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-   -   Low compression on left cylinder bank (https://forums.maxperformanceinc.com/forums/showthread.php?t=860348)

ponchorob 07-27-2022 02:58 PM
Low compression on left cylinder bank
 
Have stock 65" 389 engine... engine has extreme blow by through valve cover dipstick tube and also smoke under the intake manifold..measured compression passenger cylinder bank has solid 175 psi on all cylinder but driver side only 70 psi on all cylinders..any ideas what could be the the problem..no water in the oil..

25stevem 07-27-2022 04:12 PM
Yes, either 4 pistons with cracked compression rings and or 4 cracked pistons along with some compression rings

Scarebird 07-27-2022 04:19 PM
I had this happen with a Olds 425.

I installed a Mondello cam. Most pre-1967 Olds V8's had 45° lifter banks going to 39° in 1967. The 425 was a 1966 motor but they sent a later 39° cam.

Compression was significantly different bank to bank; 8:1 one side, 10:1 the other.

ponchorob 07-27-2022 04:34 PM
So thats interesting so you think in your case it was the cam ..despite i dont understand the reason....i also cant believe that all 4pistons or rings have the same damage since i ill have excatly the same compression on the whole bank..still wondering..

Against the Wind 07-27-2022 05:28 PM
Dumb question, but have you checked the torque on the head bolts on the driver's side?

25stevem 07-27-2022 06:14 PM
Blown head gaskets could account for low compression in two cylinders right next to each other without any fluids leaking as I have seen that many times , but even if you had a very strange happening of enough head bolts being loose to make for across the board low compression, I can’t see having the level of blow by that you have!

25stevem 07-27-2022 06:16 PM
If you have broken rings if you get a bore scope you will see scratched up cylinder walls.

Scarebird 07-27-2022 06:58 PM
Quote:
Originally Posted by ponchorob (Post 6360068)
So thats interesting so you think in your case it was the cam ..despite i dont understand the reason....i also cant believe that all 4pistons or rings have the same damage since i ill have excatly the same compression on the whole bank..still wondering..
I know it was the cam. When Mondello sent out another cam (and grudgingly gaskets) it had compression within 10 psi across the board.

If the sealing of the cylinders is nominal ie rings, valves - then the only thing affecting the compression is ratio and timing.

Thinking about this it just came to me that I would not be surprised to find the wrong pistons, installed wrong or different compression heads (or a combination of the 3!).

I wonder what a leakdown test will find?

ponchonlefty 07-27-2022 08:07 PM
Quote:
Originally Posted by ponchorob (Post 6360045)
Have stock 65" 389 engine... engine has extreme blow by through valve cover dipstick tube and also smoke under the intake manifold..measured compression passenger cylinder bank has solid 175 psi on all cylinder but driver side only 70 psi on all cylinders..any ideas what could be the the problem..no water in the oil..
take the head off. then you can check whats going on. its odd for all 4 cylinders to have the same problems. could be rings could be gasket could be the seats in the heads. hope this was helpful.

ponchorob 07-28-2022 02:52 AM
Thanks for all the input...will look at the engine closely next week..

nUcLeArEnVoY 07-28-2022 05:46 AM
Burnt valve, maybe?


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