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Originally Posted by frankkerfoot
Jim
I agree with almost everything you say, except the details about using the leakdown tester. The leakdown testers I have used on previous racing engines measured a continuous leak rate by measuring the pressure drop across a calibrated orifice (much like measuring current by measuring voltage drop across a known resistor, if you are comfortable with electrical analogies). The pressure on the input side of the orifice is set (using the regulator) to a value like 80 PSI or 100 PSI, and with the input pressure still supplied the pressure after the orifice (which is the 2nd gauge, measuring the pressure in the cylinder) determines the leak rate, expressed as either a PSI difference between the two gauge readings, or a percentage. My recollection is that if you shut off the air supply, even on a cylinder with excellent leakdown (a few PSI or less) the pressure in the cylinder goes to zero within a modest number of seconds. The technical details for the leakdown tester you picture describes a continuous flow process like I described.
By the way, leakdown testing has several other advantages over compression testing. Since the process is continuous you can listen to various ports (like the intake port, exhaust port and sump) and determine what is leaking. You can perform leakdown with the piston not at TDC (by anchoring the crank in some way to withstand the air pressure on the piston) and assess ring sealing at points other than TDC.
Frank
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Frank,
I agree with what you say. I did mention listening to the ports, valves etc. for any
hissing indicating a leak. The leak rate test you describe can be done with this tool, but you need a specified value for the % Rate of leak (Determined by the Manufacturer and not easily available), otherwise you can only determine consistency between cylinders. One thing you didn't mention was that the Cams (or Crank/Cams in combination) must be rotated so that all the valves are closed. On some engines where there is a Valve Overlap, you need to remove the cam altogether so that the lobes don't hold the valves in the open position.
Happy Motoring!...Jim'99