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compression results

2K views 9 replies 4 participants last post by  RayVoy 
#1 ·
This is probably answered somewhere here but I'll be darned to find it. Being a wannabe mechanic, I would like some opinions here on compression numbers. 1978 350 sbc...I have been told that "normal" on this engine is 150 @ cylinder. I was also told that 130 was "normal". my compression numbers averaged around 135 on all but 2 cylinders. Of these 2, one was 150 and one was 122. So, any thoughts on such a large range?
 
#2 · (Edited)
Depending upon the heads and cam config, I would guess that when new your engine would read as high as 170 psi.
Normal wear could reduce that to the 150 range; however your may have been 150 psi when new, again depends upon heads and cam. In this case, 135 would be normal wear. I just can't see a 43 year old engine still reading new engine compression in one cylinder.
The 122 however would be an expected number from an old, probably high mileage, engine. In any case, the range should be within 10 percent of all 8 readings.

How did you run you tests?
Best way is to bring it up to operating temps,
Pull all sparkplugs.
Disconnect the coil wire.
Remove the fuel pump relay.
Open the throttle max.
Crank the engine,
The gauge should climb in bumps, everytime the piston comes up on compression.
4 bumps will usually do it, but let it climb until it stops.
Jot down the readings.
Cylinder #1 is drivers side front, #2 is passenger side front.

If you still have a low reading, you need to do a wet test, this will point to worn rings, or valves.

Put a little oil in the low reading cylinder(s) and retest.
If the compression increases, the oil is sealing a bit of wear on the compression rings. If there is no change, the valves need attention. Worse case, there is a crack in the head.

EDIT:

@ryeder , @bsname just posted a good point that made me review the steps I provided.

And he is correct, the fuel will cause a problem, the fuel pump relay needs to be removed.
I will go back to my list and add this necessary step
 
#5 ·
The 122 however would be an expected number from an old, probably high mileage, engine. In any case, the range should be within 10 percent of all 8 readings.
After the compression test, you can do a "leak down" test on your lower compression cylinders to see what's bad - intake/exhaust valves, rings, intake gasket, etc.
 
#3 ·
Thanks, Ray. I'll be re-doing the test this weekend. which brings up another question. the engine exhaust is blowing a bit light gray in color which could indicate an antifreeze leak. A cracked head could cause that, but there's no indication of this milkshake in the oil. I'm beginning to wonder if throwing in a crate engine might be the simplest solution....based on the next test. Anyway, enjoy your weekend.
 
#4 ·
It could be just moisture in the exhaust, the cat action produces water.
I've rebuilt engines and it can be a lot of work, and you need to wait for things to get done, like having heads rebuilt.
And when your all done, there is no warranty.
 
#9 ·
Best way to turn engine over for compression test is with a remote starter switch - you do not want to [ suck fuel out carb days ] or injectors to spray with using key .

That alone can vary compresion .
Like mentioned the design of the engine can vary , but mostly yr. of manufacture - emissions started [ generally speaking in 73 ] in the 70s they changed ideas on compression for different attempts to play with emissions , low [ new ] could be 145 & as high as 175 .
Carbon buildup on valves or seats can do some of this , hoping for the easiest , vers. rings .
If your coolant level does not change / require coolant , you can still have a combustion leak - that does not involve the cooling system [ yet ] .
Leakdown is a good test , but easiest 1st , hardest last .
 
#10 ·
@bsname good point, the fuel can make a difference, I have corrected my post to add the removal of the fuel pump relay.

I agree, the remote switch to spin the started is a great tool, a big help if working alone. The 2nd advantage is you have "finger tip" control of the starter.
 
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