Compression test!!!!

Discussion in 'Small Block Tech' started by 72buick350, Mar 16, 2002.

  1. 72buick350

    72buick350 i love my BUICK

    I found a guy who is selling a rebuilt 68 350 GS engine. i want to take a compression test. What really is a good compression for a rebuilt engine? I want to put in my 72 skylark. Do you guys think this is something i should go for? It also comes w/ headers, will i have a problem w/ clearance when installing?


    Thanks for the help fellas!!!!!!
     
  2. cray1801

    cray1801 Too much is just right.

    A compression test is a good thing to do. Just remember a compression test does not measure how well the combustion chamber is sealed, it measures the dynamic compression which is cam dependent.

    A leakdown test is also good to check the combustion seal (rings, valves).

    Compression test, actual reading is less important than the variation between cylinders, variation should be within 7% or so for a engine in good shape. Actual numbers could be anywhere between 150 and 200 and be acceptable. For example 150+/-7% or 200 +/- 7%. Some say +/- 10% is still acceptable.

    Leakdown test, leakdown in the 2% range is good.

    I'd also take the pan off and check the main bearings, specifically #'s 1/2 and 7/8. Also check the condition of the distributor gear. Good luck.
     
  3. 72buick350

    72buick350 i love my BUICK

    How exactly would i do a leakdown test , i'm still learning
    so bare w/ me!!!!
     
  4. 72skylark

    72skylark 4 Doors of Fury!

    Leakdown test is a little more complicated than a compression test. basically you rotate the motor so that both valves are closed for the cylinder that is being tested. Then pressurize the cyl. The tester will measure how much pressure is being lost. And if it is a lot you'll be able to hear where it's coming out of. I.E. if you hear hissing from the oil fill cap, it means bad compression rings, if you hear from exhuast or intake it is a leaky valve, and if the radiator starts bubbling it's a bad head gasket.

    I'd stick to a plain compression test. Most compression gauges are marked "good" and "bad" on the gauge. And have the pressure that should be read based on compression of engine. But if the cam has alot of overlap the compression won't be able to build up that much, if it's stock you shouldn't have to worry about that. So like cray said it's mostly about the variance of readings. My old car all cylinders had around 130-140. But one clyinder had 75. So I new something was up with that cylinder, ended up being a burnt valve.
     

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