I am trying to figure out if the motor in the car I bought is any good, so I thought I'd check the cylinder compression. Anyways, WHAT SHOULD THE GAUGE READ (in terms of PSI or whatever units)??? I tested cylinders 1 and 3 and got approx. 25 each. I tested 5 and 7 and got approx. 40-50 each. I achieved this by spinning the motor about four times (by hand). Also, how do I know if the rings are junk--i.e. how little compression is "acceptable" for an engine to run "decently?"

Thanks in advance for any help.

MILT

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'83 HOmaro--doesn't run
'92 Bird--burnt to a crisp
'78 T/A 400--fast, Burt Reynold's style
'83 Buick LeSabre--the Ghetto Cruiser

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Milt,

The compression test pressure you'll get can vary widely depending upon methods.

The accepted method is to remove all the spark plugs, disable the ignition coil, attach the compression tester, wire or prop the throttle fully open (often overlooked), and crank the engine with the starter so that you get at least four good compression pulses at the gauge. That would be eight complete revolutions of the engine, or four complete cycles. Make a note of the pressure reading. Do this for every cylinder, then compare the readings. The lowest cylinder pressure should be within 80% of the highest.

I have never cranked by hand to test compression, so I have no idea if the pressures you got are correct or not. I always like to see at least 90 PSI per cylinder on a lower compression engine, with good balance. Many engines test at 80 PSI or below and run just fine. The important factor is to have balance among the cylinders.

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Later,
Vader
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Thanks. I'm going to put the engine back in anyway, after I throw a couple of gaskets and a new tranny at it, but just wanted to know if I should expect anything from this motor when I re-install it. I suppose I can re-do the compression check once the engine is hooked up...

Thanks

MILT

 

Usually turning the engine by hand is too slow, because the pressure will leak down faster than the piston moves, even with good rings and valves.

More important than the absolute value, is the consistency between cylinders. Like Vader said, some motors with unbelievably low compression readings still run just fine. Alot of factors influence the number the gauge will read. Typical numbers for a motor like yours, again depending on alot of factors, are in the 155-175 range.

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