I'm getting ready to do a compression test on all 8 cylinders so I can get an idea of how the engine is doing. It has 140K miles on it, and I was wondering if anybody had any data from the Service Manuals made by GM on what the compression should be etc. Basically I need to know how to interpret the numbers,

Any help is appreicated,

Jason

 

no real number to shoot for, the main thig is for all the numbers to be with in 10% or less of each other. a good engine should be around 130 give or take a little. i've saw as high as 180 and lows of 100. if you find one number way lower you have a problem there.

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-=ICON MOTORSPORTS=-
1st & 3rd

 

Thanks for the help Ede, but how do I know if I'm getting some excessive blow-by or if the rings are going, etc? I'm getting ready to do a heads/cam/intake swap, and I'd like to use the stock bottom end and rotating assembly. I was hoping the compression test would tell me the health of the fit between the pistons, rings, and cylinder walls. If it's bad, I'd either have the block bored and all and get some news pistons and rings, if the current fit is ok then I'd just build up on this block.

Do you know of anyway to tell the "health" of the bottom end and rotating assembly?

Any help is appreciated,

Jason

 

A compression test will give you an idea of whether you've got good seals on both the rings and the valves.

Excessive blow-by is most often caused by worn rings allowing combustion pressure to escape past the rings and into the oil pan.

Here's a little trick to give you a handle on your engine.

Remove all the spark plugs and disable the injectors so they won't shoot gas and wash down the cylinder walls. Disconnect the BAT wire at the distributor. Screw in the compression gauge and crank the engine about five revolutions.

Write down the highest reading and also note what pressure was hit on the very first crank of the engine.

Now do the other 7 and then compare. There should be no more that 10% difference between the highest cylinder and the lowest.

If one cylinder is really low, squirt a little oil in that cylinder through the spark plug hole, then pump it again. If the pressure reading goes up a lot, it's probably bad rings; if it stays about the same, then you probably need a valve job.

A better way is to use a dual gauge leakdown testor, but this is a tool normally only found it well equipped shops or with race cars. It also requires an air compressor.

Hope this helps.

Jake

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1986 Corvette Coupe, 415 CID, Edelbrock 6073s, ZZ9

 

after you take a reading squirt some oil in and if the number increases you have bad rings.

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-=ICON MOTORSPORTS=-
1st & 3rd

 

One more thing. When you crank the engine to test a cylinder, you need to tie the throttle to wide open position to admit air.

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Later,
Vader
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"No matter how hard you try you can't stop us now"
Adobe Acrobat Reader 4.0

 

Thanks, the oil test is what I was looking for. Squirting oil in will let me know if the rings are going and that was my main concern since the heads are going to be replaced anyway.

Thanks again all,

Jason