400 compression

[jprather]

so, when i run a compression check on a 72 400 2bbl, what should the avg psi be per cyl???

 

[Ron Ward]

At what altitude?

 

[jprather]

oh wow, good question, I'm in central florida, so it is low, oh and i am just running a "dry" test

 

[dieseldazzle]

I'd be more concerned that all the cylinders are within 10% of each other and not the number as long as it's over 100psi

 

[jprather]

thats what i needed to know .. all are on avg 113 +/- 5lbs

 

[Cranky]

Squirt some oil into the cylinders and see if that number comes up some. That's pretty low. If it does come up, you might have a ring sealing problem.

 

[Hemi 354]

++ I agree with Cranky for some oil in the cylinders. I would be looking for something closer to 140-150 lb.

 

[lewtot184]

that's an 8:1 compression engine at best. doubt it would ever do over 130psi with the rings in good condition.

 

[dieseldazzle]

Your findings are fine as long as they are within 10% of each other.

 

[jprather]

They are within 10%, car runs fine, no smoke. doesn't seam to burn any oil. Just trying to check things out 1 step at a time. just seams to be low on power, but with a low compression 400 and 2 bbl i guess i shouldn't expect to much !

 

[PP1RT]

A 400 with a 2bbl isnt gonna throw you into the back seat. I'm guessing you have 2.73 gears on top of it. Gear change along with headers and a 4bbl would make it funner to play with until a rebuild is required.

 

[rumblefish360]

I have a '78-400 and it is a fine runner in excellent health. I rate the engines performance as D.O.A. Due to the year and its load of emissions equipment. Very low compression @ 7.8-1 measured. Broom stick cam is a killer.

I did like the new post on the Trick Flow heads and there (IIRC) 73cc chamber. That ought to help a bit.

 

[Cranky]

Just because it's not burning oil doesn't mean the rings are doing their job of sealing compression. Another 'test' is to look at how much vapor or smoke is coming out of the valve cover oil fill hole with the engine running. How much do you see (if any) when the engine is cold vs hot? Does it have any at all? Doing the 'oil test' is just another part of checking the engine's health. Having a leak down tester is even better but squirting the oil into the cylinders to see if it makes a difference is just another way to get an idea of it's health. I've seen plenty of engines that were 10% but were still not in that great of health. I'm assuming this engine is stock and no one installed a cam? Also, you do have all the spark plugs out and the throttle set wide open during your test....?

 

[morrow_jacob]

At what altitude?

The altitude doesnt come into play during a compression test, it is actually psig, not psia. When the cylinder is blocked off from the rest of the world that pressure doesnt apply itself to the gauge.

 

[SwedCharger67]

I think the altitude comes into play.
At lower altitude the cylinder sucks in slightly more air, and the cylinder pressure will be slightly higher.

 

[morrow_jacob]

I think the altitude comes into play.
At lower altitude the cylinder sucks in slightly more air, and the cylinder pressure will be slightly higher.

Apparently I was thinking about the test when you use your air compressor (leak down test), not just a simple gauge stuck into the spark plug hole. My apologies.

 

[rumblefish360]

I think the altitude comes into play.
At lower altitude the cylinder sucks in slightly more air, and the cylinder pressure will be slightly higher.

My old girl friend was saying something about this, but all I really heard was gobble gobble ugunms mummm yum yum.

Ehh, who knows! I never really listened to her anyways.