Compression Test

    1. What's an acceptable compression tolerance between the cylinders?

    2. Given the variance in pressures is under the tolerance for the cylinders what's a max and min pressure before I should consider having someone look at the motor?

    In case #2 was unclear. Let's say for example the cylinders have a compression that ranges from 120-125. (We'll assume this is an acceptable range, which is my first question).


    What's the upper and lower limits at which point the engine needs more inspection. For example if every cylinder was in the 70psi range I guessing that's probably bad.

    Static as detailed in the shop manual. Although I didn't do mine at normal operating temperatures because I'd prefer not to burn all my hands to pieces trying to remove plugs from a warm motor.


    I found where a variance of 20psi is acceptable, but it doesn't say whats a "normal reading".

    I understand in not doing a "running" compression check, it definately hurts more! It will, however, give you a more realistic diagnosis of the condition. Engine parts will have been given a chance to expand at operating temps. A valve timing issue would show higher or lower readings based on early or late valve closing events. My worry is the almost 20% difference between Cylinders 1 and 4. Have you done a vacumm test to check for weak or broken valve springs? You may even have seats begining to burn from too loose a valve adjustment on 1, 2, and 3 with gases getting past the valves. Sorry to give you more homework to do. <img src=images/icon_smile_dead.gif border=0 align=middle> Kevin

    No a vacumm test is next on my list.


    I'll be heading over to a friends soon so he can show me how to adjust my valves as well. They haven't been done in the 5/6 years I've had the car, so I know it's crucial.


    The car has been a garage queen so I didn't worry about it too much, but since I'm trying drive it more I want to make sure it's in working order so I don't damage the motor during routine cruising. I just did a tune up and replaced the plug wires. And I'm almost finished with the Ignitor II and Coil install. Other then making sure everything is working fine I'm with the "modifying".

    here are the results from a compression check I did Nov 2008, done on a warm engine.


    21,000 miles after an engine overhaul. .020 over pistons, stock compression.


    #1 195

    #2 190

    #3 190

    #4 190

    #5 200

    #6 200

    #7 200

    #8 200


    according to Ford Shop manual (Section 8-5, page 109 ) Compression Pressure @ sea level @ cranking speed should be 130 - 170 for the 289 V-8. They don't list any variations for the different V-8 engines.


    Z. Ray


    Edited by - zrayrichter on 01/25/2009 13:33:47

Participate now!

Don’t have an account yet? Register yourself now and be a part of our community!

Register Yourself Login