Odd compression test result....

[moppit]

Hello Gents...

Did a compression test at the weekend and got some puzzling results... it's my first ever one..warmed the engine, disconnected fuel pump, removed plugs, opened the throttle fully, cranked about 7 times until reading stabilised.

I was suspecting a dodgy head gasket or crack in the block (tapping noise when warm, lots of condensation from engine). I had a failed stat and things got very hot before I realised....

However all cylinders were in the range 140-150 but cylinder 6 is 175? Was expecting something to be lower not higher - now I'm confused! The tapping is on the opposite side to the high reading. Maybe just a rogue tappet? The engine is a relatively new build so wear should not be an issue. It's a 3.5 with Typhoon cam. Maybe that cylinder just exceptionally good or are all the others knackered...

Does water in the cylinder do this? Nothing seems to spray out when I crank it with no plugs. I'm holding off taking off the heads until I've had a re-think...

Any thoughts appreciated...

[DaveEFI]

That method of testing compressions has fallen very much out of favour these days - and rightly so.

Your engine doesn't run at the 200 or so rpm you'll get from a starter - so a big variation is always likely.

Very first RV8 I ever had some 35 years ago showed odd compressions while running perfectly. Which surprised me - but made me wonder if it was to do with the hydraulic tappets.

The preferred method to check piston/valve condition now is a leak-down test.