380sl Compression Readings

[mbboy]

After doing a valve job on my 1981 380sl, the compression readings I get from all cylinders on a cold engine are between 160 and 170, suggesting that the readings would be even higher with a hot engine. However, the manual states that the compression should be app. 8.5 bar, which converts to app. 123.25 PSI. OTOH, other people with 380sl's have posted readings similar to mine.

I was just wondering whether the 116 engine could really be such a low compression engine, and the higher readings caused by oil in the cylinders from worn rings? In either case, the manual states only that the minimum acceptable compression in any cylinder is 7.5 bars (app. 110 PSI), and cylinders should vary no more than 15%. Is there a maximum acceptable compression?

[Mike D]

Ahhh yes, the eternal compressions readings post.

You cant go by listed p.s.i. readings unless you are using a factory gauge, at sea-level, 70 degrees F, 50% humidity and Aquarius is in it's decline.

Too many variables in the equation. The lowest cylinder should be within 80% of your highest.

Minimal readings can be used as a guide but maximum or "optimal" can't be trusted.

You are getting the higher readings because your machine shop probably planed the head surface to get it flat. This reduces the combustion chamber size but the piston is still compressing the same volume of air into a smaller area. Squeezing a tennis ball to the size of a ping-pong ball kind of.

You might have to go to a higher octane fuel or retard your ignition timing if you run into "pinging" under load.

[mbboy]

Originally Posted by Mike D

Ahhh yes, the eternal compressions readings post.

You cant go by listed p.s.i. readings unless you are using a factory gauge, at sea-level, 70 degrees F, 50% humidity and Aquarius is in it's decline.

Too many variables in the equation. The lowest cylinder should be within 80% of your highest.

Minimal readings can be used as a guide but maximum or "optimal" can't be trusted.

You are getting the higher readings because your machine shop probably planed the head surface to get it flat. This reduces the combustion chamber size but the piston is still compressing the same volume of air into a smaller area. Squeezing a tennis ball to the size of a ping-pong ball kind of.

You might have to go to a higher octane fuel or retard your ignition timing if you run into "pinging" under load.

Interesting theory, Mike. But I got the same or very similar readings before the heads were milled. So chamber size-reduction can't adequately explain the higher readings.

OTOH, I've always had to use high-test, even though the manual calls for regular. because regular gave me a noticeable amount of pinging. I've never been able to explain why, though

[Mike D]

Not a theory, it's a fact. You can't reduce the size of a compression chamber without increasing the compression. That's why they sell "oversize" headgaskets to restore the difference from head surfacing (that and for those of who went overboard trying for higher compressions to get valve clearance back) and lowering the compression to "liveable" levels. This of course doesn't apply to "flat" faced heads with no combustion chamber.

It doesn't explain your initial readings. Have you tried a different gauge?

Maybe you had a set of the Euro high compression heads all along? What's the specs for those?

In a different vein, not related to the compression readings, how much or did the head surfacing affect your valve timing? I know they make shims for the cam towers to offset head surfacing.

[mbboy]

Originally Posted by Mike D

Not a theory, it's a fact. You can't reduce the size of a compression chamber without increasing the compression. That's why they sell "oversize" headgaskets to restore the difference from head surfacing (that and for those of who went overboard trying for higher compressions to get valve clearance back) and lowering the compression to "liveable" levels. This of course doesn't apply to "flat" faced heads with no combustion chamber.

It doesn't explain your initial readings. Have you tried a different gauge?

Maybe you had a set of the Euro high compression heads all along? What's the specs for those?

In a different vein, not related to the compression readings, how much or did the head surfacing affect your valve timing? I know they make shims for the cam towers to offset head surfacing.

I doubt if the gauge is wrong, since other people have reported similar readings, and I'm pretty certain I don't have Euro heads. Nor do I think the heads were shaved to any significant degree, because they weren't warped, and I can't really say how much my valve timing will be affected-- if at all -- because I haven't put the engine back in the car yet.

I did, however, check the rocker height, and they're all right on the money -- even though I put them back in different positions, for reasons I discussed in a Tech Help thread about a week ago. So, I'm hoping that I won't have any significant valve clatter when I finally do get the engine running again, but I guess only time will tell.