Stock compression for an 87 5.0 HO???
 

Well let me start off by saying HI to everyone. My friend picked up an 87 5.0 HO to put in a 66 Bronco that he’s doing a frame up restoration on. We thought we would do a compression test to see what shape the motor was in before we started. The Mustang that it came out of was rolled and was upside down for a while so one cylinder shot oil a crossed the garage. The compression test went from 140 psi to 190 psi. on one side and 180 psi to 195 psi on the other. I haven’t seen a stock motor have that kind of compression of I figured I would ask the 5.0 experts here on the board.

What should the stock motor have for compression?

I’m thinking the motor has a lot of carbon built up on the pistons that it causing the high compression.

Thanks for your input.

From what I know its not the actuall number that is the most important. Whats most important is that the value you get for each cylinder is about the same. I think you want to lowest number to be no more than 10% off from the highest. I recently peformed a compression test on mine. I did three tests for each cylinder and out of all the tests on each cylinder the highest I got wast 155 and the lowest was 145.

when i tested compression in my brothers mustang (88 gt) bored .040 over with just standard pistons (about 9:1) and the stock heads milled .030 , he had 160 psi +/- about 5 psi in all cylinders , i think that his compression ratio was about 9.5:1 to 9.75:1 . hope this might help . sounds like you got a lot of compression .

What should the stock motor have for compression?

Stock 87HO motors in mustangs have a 9:1 compression ratio