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So now you made me look and see the relationship between altitude and barometric pressure. What I found was way more math that I think I need.
http://en.wikipedia.org/wiki/Barometric_formula
I will stick with the formula, it is simple and easy. I am thinking altitude effects the compression gage more then barometric pressure does but I think I will pass on doing the math to figure that out.
 

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Discussion Starter · #5 ·
The reason I'm asking is I performed a compression test about 4 runs ago on my engine. Average cranking compression before was probably 125 psi. Cranking compression after the 4 runs fell about 5 psi per cylinder, but it was raining outside, so the available atmospheric pressure to push air into the engine is less than before. How much? I don't know. I would just do the compression check again today to see what the difference is, but it's raining again today! lol
 

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5 psi can be a low battery or the oils off the cylinders

I look for all cylinders to be within 5-10 lbs of each other if ones hurt its going to be off by quite a bit, then leak that one down
 

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Discussion Starter · #8 ·
If you use the link I posted how close are you to where it calculates? If your off a little I wouldn't worry, if you are off a mile do a leak down test.
I'm getting close to 110 psi on that calculator using 4.145" bore, 4" stroke, 6.2" rod, 9:1 compression, 600 feet altitude. I'm guessing on the intake valve closing. I'm using a cammotion solid roller intake lobe with 298 degrees duration at .020", installed at 116 intake centerline with 3 degrees advance, which I'm estimating puts my intake valve closing event at 82 degrees ABDC unless I'm doing my math wrong. Cranking compression on the engine currently is averaging 120 psi.
 
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