Are Compression ratio's and compression guage readings directly related?

I ask this because, when I had my 305 and put 350 heads on it, my compression went from ~145psi to about 120psi. the stock 305's came with 8.5:1? so I figured I lowered it to about 8:1 based on my guage readings. Then I put in my newly rebuilt 350. the engine shop said it has 9.4:1 compression. But when I tested them with my guage they all showed 135psi. So this confused me. I was expecting 150-160psi based on what my 305 showed.
A friend of mine just put in a used 350 into his truck, we tested the compression to find he has 150-170psi, a completely stock TBI 350, all carbon buildup removed..

so is there a difference between compression ratio and compression found with a guage?

My 350 in the camaro was bored .020, would this cause a lower compression since there is now more space in the cylinder?

 

A camshaft with alot of overlap will lose
some cylinder pressure because the exhaust
valve stays open longer allowing some of the
intake charge to escape.

 

ah yes, that makes sense.
Was I correct when I assumed that compression readings on the guage will raise when CR is raised?

 

Compression ratio is the ratio the cylinder is compressed. It has nothing to do with psi. When the piston is at TDC the area above the piston is equal to 1. When the piston is at BDC the area above the piston is divided by the area at TDC. If this number is 8 times larger than at TDC then the compression ratio is 8:1. Higher compression ratios require higher octane fuels to keep the fuel from self igniting while under pressure.

Cylinder compression depends on things like valve and piston ring sealing, cam profile etc. It is how much pressure that is built up in the cylinder at TDC. A typical engine should be around 150 psi regardless of engine size. Anything under 100 psi is a very worn out engine. Higher pressures just mean less gasses are escaping promoting better burning of the air fuel mixture. Lower pressures usually mean something is worn out.

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87 IROC-Z SuperPro ET Bracket Race Car
461 naturally aspirated Big Block (times are for the current engine)

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87 IROC bracket car, 91 454SS daily driver, 95 Homebuilt Harley

 

That was very helpful. Thanks