Reply To: compression 57 3 hp Johnson

[Don]

OK Bill, let me try to relay how I have attempted to make sense of this issue. Let’s just say an engine showed 100PSI on a compression gage after cranking it vigorously with an electric starter.. Now, let’s just say we added another head gasket to the block and put the head back on, then rechecked compression….Can we agree that the compression reading would go down when measured with two head gaskets installed?
Now let’s relate the above example to the little JW powerhead. The compression gage hose adds some volume to the JW’s cylinder/combustion chamber when the check valve is up by the gage, kind of like our example of adding another head gasket to the engine and rechecking compression..
So next let’s try to understand why this hose is such an issue for the JW powerhead, but not a big deal on larger powerheads….Let’s just imagine we are checking compression on a 350 V8 engine with this gage/hose set up. We probably wouldn’t see any difference in compression readings with a gage with no hose vs a gage with the long hose and valve up by the actual gage. Theoretically, there would be a difference in readings, but it would be so small we wouldn’t notice it on the gage. The volume lost in the hose is so minor when compared to the large combustion chamber explaining why a change in compression readings would be just about undetectable.
On the other hand, the volume in that hose is quite a bit when compared to the size of the JW combustion chamber, which explains why we see the lower readings…
Perhaps my explanation/interpretation is incorrect, but it makes sense to me….