Quote Originally Posted by Mark75H View Post
As stated, gages vary too much for a specific number to be quoted.


Compression will not work out as described because 2 stroke exhaust ports are open for much of the first half of the compression stroke.
----------------------------------------------------------------------------------------------------------------------
Sam as you correctly state , and I am aware of, the exhaust timing will effect the compression psi and the formula was stated for the full cylindere volume. As you know the compression ratio will be lower if based on cylindere volume from TDC to the top of the exhaust port thus a lower compression psi will be the result. The formula derives from the general gas law and is used in many forms by others.

No formulas that I am aware of are completly accurate because they rely on some degree of emperical measurement. The intent of the formula was to state a means to calculate a relative expected measure of compression psi to be compared to a gage. Certainly different compression ratios can be used in the formula.

Based on my experience with stock 2 cycle outboards the formula yields fairly close results after several hand pulls compared to compression gage readings. Also I have noted that the first pull compression gage reading comes fairly close to the calculated compression pressure based on the exhaust port compression ratio. Which is why I always note the gage readings at each pull.

The 14.7 is local atmospheric pressure and the minus one ( -1) in the formula adjusts the calculated pressure from absolute pressure to gage pressure to match what a pressure gage is scaled to read.