Engine compression test

[34fordy]

If air pressure at sea level is aprox. 14.7 lbs. per sq. in and your old
flathead had 6.3 to 1 compression ratio, does this mean that a compression test should yield about 92 to 93 lbs. if all is well in that cylinder? Or have I been breathing too many exhaust fumes?--I realize that valve timing and other factors have an effect also but just trying to see if there is a (norm) that one could use to compare--If a fellow writes in that he has 140 lb.s in each cylinder after hi-comp pistons someone else with a stock engine wonders what is wrong with his 90 lb. readings-- Don

[Tinker]

If there is a difference of ten pound across the cyclinders then worry.... Maybe... Well if they are all at 60 lbs then worry too.""

[Binx]

Most people like me use a cheapo pressure tester. The needle bounces up and is captured by the check valve. It really isn't a true reading unless it is calibrated by a precision measurement lab. I have one that reads really high and showed my son that my '93 Toyota Tercel has 210 psi which really drove him nuts since he built a 300+hp Nissan 4-banger that isn't even close to that. Two things that are important are no more than 15 percent variation between the cylinders and REAL pressure of 90 psi minimum. It will run with less, but really inefficient.

Lonnie

[Diavolo]

I read somewhere that lower than 70 psi is almost impossible to start. When I fisrt got my 52, I got 75 psi and it was not a fun experience to get it started the first time.

[Ross F-1]

At mile-high altitude, atmospheric pressure is 2.5 psi or so less (12.2). On a good day, my freshly rebuilt flatty puts out 95 across the board. I had a similar reaction when I went on the net and saw people with 120 on a well-used engine.

[Bassman/NZ]

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With a regular cheap compression tester on my 59A with new rings and valves lapped, I'm getting 135lbs on 1-4 and 125 on 5-8. This with milled heads and 50thou clearance over the pistons. I guess the 10lbs variation is due to the 1-4 bank being slightly different configuration? By my calculations (compression divided by atmosphere) would come to approx. 9:1 on 1-4 and 8.5:1 on 5-8. Hmmmm... kinda dubious about that.