Compression test 3.8..-9

Just did a compression test…engine at temp, throttle open…breather off…screw in tester…this is a 3.8/9 with about 1500 miles…runs great…
155-160 psi on all cyl…
this is low…should be 180…we did use a slightly thicker comtec head gasket Bill Terry…could that be the issue??


Without cc’ing the head, and measuring the deck height, it is impossible to say. Shaving the head and deck, changing pistons, changing head gasket can all EASILY change the actual CR by a full point or more.

Did you do both wet and dry tests?

Ray L.

Hi just dry…thought wet was for leak down

all within 5 lb?? don’t worry , be happy.

1 Like

That’s pretty much spot on. 180 is too high.

1 Like

180 is correct for 9-1 pistons.

You’ll struggle to get it to run properly with compression as high as that.

should be ok where it is…but do the wet test just for info as to what it will read…is it running ok, any blue smoke, or blow by? If not all is just fine. If it was higher comp, still would be ok…tuning just becomes touchier to be more precise…but precise is good no matter what.


Purrs like a kitten…no smoke/missing/pinging…power is adequate…not going to screw with it…thanks

1 Like

Very wise.