81 malibu compression test

malibu
chevrolet

#1

i did a dry compression test and they are 85 threw 90 psi is this to low for a 81 chevy 350 bored over 60 with a compcam in it.


#2

this is the engine


#3

You’re in trouble. Those numbers should be about double that.

Go back and perform a wet compression test; meaning a small squirt of oil into each cylinder as it’s retested. If the readings take a large jump up then the piston rings are gone. If the numbers remain about the same then the valves/valve seats are gone.


#4

Long duration camshafts will destroy compression at cranking speed. One valve or the other is open virtually all the time!

You did have the throttle held wide open during the test, correct?


#5

I agree that a long duration cam with a lot of overlap could drag the readings down but it’s hard for me to see it being drug that far into the gutter.
One of my cars here had a Ford Racing cam (F303) and even with lowered compression it was still huffing out about 140-150 on all cylinders. I don’t remember the specs on that one but it was about 228 on the duration and lift was over .500.

Readings spiking way up during a wet test will show one way or the other. A vacuum gauge reading might not hurt either.


#6

Did you keep cranking until the needle hit a peak? I could understand such low readings if the engine were not cranked until the reading peaked. This normally takes four or more jumps of the needle on the gauge to get an accurate reading, possibly more with a really big cam. More won’t hurt. If you did the test incorrectly, you should check it again. Does the engine run reasonably strong? If the compression readings are that low across the board, the engine would probably barely run and would have enough power to move the car, and that’s about it.


#7

it seams all but on is at 80-90psi. the number one cyl is at 60 psi. i havent ran the car for 2 or 3 months, but thats what i have so farr, i cranked the engine 5 times a piece for these new readings. (dry test)


#8

Since you haven’t run it in so long, get it running and well warmed up before you do a repeat compression test. Then if the compression is still below 120 psi in all cylinders do a wet compression test. If you have the ducets buy a leak down gauge and test each cylinder for leakage numbers; post them back; and let us know where the air is going.

Hope this helps.


#9

i just took off the edelbrock carb and there was gas on my carburater base gasket, so that means i had a vacuum leak huh. (i havent started the car since yesturday).


#10

With the carburetor off now is the time to run a wet compression test. Since it’s low across the board just pick one on each side and run the test as a controlled experiment of sorts.

You really need to weed this out right now before spending any more money and time on this car.
If the 2 cylinders you pick take a substantial jump upwards on the readings then it’s time for an engine rebuild; unfortunately.