hey i ran a compression test and the four cylnders on the left side of the motor all had differant compression ratios and were lower then they should have been..

119
165
170
159


the right side of the motor lookd fine

174
171
176
176

whats wrong?

170 is about right for 9:8.1 compression?

 

ok.. i'll add a little more info

this is on a 383 everything is new probably has about 380 miles on it.. new heads etc.. bored .30 over.

sparkplugs didn't look to bad, it was running a little rich so it was hard to tell

 

You can't automatically assume that the readings you get on a compression gauge will have anything to do with the static compression. The cam will have a HUGE effect on the readings.

You should be looking for all cyls to be within 5% or 10psi of each other. If they are, then either they're all good, or they're all failed exactly the same severity in exactly the same way..... a pretty low probability.

They all look fine to me, except for #1. It's way out of whack from the rest of them. That one has an issue.

 

yeah one is at 159 thats also low.. is it possible the head on that side of the motor is cracked? or what els could it be valve seal?

 

I wouldn't worry about the 159 one.

Your problem is the 119 one.

Pick the low-hanging fruit first.

You could shoot some thick oil into it, test it again; if the reading goes up when wet, that's a pretty good sign it's a ring problem. If it doesn't, it's mosr likely a valve malfunction. Could be apoorly lapped seat, warped valve, ropcker set too tight, etc.

Make sure the throttle is blocked wide open for the test.

 

See, I'm a tad ****/paranoid, I'd say run 'em two more times. First the same way, second using wd-40 to check variances. Then I'd pull out the heavy oil if I still were having an odd number. I like rechecking my work. :shrug:

 

RB is right. Look after that #1 hole first. Another method would be to do a cyl. leakage test. Buy or borrow a tester, put #1 at TDC and pouer the air to that honey. If you have greater then 10% leakage remove the rocker arms and tap both valves with a plastic hammer and retest. If the reading goes down you may of had a misadjusted valve.

Auggie

 

adding oil this time compression increased ALOT on that one cylnder


BEFORE/AFTER

119-171
165-171
170-172
159-174

i take it this means the rings are shot?

 

Did you re-test before adding the oil, just to make sure you hadn't had a problem with your procedure the first time?

Did you add oil to all of them, to make them all come up that much?

What makes you think it needs rings in the first place? Why are you in such a hurry to jump to that conclusion?

 

yes i retested them and yes i added oil to all four.. what els is going to cause the compression to raise only when oil is added? i dont see how i jumped to a conclusion

why do i think it needs rings look at the compression..

 

Maybe you just did it wrong on that one the first time? Like, maybe you didn't hold the throttle all the way open?

 

That's not what i meant....

Why were you testing compression in the first place?