I've never done this before, and undertook it for the first time today, guided by books, google, and YouTube. My truck's a 4 cylinder. I did the first test after driving it for a while to warm it up. I got 150-150-146-140 psi for the four respective cylinders 1-2-3-4. The instruction I was following was to crank the engine for about 5 seconds on each cylinder. After reading some more, I found a site that said you should only crank 6 revolutions per cylinder. Otherwise you can falsely build up acceptable pressure in a cylinder that might actually be worn or have worn valves. So, I redid the test, trying to count 6 revolutions by ear. This time I got 120 plus or minus 2 psi on all four cylinders. However, the 6 revolutions seemed to me to take about the same time as my previous 5 second count. The difference may well have been due to an hour of cooling off time: I didn't warm the engine up before the second test. Anyway, my real question is about acceptable readings. I have two separate manuals for the car, one Chilton's, one Haynes, and neither says what acceptable readings are supposed to be. (I checked them both over for a long time.) Both books explain how to do a compression test, but neither tells you what the readings you get may mean. A website I found says anything above 100 psi is acceptable. 135 or more would be considered excellent. The implication was that this is true of all cars. Is that the case?