Pro Builders Help - Very High Compression for No Reason

[SkiDaddy]

You could put the stock head on for safety until you get to try another head (or domes). It's possible someone changed the inserts on the CNC lathe & entered the wrong cutter compensation value, leading to undersized chambers....

If it really rips off the bottomit could be from the high compression (a good thing if you have the right gas).

 

[SUPERTUNE]

Another reason could be the oil used with the piston rings when motor is assembled. Engine needs to be run on the water with a load then checked with a compression gauge after running in.
As an engine builder my whole life, I always use measured compression ratio's with a cc buret with the motor pre-assembled and calculate fill volume by the displacement. This is the the Best way IMO as I build my engines by math spec's as I need them to have the compression values I need when they get to their destination.

 

[Sanford]

Another reason could be the oil used with the piston rings when motor is assembled. Engine needs to be run on the water with a load then checked with a compression gauge after running in.

Thanks for the feedback. Yes, I'm aware of this...motor has been tested dozens of times in the field at operating temps. I've even "washed" cylinder walls with plain gas (then air dried) in order to eliminate oil viscosity boost.

Today I might go out and buy an industrial "lab" spec (1% full sweep) gauge (at Grainger) and put it on my standard compression gauge connectors. Because of the different reading depending on what/whos gauge I was using (5 gauges between 190-215 psi), I want to completely eliminate a bogus gauge.

Ran ski again yesterday and it runs so good, cool and without any engine noises. While I know what/how to listen for detonation on a 4-stroke, I don't think I've even been able to hear it on a 2-stroke. Would a billet dome get pitted or show signs or is the billet too hard for that to happen? Pistons are too clean to show anything.

 

[Sanford]

Today I received my 4.5" 1% full sweep accurate pressure gauge. This gauge is supposed to be lab calibrated and is brand new!!

Between 185psi and a peak of 187psi (with 37cc domes). Now this seems the most logical figure based on my larger 81.75mm bore. If this was a standard bore I'd be down another 4 to 7psi.

Next I'm going to test it with 35cc domes.

 

[Matt_E]

Sounds right.