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So had bad are these compression check numbers?

1728 Views 14 Replies 8 Participants Last post by  Bummo
I did a compression check today on the front 4 cylinders (ran out of time and light) and ended with problems (see Vehicle Help post about Oil pump for BB) but thats for tomorrow. Anyway the cylinders ranged from about 125 to 135. All plugs out, engine warm, cranking speed could have been faster but OK I think. I added oil to one cylinder and the numbers went up by 5, but it my have just been the cranking time on that one. Sooooo, this is a 400, 1977, maybe (maybe) 86,000 miles. What do you think.
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pretty good, I'd say. Sounds like rings are sealing.
that's good no problems
Yeah, at least all of yours are above 40. hahaha
I think the bottom number from MOPAR is 100 psi and the individual cylinders shouldn't vary by more than 10% from each other. Check the other four when you get the chance, heck check all 8 at least twice so you get a solid number. So far I would say you are ok.


Frankly Bummo, it's an incomplete test. It really tells you nothing. For everyone that said it's good, no problems, what happens when he gets to #8 cylinder and gets a reading of 10psi? One dropped cylinder kinda makes the other numbers worthless, doesn't it.

Do all 8 and then make a conclusion. If they all remain in the 125 to 135 range, it's not bad for a driver.

It doesn't matter now anyway PG he has no oil psi anymore so cyl psi is the least of his problems.
You are dealing with probably one of the lowest compression smog motors Chrysler ever put out. It was typical for a smogger 400 to have 7.5:1 compression.
So on a low compression smog motor, what sort of numbers would a compression test on a new 400 be? And geeezzzz, the motor aint dead yet (i think). Glad some of you guys arent doctors. "he's dead, bag em"
Lmao oh i think you did it in doctor, the #'s would be about what you have now
So far your cmpression readings are o.k. Check the other four and post the results when you have time (and daylight) :)
He has oil psi problems, so until that is straightened out there won't be any other comp readings, we'll have to wait until there is good news in the VH board post
Got a cheap oil pressure (mech) guage at walmart and made a oil pump shaft to fit my drill by taking a $8. "paint mixer" from Home despot and grinding off the swirl / vane end. If I can pry my ass off this chair I will go and see if its crying time. Might wait till tomorrow (God 's day) and use it as an excuse to not go see my brotherinlaw and family. They are ok but i'd rather do it tomorrow. Hope this shaft is long enough (dont we all).
I'm no expert, but being in the same kind of boat with my tired out 440, i did a compression check on all 8 cylinders and had 2 that rated 100. well, bought a manual and it said 100 Minimum and max differential between any was 25 psi. sooooo, i'm gonna drop out the 440 and put in my 400 til i get the 440 rebuilt. supposedly the 400 was rebuilt less than 100 miles ago. i ran it in my charger awhile back, and geez,,,,did it lope. hmmm....a wee bit worried. but otherwise...55 lbs of oil press at idle running 5W30 crap oil. we shall see if it works next week when i can finish the project. woohooo...good luck dude, you are better off than I. :)
If the lope is from the cam you could always swap the cam and lifters from the 440 when they are both out of the vehicles. Probably better for the truck than the more aggressive (ie higher rpm cam).
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