Compression Testing Activity Running Compression Test When a standard compression test does not reveal any concerns a running compression test can detect: • Broken valve springs • Worn valve guides • Bent pushrods • Worn cam lobes • Sticking valves • Intake or exhaust manifold restriction Theory behind this test: When you perform a standard, static compression test, you are checking cylinder sealing, not breathing (volumetric efficiency). This test looks at an individual cylinders ability to breathe. Say the engine is running at 18 inches of vacuum, barometric pressure is about 30 inches, so the difference (30 – 18 = 12) is what the engine is inhaling. 12 inches of mercury is equal to about 6 psi absolute air pressure. Compressed to about 9:1 ratio, you should get 6 X 9 =56 psi pressure if all the air makes it into the cylinder and then gets exhaled. Therefore, your idle reading on a running compression test is about 60 psi. Note: Leaving the Schrader valve in will affect your reading. An engine idles at about 600-900 rpm, and the starter motor obviously cannot crank the engine as fast as the engine idles. Most manufactures’ specifications require the engine to crank at 80-250 rpm. Therefore, we are checking the compression of an engine at cranking speed to determine the condition of an engine that does not run at that speed. Some would think that the compression would be substantially higher because the valve overlap of the cam is more effective at higher engine speeds and this would tend to increase the compression. Actually, the compression pressure of a running engine is much lower than cranking compression pressure. This results from the volumetric efficiency. The engine is revolving faster, and therefore there is less time for air to enter the combustion chamber. With less air to compress, the compression pressure is lower. Typically, the higher the engine rpm is, the lower the running compression. • Compression (cranking) 125-160 • Compression at idle 60-90 • Compression at 2,000 rpm 30-60 Similar to the cranking compression test, the running compression of all cylinders should be equal. Therefore, a problem is not likely to be detected by its value, but by how far the running compression varies from the other cylinders. Use the procedure below to perform a running compression test.
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