GodSpeed

hey i want to do a compression test. this will be my first atempt at this so i was wondering if there are any special tricks i should know about. i have a compression gauge already are there any other tools needed? also what should the cylinders be and how diffrent can they be before its a problem?

thanks for any help you can provide

75ohm

I pulled all four plugs and disconnected the coil to the cap... used antiseize on the threads of the fitting (and the new plugs) for the compression tester and cranked 5 times on each cylinder to get a reading. I had the magic # of 186 that I referenced... you're going to want all about the same +/- a couple... read once that 10% but I would be concerned if that was the case. All real close to each other is good... all close to 186 is better! Good luck, let us know.

Dennis Wilson

Don't forget to block the throttle open. The ideal compression is about 15 times your compression ratio. If one cylinder is too low, squirt some oil into the clinder and repeat the test. If the compression comes up, you have broken or weak piston rings. BTW if your compression is too high, this indicates there is high carbon buildup on the pistons and/or the combustion chamber.

Dennis

eohrnberger

I've read someplace, on here I think, that an fresh S2 engine should be up around 180 PSI per cylinder. On my PPI I insisted on a compression test, two years ago now, which came up 172 +/- 2. I guess that's pretty good, considering that the engine had, at that time, 175K miles on it.

Does anyone have any more information specific to what the fresh versions of our engine should have? Or is it just some math calculation that eludes me?

Laust Pedersen

It is a good idea to remove fuel pump fuse during the test, to prevent washing the cylinder walls partially dry with fuel.

75ohm

Agreed, pulled the fuse...

Dennis Wilson

The compression on a fresh/non carboned engine should be 1 barometric pressure (14.6 lbs) times the compression ratio. As stated earlier, the compression ratio can be raised due to carbon on the pistons/head combustion chamber or due to machining the head which reduces the combustion chamber size (increased compression ratio).

Dennis

Laust Pedersen

True as a first order approximation, but the compression is closer to being adiabatic (the air will be heated due to the compression alone, increasing the pressure further).
The 951 manual lists 145 psi as being normal and it only has a 9.0:1 CR.

Laust

Dennis Wilson

Laust,

Yes the one bar times compression ratio is just a starting point. The variables are intake restrictions (more on a turbo engine), altitude, cleanliness of engine and any head or block surfacing/machining. Also, how many compression gauges are recalibrated frequently? Mine aren't. This is why most spec's will give you a plus/minus or minimum allowance. It ain't perfect but considering the variables, it's all we got. YMMV

Dennis