Performing a static leakdown test can be very helpful in determining if piston rings have begun to leak, if seated vales are sealing, and if the head gasket is allowing compression to leak into the cooling system.

The Longacre Leakdown Tester includes: two gauges, a port for connecting to the compressor, a port for connecting to the spark plug hole, and a regulator. Leakdown testers can use digital or analog gauges.

Testing should be done with the engine warmed up to normal operating temperature. This is done so that the rings have expanded and will seal better, which should provide more meaningful numbers.

With a spark plug removed and the piston at top dead center, pressure (normally 100psi) will be fed into the cylinder via the spark plug hole, and the air flow, which represents leakage, can be measured. For example, if the left gauge is at 100psi, and the right gauge reads 95psi, you have 5% leakdown.

If you have high leakage, you should listen for air hiss in the following areas:

Exhaust pipe – May indicate a burnt or stuck exhaust valve

Carburetor – May indicate a bent or stuck intake valve

Oil filler or dip stick – May indicate a possible broken ring or piston

Radiator cap - Bubbles in the radiator indicate a leaking head gasket or cracked head

Generally, racing engines would be in the 1-10% range for optimum performance, although this number can vary. Ideally, you would want to complete a leakdown test when the engine is fresh and use those numbers as your baseline.

Remember, like tire pressure gauges, you should use the same leakdown tester on the same engine for a true comparison.