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Compression vs. Leakdown

Discussion in 'Technical Q&A' started by JSessions, Jun 20, 2005.

  1. JSessions

    JSessions Guest

    May 23, 2005
    78
    SC
    Full Name:
    Jason Sessions
    What is the difference? Is one better than the other? If compression test is good do you need to do a leakdown... vice versa?
     
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  3. Michael B

    Michael B F1 Rookie
    Silver Subscribed Owner

    Apr 28, 2004
    3,649
    US of A
    Full Name:
    Michael
    I am sure a "search" would give you many more thoughts... But:

    Laymans terms:

    Leak down test can show you the amount of pressure that is "leaking" out of the cylinder and or valves/head. You pump a set PSI into the spark plug hole when the piston is at TDC for that cylinder, and then read a gauge on a leak down tester showing the % that is either leaking by the rings, or the valves.

    Compression test is when you turn over the engine (allowing the piston to sweep through the cylinder) with a compression gauge plugged into the spark plug hole. This gauge will give you the pressure that the piston makes in the cylinder when it goes through the compression stroke.

    A compression test gives you a figure for each cylinder for you to compare. If all of the cylinders are close to the same (say within 10%), then you can say you have a fairly good (or equally worn) engine.

    The leak down test can also tell you if the engine is worn in each cylinder by reading the % that leaks out of them. However, the draw back of the leak down is that it is only testing that TDC section of the cylinder wall. On the other hand a compression test will test the whole stroke of the piston. If you have a cylinder bore with an awful taper, the leak down test can miss that.

    If I feel the car needs to be tested - I use a compression test 1st, to see if there is an issue to begin with, and if there is, then I proceed with a leak down test to pin point the issue (rings or valves etc.).

    Consult a competent shop.
     
  4. JSessions

    JSessions Guest

    May 23, 2005
    78
    SC
    Full Name:
    Jason Sessions
    Thanks for the quick review. My mechanic in SC and the mechanic where my potential car is in CA told me to do the compression first, and if it passes, not to waste money on a leakdown.

    It seems like a lot of fchat people talk about leakdowns, but from what you and two other ferrari mechanics say, everyone should be worried about the compression.
     
  5. Husker

    Husker F1 Veteran
    Silver Subscribed

    Dec 31, 2003
    8,674
    western hemisphere
    It all depends on who you're asking. A Urologist would give you one answer, a neurosurgeon another, and a mechanic, yet another.
     

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