Hot Rod Forum : Hotrodders Bulletin Board - View Single Post - Compression Testing ....(Hot vs Cold)?
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Old 06-30-2006, 12:01 PM
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Actually the difference between hot and cold is usually caused by piston expansion. A cold piston is smaller and rocks back and forth in the cylinder more which allows the rings to not mate to the cylinder walls as well. That is why the oil helps. The ring end gap certainly changes but is not the primary ring compression loss. Some piston materials require a larger clearance and a larger clearance will flop more.

A hot test using the differential pressure testing method is preferred and will indicate where the loss is occuring.

Of course you had all the spark plugs removed and the carb venturi full open when you did the test.
Using the hot cranking method and letting the cylinders pump 5-8 times your readings are acceptable. You should also note how many pumps it took to get the highest reading on each cylinder.
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