Compression test

johnk

johnk

Member
Morning all,

Should I do a compression test when the engine is hot or cold, or does it not matter? Also, what pressures should I be looking for?

The engine is a 1972 2000 tc that has been converted to sc spec by a previous owner. The timing is spot on, the mixture is good and it doesn't burn oil or smoke but it has always been down on power. It will only do 70 flat out and the speed drops at the slightest sign of a hill.

Thanks.

John.
 
Hot so it takes up the clearances to normal running. And mostly its that your are looking for a narrow spread of pressures, ideally 5% variation max. The wear won't normally be even in a tired engine because one part tends to get a better feed (nearer the pump) than the other or there is a specific issue on one cylinder. The absolute reading will depend on the CR and if you look in the factory manual it'll tell you what to look for (I have a V8 so don't know).
 
The spec on a 9:1 is 160 psi. Having said that I believe that issues such as a tired starter motor will cause lower compression readings, as will the type of adapter on the compression tester. Hence you are better looking for lack of variation rather than the absolute number to indicate that you have a good motor. I have seen compression numbers become progressively worse going through from the first cylinder to the last cylinder. A set of jumper leads to a good battery resolved that, the compression readings were then good.

Always test on a hot motor with throttle open all the way, and a freshly charged good battery.
If you get a low reading on a cylinder then repeat with some oil down the cylinder bore. Squirt the oil in with the piston at bottom dead center, so the oil goes on the cylinder wall, and not on top of the piston. If the compression number rises, you have a bad rings. If the same, then you have a valve problem.

A leak down test is a better way to go, but not many people have the tools to do that. A leakdown test is the only way an aircraft piston engine is compression tested.

James.
 
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