Finally got in the garage and compression tested my cars 1972 400cu-in engine, factory 8.2 to 1 compression as far as i know, 128,000 miles.
I thought i may have a bad timing chain, have not pulled timing cover yet but am 1/2way there, block drained, etc - previous post here --https://board.moparts.org/ubbthreads/showflat.php?Cat=0&Board=QuestionAnswer&Number=7783679&Searchpage=1&Main=7783679&Words=+ignoble&topic=&Search=true#Post7783679

So a test showed on the gage 148-135-138-132-131-135-140-125 psi. Is the rule of thumb 10% variation or more, is too much difference? With only cylinder #1 still at a high psi#, i wonder if its time for a machine shop evaluation or other plans in addition to a probable loose chain ? Thoughts ? Thanks.

As I kid, I grew up around Mopars, I remember old-time mechanics (guys who set points w/their fingernails) saying that 25lb spread was OK. It'd probably idle better with closer pressures, but I think that's not bad personally

A few questions? Was the engine warm and was it still warm on the last cyl test? Did you have the carb wide open to get full air flow for the test? Did you spin each cyl the same revolutions for the test? For a 400 with those miles I'd be happy though.

The engine was cold throughout, intake is off so full air available, i tried to do same amount of revs thru each cylinder, enough to bump the gage to its high mark for each, plus a couple. -- the 125 psi cylinder [#8] went to 127 with twice the # of revs as the others.

I recall an article awhile back that stated a 10% spread is within acceptable limits for a street engine. 10% of 148 is about 15 PSI. Your difference is 23. For high performance street/strip engines, 5% was the goal. For reference, the last test I did to my 2500 mile 493 was 185-192. Thats 3.6%.
Mine idles a bit rough though. Should idle a bit rougher with the bigger cam!

If the engine does not smoke, shake or have bad street manners, run it. Enjoy it.

Just drive it. That engine is fine timing chain should be changed but don't use the crap ones from Summitt the store brand is junk!

A point needs to be made here. A timing chain won't make the compression readings vary like that.

You're either looking at rings or a valve that leaks on the low cylinder. If it was a chevy smallblock from the '60s or '70s it might be a bad camshaft lobe.

Put a spark plug in just that cylinder and spin the motor. Listen for the leak. An intake leak will be easy to spot, an exhaust leak a little more difficult.
If there isn't an obvious leaking valve, squirt a little SAE 30 motor oil in the affected hole and redo compression test on that cylinder. If compression comes up markedly, it's rings.

If you find a bad exhaust valve the head should come off. Burnt exhaust valves can torch a hole in the valve seat area.
A bad intake valve you can live with better.
A bad ring seal can also be lived with.
There are downsides to any of these "living with it" scenarios, though.

Now another point: Your 8.2:1 400 is really more like 7.2:1 if you were to measure everything and calculate. I'd suggest starting to plan an engine rebuild. Tech advice from this board is free and mostly very good.

R.

Put the sparkplug back into # 8 cylinder and cranked it over, did not hear any air leakage.
Put 5W30 oil into Cyl # 8 and cranked for about as long as it took too get to 125 psi last time [dry] , this time with oil its closer to 133 psi.
With a lot of cranking and more oil it pushed almost to 145 psi
Thanks everyone !

could some of those cylinders be hi for that engine?
lots of carbon?

That is a real possibility and one I hadn't thought of. Good catch!

R.

Get the motor nice and warm, have someone run the engine about 3000 rpm while you dribble water into the carb. Do this where the junk that comes out of the engine won't stain the walls! You'll be surprised at what you get. Or for even more fun, throw in a couple bottles of Hydrogen Peroxide.