I'm not following. The numbers I gave are for a .040" head gasket which is what I have in the engine.

 

Yeah, I got it wrong.. The numbers you gave are for a 0.040 headgasket. So you have 0.050 squish.

 

So I finally was able to get the bike out and start it. It runs well. I let it idle for a while and road it around the block a few times. I didn't want to go farther because the main streets are full of winter salt. Anyway, I pulled it back into my shop, let it cool down enough to not burn myself and did a compression test with it hot and freshly ran. Guess what. Both cylinders pumped up to right at 200psi. Thats very close to the calculated compression from Big Boys and Hammer Performance. Moral of the story, the engine needs to be hot to get an accurate cranking compression reading.

 

So you got the original high numbers without running the motor before hand?

 

Divide gauge pressure reading by your location atmospheric pressure, to get real compression for your altitude, that includes what cam is helping as well.
https://www.engineeringtoolbox.com/a...ure-d_462.html

As for actual thing that are going to change compression, its the amount of piston squish to top of cylinder, and the amount of head volume, over what the cam is helping as well.
If squish is now at .030" to top of head, but running .030 head gasket, then squish is really at .060" instead.
Base gasket only comes in to play for thinness, if it going to decrease the amount of piston squish.
As for back to piston squish, you can run less, but keep in mind that wrist pin wear to connecting rod bushing slop and even lower rod to crank shaft slop, can get you into trouble with less than .030 squish as the engine wears in, since piston may still have enough slop on up stroke off compression stroke, to contract head gasket if any over hang, or even bottom of heads with D type heads, or domed pistons.

Simply put, no replacement for displacement, and with such, don't have the have the compression so high that the motor will no longer run on pump fuel, or piston to heads so close, that common wear in the connecting rod top bushing/lower bearings, can cause the piston to start making contact to the heads in the end.

 

Simply multiplying by the pressure ratio to STD pressure is a good first order approximation but it's really exponential change on higher CRs.

When you say " .030" to top of head", I think you mean to top of cylinder.

"Base gasket only comes in to play for thinness"

Doesn't also affect compression ratio?

I wouldn't worry about wear, I'd worry about flex.

 

Yes. I think the reason is as follows.
1. The bike was cold so the oil is thicker, creating a better seal on the rings.
2. I knew I was going to turn the motor over to get the oil flowing and do a compression test. Before I bolted on the heads, I gave the cylinders a light coating of oil. So the cylinders had more cold oil in them than they do hot oil after running the engine.

 

I's suspect that what increased the CCP was the additional oil in the cylinders.. You want a perfect seal on the rings and should get only about 2% on a fresh set after rings are seated. Next time run the motor first..