Compression Pressures

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Jan 10, 2007 | 10:30 AM

rdw516

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From: Powhatan, Virginia

Compression Pressures

In the Chilton's and Haynes manuals I have, they state the compression should be no less than 75 to 80% of the highest reading obtained/ ( from the lowest pressure to the highest pressure of the cylinders) All said and done, it doesn't take a brain surgeon to understand this. Say for instance, if the highest pressure is 200 PSI, then the lowest should be 150 PSI. But, if one goes by the percentage ratio, then 7.5 psi is acceptable IF the highest reading in 10 psi.(75% of the highest reading.) So this ratio statement in itself is BS.

Can someone tell me WHAT the actual compression pressure should be in a normal, operating engine should be. THEN the percentage ratio could be effectivly applied to determine if all cylinders are up to par.

Jan 10, 2007 | 10:54 AM

n9lhm

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From: South Bend, IN

No, it's not BS. The absolute value of compression pressure is dependent on a dozen different things -- primarily cranking speed and altitude. Typically it is 100PSI or more, but again that is not always a 'repeatable' measurement in the field, and is why compression specs are normally given as a percentage.
On the old Detroit diesels, compression pressures were specified to be measured with the engine running at a particular idle speed -- and you were given a range of pressures that were acceptable depending on the altitude the engine was being operated at.

Last edited by n9lhm; Jan 10, 2007 at 10:57 AM.

Jan 10, 2007 | 12:01 PM

Silver Streak

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From: Broken Arrow, OK

Quote:

Originally Posted by rdw516

But, if one goes by the percentage ratio, then 7.5 psi is acceptable IF the highest reading in 10 psi.(75% of the highest reading.) So this ratio statement in itself is BS.

But the simple application of a little common sense will tell you that 10 psi isn't enough. There are very few engines that would produce 10 psi anyway. Typically the numbers will either be high enough to run or they will be zero. Getting compression below 60-70 psi that isn't zero doesn't happen that often. Most healthy engines will fall between 100 and 150 psi.

The ratio rule is intended to find a problem cylinder, or evaluate engine condition. It is not to diagnose a dead engine. It has been generally accepted that the compression can vary considerably between cylinders of a healthy engine. 25% is just an arbitrary number to tell you when one cylinder isn't pulling its weight.

Feb 28, 2007 | 06:54 PM

MonatheGrona94

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From: Hampton Falls, NH

Make sure when you are taking a compression reading that you have the throttle propped completely open and all the spark plugs are removed and when you crank you can see the guage and watch fro the highest reading, once it stops rising on the guage that is the pressure you take. all my cylinders read out to be 165