You would want to check the vac pump to make sure no vanes have stuck for starters. Measuring crankcase blowby at 7000 rpm under load will be much more accurate.
How To Do A Leakdown Test Mobil
Acceptable leak down test results. The procedure is similar to a compression test to a point. An engine between 10 and 20 percent can still run ok but itll be time to keep an eye or ear on things. Like joe said 15 20 doesnt mean much. Even for normally aspirated engines respectable leakage numbers would be anywhere from 8 to 12 percent with a variation between cylinders of 4 to 5 percent but its possible the variation could. I might break out my leak down tester after i find a cylinder with low compression or i might start with a leak down tester when i suspect a mechanical problem with the engine. Engine compression atmospheric pressure at sea level is approximately 147 psi.
So the great thing about the cylinder leak down test is that. In simplified terms if air is pumped into the cylinder at 100 psi and the gauge reads 97 psi then the leak down percentage is 3. The cylinder leak down test reading is expressed in a percentage. The valves rings and head gasket seal the combustion chamber. Leak down needs to be taken lightly. The tester is connected to an air source and is attached to the engine through the spark plug hole.
The intake valve leads to the intake manifold and the exhaust valve leads to the exhaust tailpipe. Above 20 percent loss and it may be time for a teardown and rebuild. Consequently good cylinder leakdown readings should be below 5 8. No engine will have perfect sealing with 0 percent loss. To get back to your question as long as the cranking compression is relatively equal i wouldnt worry much about the 40 percent leakage. Doing a leak down test is a fairly simple matter.
Five to 10 percent loss indicates an engine in great to good running order. One test you could try is to pump a shot or two of engine oil into a cylinder crank the engine over for a dozen rotations or so and try the compression test again. It deals with how well the cylinder is sealing and nothing else. To perform the test you must first remove the spark plug on the cylinder youre going to test. If you have lost ring seal then you will make less power go slower. Perform a cylinder leakage test by injecting regulated shop air into the cylinder heads spark plug hole then visually searching and listening for a leak.
A 91 compression ratio cylinder is compressing the air and fuel mixture to about 132 psi at sea level 9x1471323.
Gallery of Acceptable Leak Down Test Results
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