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Good compression numbers?

7124 Views 7 Replies 4 Participants Last post by  Bill S
I actually can't find compression numbers in my service manual......on my '83 Spider, what would be considered good compression numbers? I was getting somewhat bogus readings for a while last night when checking, because the battery was low and thus the engine turned over slowly...this is an issue because my cheap Walmart compression checker seems to lose pressure fairly rapidly (like 5 psi in just a few seconds), I'm HOPING my cylinders aren't leaking that fast......anyway, after 7 or 8 compression cycles, I'm seeing around 160psi on three cylinders and about 150 on #4. I tried the oil-in-cylinder thing and it doesn't seem to affect compression on any cylinder (at least the 1 or 2 psi variation I occasionally saw was within the "noise" margin for this test, and the numbers with oil in weren't always higher either).

Sound about right? The engine runs well and pulls nicely, and no smoke whatsoever. I'm really hoping my 10psi drop on #4 will just be a valve adjustment issue.

I should also add that I did the test with the throttle held wide open, and engine warmed up well.
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Was #4 the last cylinder you tested? The battery may have been wearing down by then...

I get ~155 psi in all cylinders of our '84. Rebuilt engine well broken in.

A step above a simple compression test is to do a leak down test. That'd give you a good number for future reference (especially if you do the test every so oftne using the same tester). And, if there is a problem, it lets you know more precisely where (rings, valves, head gasket).
I ran the test multiple times, in different sequences, so the final numbers pretty well represent the state of things.

155 aye? Yeowza, maybe my baby's been babied. :) Actually my el cheapo gauge may be off a little, I suppose. FWIW, the car has about 90k miles on it, don't know what's been done to the engine in the past.

A leak-down test is something I've considered, I may be looking into it once I get #4 squared away (or if I have more problems with #4 after I get in there).
Umm, correction - the last compression test I did saw readings of 175-180 psi. (note to self: stop relying on memory...)

Anyway, what is usually most important is that the cylinders are approximately equal. And the fact your tester won't hold a reading probably means its valve isn't working right. The valve looks like a standard schrader valve (like the ones in your tires) but I think it's supposed to be a heavier-duty version.

A 'real' leak down tester is a worth-while investment. You can also try my eL-cheapo version. It is just a hose with adapter that screws into the spark plug hole. The other end attaches to an air hose. Position the cylinder to be tested at TDC on compression (valves closed) then pressurize the cylinder to 60-80 psi with the adapter.

BE CAREFUL - the engine can suddenly turn. (keep fingers away from things that could move)

With the cylinder thus pressurized listen at the intake, the exhaust pipe, the oil fill and check the radiator. It is normal to hear a little air hissing past the rings (audible at the oil fill opening) but the valves shouldn't leak and there shouldn't be any bubbling in the coolant.

A real leak-down tester allows you to do all the above plus it gives you a reading (usually as a percentage) of how well the cylinder holds pressure. That percentage reading is useful for keeping track of an engine's state of health over time. But my eL-cheapo version will let you pin point problems (for a lot less money...).
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Here are actual compression test results...given the following numbers, what can be inferred?:

The oil added for the 'wet' test helps to (temporarily) compensate for poorly sealing rings. It won't make any difference for poorly sealing valves. Thus the dramatic increase from 'dry' to 'wet' indicates the rings/cylinders are not sealing as best as they could.

The readings are fairly close thus all the cylinders are approximately equal. That is good.

How does it run? If it runs well and doesn't blow clouds of blueish smoke, I wouldn't do anything for now. (meaning I'd drive it and enjoy it)
Thanks Eric. That was an engine with 140K miles on it. What numbers should I expect from it after a complete bottom and top end freshening?
Sound about right? The engine runs well and pulls nicely, and no smoke whatsoever. I'm really hoping my 10psi drop on #4 will just be a valve adjustment issue.
If by valve adjustment you mean adjusting your valve clearence, it really shouldn't have any affect on compression readings unless you have no clearence at all and the valve is never completely closing - highly unlikely. If the engine is running okay I wouldn't worry too much about those readings. Worth keeping an eye on but certainly not worth tearing the engine down for. I consider 150 psi worn but not worn out.
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