How much compression does a 4.0 need to run good?

winterbeater · Feb 22, 2009

I know that the FSM says that compression should be 120 to 150 PSI. The old standby used to be that you needed at least 100 PSI (for old beater motors), but I've also seen motors that wouldn't run at 100 PSI until they were re-ringed to raise compression back up. What is the lowest compression anyone has checked on a 4.0 that still runs good?

Sponge Bob · Feb 22, 2009

you want all cyl to be within 10% from high to low so you could possibly have 50psi and run good but no power. tell us what you are finding

ChevelleSSLS6 · Feb 22, 2009

some old engines had 6:1 compression (old flathead engines). That's low enough they could almost run on kerosene/diesel (via spark ignition. diesels use the hot comressed air from 20:1 or so compression to instantly ignite the air as it injects fuel at compression TDC)

veedubshafer · Feb 22, 2009

my buddies rig has only 70-75 psi per cylinder - and we even checked the gauge on another motor for acuracy and it runs like a top - its an 88 4.0 with over 500k on the unrebuilt motor - I mean its by no means a power house but for the past 150-200k its been running 35's and it still gets up and goes - oh, and it gets about 15MPG

winterbeater · Feb 23, 2009

Wow! That's good information. I figured they'd be dead under 100PSI for sure. I guess that's why they run so long.

winterbeater · Feb 26, 2009

Sponge Bob said:
you want all cyl to be within 10% from high to low so you could possibly have 50psi and run good but no power. tell us what you are finding

I haven't needed to check mine, but one guy reported his poor running 4.0 had 120 PSI and most other reports have been higher. I wondered if that could be his issue, but I doubt it now.

veedubshafer · Feb 26, 2009

winterbeater said:
I haven't needed to check mine, but one guy reported his poor running 4.0 had 120 PSI and most other reports have been higher. I wondered if that could be his issue, but I doubt it now.

must be another issue because I have personally seen it at 70-75 and still running good - probably because they are all that low so its equal

winterbeater · Feb 26, 2009

Thanks for the feedback. I agree with what you're saying. It kind of shocked me, but I believe.

How much compression does a 4.0 need to run good?

winterbeater

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