[rotorwrench]

Lycoming has their own Service Bulletin on differential cylinder pressure testing. Input test pressure is regulated to 80 psi. Minimum test gauge pressure is 65 psi with no more the 15 psi spread in any given cylinder. This is what the industry uses for all engines whether it is a Continental A-65 or a Pratt & Whitney R-4360.

Having done many a differential test, I have the following observations. Carbon particles in an engine can come loose and get into valve seats causing leakage at the valve. This will be evident by air leakage noise either at the carburetor inlet or the exhaust pipe. Leak by the piston rings will cause build up of pressure in the crank case. On a model A, a person can hold their hand over the oil filler tube opening and feel the pressure build. Leaky rings can be cause by deteriorated rings, broken rings, or carbon fowled rings causing them to stick. The engine can be turned to back the piston off off TDC and then forced back up to TDC to check for sticky rings on an airplane application with a propeller but it is tricky. The prop is hard to hold even with only 80 psi in there. On helicopter applications a turning tool has to be used to do this but it's just as tricky. A person can get injured if they can't hold it or it may cause damage to the aircraft if it hits something.

This stuff can be done on car engines but turning the crankshaft under pressure may not be possible in some applications. The model A hand crank will not have enough leverage to hold the crankshaft by hand and under pressure. I don't turn them off TDC unless the compression is low and that is only to see if I can gain a pressure increase by varying pressure acting on the rings.

This type of test will not tell the technician how much compression the engine's cylinders will generate. It is only a test of how much pressure the piston and rings will hold with the limit of 80 psi test input pressure.