does that even tell you anything at all?

Leakdown may also reveal more....

 

If the compression check was done on a warm motor and the same number of strokes on each cylinder - 6 is a good number - then, IMO, the readings are not acceptable. On a cold motor, a compression check is not really valid as an engine health check unless you are looking for some obvious faults - blown head gasket, etc. where one or more cylinders show no or very low numbers. Cylinder walls change shape as they warm up and depending on each individual cylinder block, they may seal well at operating temp and less so when cold.

If that check was done warm/throttle full open, I would say the numbers are not acceptable because there is too much difference between the high and low cylinder. OTOH, if the car was running OK and I was just doing the test "for grins," I would probably just leave it alone. BUT, since you are pulling the engine, I would take the appropriate steps to re-ring.

 

Ditto.

A compression test is a "something might be (or is) wrong" test. Doesn't tell you specifically what. Could be rings, could be valves, could be something else.

You can drop a bit of oil in low cylinder and see if compression improves, but I'd definitely do a leakdown test either way.

Also, I wouldn't try increasing the base power without pulling the heads anyway and inspecting everything.

 

IMHO if the motor is out then separate it from the transmission case and rebuild it. Once the block is separated you will see how easy it is to break down the motor and do it right. Just saying.....