View Full Version : Compression results!!

09-04-2010, 01:56 PM
Just did my compression test here are the results:

#1:125 psi

My plugs are black and you can smell the fuel on them.I just put them in 15 hrs ago.

09-04-2010, 05:16 PM
Too rich, nice numbers tho...

09-05-2010, 11:55 AM
You may also want to do a leak down test. The #'s seem fairly even depending on how you tested it. But an overly rich condition can wash down the cylinders & cause premature wear. I don't know if your engine was rebuilt before you got your boat or not, so I don't know what the desired compression was on that build. Depending on if it was rebuilt with a slightly less than stock compression ratio or if it is a stock engine your compression may be alright or a little low.. Again I am not sure of your engine's age or prior build status. :)

Dr. David Fleming
09-06-2010, 12:17 PM
Compression variation is not supposed to be more than 5-10psi per cylinder. You are supposed to check the compression with a good battery, ignition disconnected and throttle wide open. The Merc service manual should give you the exact figures but generally 120psi give or take the 5-10psi are all good numbers.

"Leak down test" is a whole different issue. This test is used by the pro's - race engine people to check any loss of performance of the engine and tells how fresh the engine is. Piston is at top dead center - you have to locate this with marks on the crankshaft or turn the engine over by hand until the piston is at top. A special leak down tester is installed in the cylinder and feeds high pressure air into it - you have to have a supply source of compressed air - compressor etc. - by watching two gauges the rate of leakage by rings, valves and head gasket is measured in a percentage. Test is measured in x % pressure loss. This test is supposed to be performed on a warm engine and every cylinder is to be checked. Testers can be bought from Snap On etc or on line from Eastwood tools.

I got one to use on my Harley - was going to keep checking the motor every few thousand miles and see how the engine was holding up. Never used it because I felt the Harley engine had no handy way to be turned over to TDC (top dead center). Merc you would have to hand crank the engine and locate the TDC of each cylinder - mark this on the crank pully - then perform the test on the compression stroke of each cylinder - four stroke engines turn over twice to hit the compression stroke

I believe this would be a good test for a blower motor - especially a 310 hp big block - because the engines are weak to begin with and the blower really puts the screws to them - any problems would show up early with the leak down test.