As a general rule a compression of 135 psi or better is excellent. After running the first test squirt a teaspoon of oil into the cylinder and rotate the crankshaft six more turns.
Large Text Compression Benchmark
Acceptable compression test results. Nick ferrari a compression test reveals the condition of your engines valves its valve seats and piston rings and whether these parts are wearing evenly. Also when you do a cylinder leak down test you. Running compression at idle should be 50 75 psi about half cranking compression. Worn piston ringscylinder walls. Engine compression atmospheric pressure at sea level is approximately 147 psi. When you test engine vacuum at the manifold you are looking at the breathing of the entire engine by checking vacuum at a common plenum source.
When you do a cranking static compression test you are checking cylinder sealing not cylinder breathing. Doing a compression test on your ford or mercury taurus sable windstar equipped 30l and 38l v6 is an easy test to do. Snap throttle compression should be about 80 of cranking compression. The most desirable situation is that all cylinders give the same or close to the same reading and that reading is above 135 psi. The results of a compression test can be quite telling. The most desirable situation is that all cylinders give the same or close to the same reading.
Lets analyze the results from the following sample test readings. Furthermore that reading should be above 135 psi. The results shown below indicate the particular area that needs attention. This article will take you thru the test and interpretation of the cylinder compression results in a step by step fashion. The relative compression test results depend on the battery starter motor and wiring condition. To get good and reliable results from the relative compression test the battery and starting system have to be in good shape.
Similarly a compression of 85 psi or lower is extremely bad. If the compression increases the piston rings are at fault. Healthy engines should have compression. As a general rule a compression of 135 psi or better is excellent and a compression of 85 psi or lower is extremely bad. In example 1 cylinder 1 has a snap test reading that is much lower than the other cylinders. This test should not be confused with similar tests that are used for different results.
A 91 compression ratio cylinder is compressing the air and fuel mixture to about 132 psi at sea level 9x1471323.
Gallery of Acceptable Compression Test Results
