Quote Originally Posted by filthy phill View Post
Zul8tr
you said
"Higher exhaust porting yields less static compression pressure because less cylinder volume from TDC to top of port. Most stock 2 cycle outboards are about 6.5 compression ratio (CR) relative to top of exhaust port, if you raise the exhaust ports that lowers the CR relative to exhaust. "

to counteract the raising of the exhaust port (40 thou) many things have been modified.
The idle reliefs have been closed off ? this should raise compression.
the head being skimmed ? this should raise compression.
and the combustion chamber itself being reduced to about 2/3rds its normal size ? should dramatically raise compression on its own..
these 3 factors should of raised the compression to far exceed that of the OMC SST60.

I think another known to be reliable compression gauge would be a good idea to know what is realy going on, simply because 145 psi is not
even close to what is considered a high compression OMC 56.

I would of been expecting atleast 175 psi, then bed in and make around 180 to 190 psi .
In my statement I was referring to a relative change in pressure with previously all else remaining the same. You are correct other things were changed so why the low psi? What was the compression pressure before all mods?

Not being familiar with your engine internals not sure how idle reliefs would affect compression psi.
Yes on head work to increase compression psi.

Redo the compression test with the cylinders oiled up to better seal the rings. If great difference the rings leak a lot, if not it is what it is by that gage.

Do a leak down test.

Yes possibly a different compression gage would give higher psi? Regardless because different gages will read different. I only use Bourdon mechanism type gages for relative differences in psi rather than absolute pressure psi and I always use the same gage to preserve the accuracy of relative psi measurement differences. Bourdon type gages are simple diaphragm expanding mechanisms that are calibrated to a standard using a dead weight tester or other accurate reference testers. Unless calibrated you will find differences among gages since you will not know the past history of the gage treatment and even new identical gages will read different.