Compression test results for 140k mile motor

Discussion in 'Fox 5.0 Mustang Tech' started by Nottoofast50, Dec 19, 2003.


  1. Nottoofast50

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    140k miles on the motor here are the resutls:

    1: 135
    2: 135
    3: 140
    4: 135
    5: 140
    6: 140
    7: 145
    8: 135

    I want to put a blower on. How are these results? I heard within 10% of each other is good so it looks good right?
     
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  2. retro50

    retro50 New Member

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    The dry compression test looks good. Now do a leak down test. This determines how much leakage you have going down the rings and into the crankcase.If the leakdown rate is acceptable then put on the blower.If the leakdown rate #'s are good and they are within 10% of each other you are good to go. You can have good dry compression tests but the leakown rate could show 40-50% by the rings and this is no good.You generally want less then 10% leakage down past the rings.
     
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  3. Nottoofast50

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    So basically the compression test is useless?
     
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  4. retro50

    retro50 New Member

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    The compression test is not useless. It just tells you that you have good compression all across the board. Meaning a healthy engine and the valves are sealing well.But the leak down test is a more accurate way to determine if the rings are leaking by into the crankcase. To do a leakdown you will need a leakdown tester and a air compressor. You run the engine untill it is warm. Then pull all of the spark plugs. Get the #1 cylinder on T.D.C. on the compression stroke and install the tester to the spark plug hole. Plug in the air hose to the tester and read the guages. They will tell you the amount of leakage in % on that particular cylinder. Repeat this 7 more times for each cylinder and then compare the readings. The engine has to be warm to do this test as the ring seal is better on a warm engine. You will get more accurate readings.
     
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  5. Nottoofast50

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    Ah.... unfortantely i dont have an air compressor, how much you think a shop would charge for that? Do they rent air comporessors at auto zone?
     
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  6. cjshelby

    cjshelby New Member

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    I'm not sure if it is typical or not but my dry compression test last month at 130,000 netted all 8 cylinders from 170-180.
     
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  7. blown5.0

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    yeh the leak down test is more accurate but I wouldnt bother, your compression test looks good for a 140k motor, if see runs good and doesnt burn oil, go ahead with your mods.
     
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  8. ej95cobra

    ej95cobra Member

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    My engine had 100,000 miles and I checked compression before the SC upgrade. Numbers similar to yours, 145 - 155. I didn't do a leak down test. I put the KB 1500 on a stock cobra long block and love it! It's been on about 6 months (7,500 miles) and loves the cold air up here in New England. Go for it.

    Supporting upgrades include: CAI, 70mm TB, 75mm Pro-M MAF, equal length shorties, 2 1/2" h-pipe w/ cats, 2 1/2" cat back, 255 intank fuel pump, KB FMU, MSD 6 BTM (+capacitor). Need to upgrade 3:08 rear, it red-lines in 3rd just before finish line.
     
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