Join Date: Aug 2010
Location: Ulster County, New York
Bronco Info: 1978 Cummins Bronco
With a stock/mild cam and 9:1 compression ratio, you'll get 140-150 psi. If I recall, I think a stock '75 302 with 8:1 CR I tested awhile back had 118psi. Like UT mentioned, consistency is more important than peak psi. It's an indicator of a healthy engine; that all cylinders, rings and pistons as well as valves/seats are wearing fairly evenly.
90 psi is on the low side, especially considering the other one is 20 psi higher.. But it may be leaky valves causing it, rather than rings.
Are the spark plugs oily when you took them out? If so, that's an indicator of ring issues.
Have you wet tested the compression yet? (squirt a couple pumps from an oil can into each cylinder before checking compression) And then compare the results to the dry tests. If there is a drastic difference, the rings are shot, or the bore is out of round or just worn too large for the rings to seal properly.
If the wet and dry tests are faily close in psi, you can check the valves and seats by bringing each cylinder to TDC of compression stroke, and then applying an air compressor hose to the cylinder, with a special fitting that threads into the spark plug hole. Once the cylinder is pressured up, you can listen for air leaking from the valves.
1978 Bronco Custom, 6" skyjacker & 3" body lift, 37x12.50R16.5s, posi front diff, 3.50 gears, Dynamic Racing C6 w/billet 1200 rpm converter, upgraded brakes, '91 Cummins 12V, Denny T fuel pin, injection pump maxed out & timing advanced, 3800 gov. spring, 60# valve springs, ARP head studs, 4" exhaust.