I must admit, I do not understand the value of a leak down test for compression on a non-closed system. I understand why leak down tests are performed on systems such as AC and why you would want to pressure check the lower unit but why an engine? An engine will always show a loss of compression during this type of test. Tolerances or gaps between parts are engineered into the motor so it doesn't self destruct going from cold to OP temp. I was always under the impression that the compression an engine could develop and maintain just prior to ignition was more important than how much compression an engine loses over an exponentially longer period in comparison.

Just tring to learn something here.

Thanks

It is just another way to check the integrity of the rings, but if you want a more detailed answer: A leak down test tells you what kind of shape your rings are in. You may have good compressions on all cylinders, but not all cylinders are exactly the same. A leak down test could reveal which cylinders are weaker than the others in terms of combustion. What you need to realize is how well a given cylinder can harness the power from combustion is dependent upon how well it can keep it in the cylinder. The more blowby a set of rings allow, the less work you get from that cylinder. The result is a loss of overall performance of the engine.

It was actually developed for aicraft for preventive maintenance to catch small problems before they become big ones. In performing a leak down test on any motor you are putting a known regulated over pressure factor in the cylinder, say 80psi with piston on TDC (with flywheel lock) and expecting a lesser under lets say 75psi that you subtract from over psi = 5psi then divide 500 by 80psi = 6.2% leak-down (under 10% considered good). Any cylinder test should be done with motor warmed up after a carbon cleaning. Like gclark said if you could get a motor where all cylinders had the same low % leak-down it would be a running son-of-a-gun.

i asked the same question before.

the leakdown tests the rings and cylinders at top dead center. that's where it's the most important!
top dead center is where the "explosion" happens and where the power comes from on a 2 stroke.

steve.