Compression Test Q's

[chirs]

I recently rebuilt my 302, bored 30 over, and used a factory summit piston and ring package. I ran a compression test on one cylinder, and it came up 130. This was slightly alarming but.........I have a few questions. I am at sea level and i believe that the standard pressure is 14.7. The stock replacement kit said the pistons would have 9:1 compression, but i am running gt-40 heads which, if im not mistaken, have a larger combustion chamber and thus would lower compression. The rings have not yet seated, less than 50 miles. I figure my compression ratio is around 8.5-8.8:1. So , i would take 14.7 X 8.8 = 129.36. Correct me if im wrong, but does this look right? Also, will my compression go up further when the rings are seated? I will test all the others tomorrow, was dark here, but should i be alarmed or as long as they are all the same then i should be ok right? Thanks,

Chris

 

[abusedstang]

my 94 gt has 143k on the ticker and all 8 cylinders showed 125 psi plus or minus 3 psi. Wasn't sure if the pressure was good. I was just relieved that all 8 were even!!!

 

[HISSIN50]

That test tells not too much since there are too many variables and not enough data. Comp testing is only decent for relative (cylinder to cylinder) comparison IMHO. There are too many methods and variances in testing to compare absolute numbers. And extreme cams can lower compression.

If the motor is not broken in, that is another variable. I would not sweat it myself. The compression can go up a bit once rings seat.

Good luck Chris.

 

[chirs]

Thanks, upon further reseach, i messed up a few things. One the throttle body is supposed to be held open during the test and the engine is supposed to be a normal operating temp. Neither of which were done. According to the same website, without performing both of those steps, one would experience significantly lower numbers.

 

[HISSIN50]

Thanks, upon further reseach, i messed up a few things. One the throttle body is supposed to be held open during the test and the engine is supposed to be a normal operating temp. Neither of which were done. According to the same website, without performing both of those steps, one would experience significantly lower numbers.

Yep. Those are a couple of the variances I was talking about. Having the motor warm does help a good bit (not as much as with forged pistons, but still makes a difference). Also numbers can go up a hair from having all plugs pulled (starter can turn the motor over more efficiently for a given amount of cranking interval).

Relative numbers will be much more important.

Good luck.

 

[chirs]

Thanks again, im not too worried now.