Smitty

Wondering how many lbs of compression my motors should have.

540 Ci, 9.0 to 1 comp roller cam .641 lift and approx 242 degrees int dur and 248 ex duration on 114 lobe separation.

I was thinking 160-170 psi but I am being told possibly less ???

articfriends

My 8.4-1 540 has 155-160 w/a cam 4-6 degrees bigger,Smitty

jeff1000man

What is more important on a comp check is that they are consistant. Are you trying to find a problem? Why the question?

clpbs

I agree with Jeff every gauge is differant what you realy whant is that they are consistant if you are trying to find a problem.

Smitty

I was just curious where it should be because I have someone interested in buying my motors. They are all at 140 cold compression, but the guy said he thought they should be at 160 or more. I guess I should run it and get it hot and then check again ?? Maybe it will be higher when hot ???

jeff1000man

Sounds like he is clutching at straws looking for a way to make you knock money off. If they are all the same, then there is nothing wrong inside. The fact that they are all so close to the same tells you that your engines are in great shape still.

SB

Sometimes compression will be tad lower with cold engine.

You'll have lower than normal readings if you do not have the throttle blades all the way open during testing.

Cranking rpm can have a good influence on readins too. Faster cranking = more cranking compression. A restricted compression rule engine will more than likely have a slower starter to help pass tech. So, yup, a fresh battery will cause more psi than a weak battery.

Most gauges read different from each other too.

The cams you have along with your static compression will tend to be around where you see it. You could always put in truck cams, have the cranking compression skyrocket to 180psi, but will that make him happy ??? No way !!!! LOL.

All my opinion and experiences.

jeff1000man

Agree

H2Xmark

your compression will be lower on a cold motor with forged pistons cause you run a little more wall clearance than cast pistons. do another comp test on a fully warm motor, use a hot battery for good cranking speed and have the throttle wide open, bet it will be higher

Griff

Cams with overlap will bleed off some compression. My compression #'s went down on some cylinders after a valve job and install of a bigger cam in my 525SC. It had 7.5 static and was around 130# with a Crane 731 cam. Before the valve job/cam, I had a couple cylinders at 120 and the rest at 140.