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Time to do compression check on 2 motors

Old 01-21-2004, 03:20 PM
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Well a compression test is really a factor of the compression ratio. For example, a 40's flat head with a compression ratio of 7:1 will show a lower pressure reading than a modern diesel engine with a compression ratio of 22:1. That's why the numbers are diluted-it really depends on the compression ratio. Now, cam profile can also play with this a little as there could be some pressure bleed off. None of this is a factor with a leak down test. The cylinder is either sealed or it isn't and if it is leaking, then we know how much and were.

The only thing that you can really look at with a compression test is consistency between the cylinders. Beyond that the numbers really don't mean anything. For example, all of your rings could be glazed and not sealing, but because all the pressures are equal, you really wouldn't know it. A leak down test will show the rings are leaking and how bad.

Remember my comment about a leak down test being bruttally honest. This test does not lie. It will reveal things that you didn't even know were wrong.
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Old 01-21-2004, 03:28 PM
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I would not pay anyone for a compression check.

If you are going to fork out your $$$ make sure he is doing a leak-down test.

Only a leak-down will tell you the health of the motor
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Old 01-21-2004, 03:35 PM
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OK, now I remember, there is a check valve in the quage, so it holds the max reading, otherwise between revolutions it would all leak out, and you wouldnt get a reading if you stopped cranking, anywhere but TDC, the guage is not relly holding the pressure in the cylinders, its just reading the max amout that was there.

OK, Me stupid
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Old 01-21-2004, 03:47 PM
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I have not done a normal compression check on anything other than a lawn mower. The leak down check should only take 1 hour for a V8 engine. Anymore than that get a tester and do it yourself. it takes a lot longer then the normal test because you have to turn the engine over per cylinder. If you have any questions PM me and I will send you detailed instructions.

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Old 01-21-2004, 04:14 PM
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Like was stated earlier, a leakdown test will tell you the percentage of compression that is somehow escaping your combustion chamber. There will ALWAYS be some % of leakage but all the cylinders should not differ more than a few percent, (that's in a perfect world). I am assuming the engines are fairly factory stock, correct? then if they are supposed freshly redone with very low hrs, you should get a % # somewhere around 3-4 maybe 5 % of leakage. On a well cared for couple hundred hr engine, somewhere around 8-12% would be acceptable. If any 1 or more cylinder differs by more than 6-8%, there IS a potential problem starting with that hole. Then while pressure is on that hole you will be able to hear whether the leakage is coming from the crankcase, the carb, (intake valve) or exhaust,(exhaust valve). This past summer we did a leakdown on a pr of blown 588's that had 12 hrs on them from a VERY reputable engine builder, customer complained that every time he went over 4000 rpm and into boost, it would blow both breathers off both engines!!!!! I was in the boat and seen it myself. The engines had between 115 &135 lbs of compression, BUT the leakdown was 35% and above!!! So do yourself a big favor and while they have the plugs out, spend a couple extra bucks and have them do the leakdown. It could save you thousands in the future. Been there, done it just my $.02
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