Hello all,

I search for engine pressure and engine compression in this room, but did not see any related article or specification. I maybe using the wrong key word, and not sure what is the technical term to call it.

I measure all 8 engines on a 1972 350sl; the pressure read around 100 to 105psi. Is this normal for 1972 engine?

Thank you,

Tom.
I

There are two methods for checking the mechanical health of an engine ...

1) Engine Compression
2) Cylinder Leak Down

Both require the engine to be at operating temperature.

Engine Compression ideally requires valve adjustment for accuracy. Given this method involves cranking the engine, it is dependent on a healthy battery.

The better test is to perform a Cylinder Leak Down ... effectively injecting 100PSI of compressed air into a cylinder with the piston positioned at TDC (Top-Dead-Center), and reading the pressure sustained by the cylinder. This method has the benefit of being able to listen for air leakage.

Air sound: in the Exhaust = Exhaust Valve ... in the Intake Manifold = Intake Valve ... in the Oil Filler (Valve Cover) = Piston Rings ... or bubbles in the Radiator = Head Gasket. The engine must be locked (in gear) to prevent the air pressure from causing the engine to turn

Read up on both methods before performing these tests.

The % difference between cylinders is more important than the absolute value(s) ... unless all read very low.

I do compresion testing on a cold engine. This what you have when you are trying to start from cold. 100 PSI of compression is rebuild time.

I also use a vacuum tester which is far more acurrate than a compression tester. As long as the valves are closed the cylinder should hold about 17'' of vacuum. A hair under the valve will cause a vauum leak during testing - it's that sensitive.

The advantage to using this method is you don't even need a running engine or the starter to work. In remote locations I bring along a portable air tank to run the tester which needs compressed air to make it work.