Compression test SBC 350

I did a compression test (my first time) on an SBC 350 in a 1977 Corvette.

Posted are the results. To each corresponding cylinder number, the first value being dry test and the second value after squirting a bit of oil inside the sparkplug hole. Then a description of what the sparkplug looked like.

  1. 150 150 clean sparkplug
  2. 150 145 clean sparkplug
  3. 140 150 sludge on sparkplug
  4. 150 150 mostly clean sparkplug
  5. 150 155 clean sparkplug
  6. 145 150 clean sparkplug
  7. 155 155 clean sparkplug
  8. 140 140 clean sparkplug

I noticed I was burning oil and I figured I should do the compression test. I wanted get a good idea on the condition of the engine.

What do these results tell you? Not having done a compression test before I’m not too familiar about what these results maybe telling me.

And I do want to clarify that they are correct cylinder numbers. So cylinder numbers 3 and 4 would be on opposite sides.

You are just beyond 10% variation low to high. That’s pretty good.

I’d say from the test, the rings in #3 are a little worn and the the valve seal in #8 is a bit weak. There is some, but not a lot of, variation dry to wet in #3 which suggests the valve seals -specifically intake - on that cylinder need replacing. #4 suggests the same IF that “mostly clean” is a bit of burned black oil residue.

If the oil-burning presents itself as a poof of blue-white smoke out the exhaust when you drive away after sitting at a light, that is confirmation of valve seals. Those seals can be changed on the car. It’s a bit fiddly and tedious but quite possible with the right tools.

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Thank you for the information. I’m going to do some reading on valve seal repentant.

The numbers are headed downhill. Ideally, you should be looking at 180ish or so.

I will also note that it’s possible to have great compression (say 190 or so) and still have a piston ring problem. The compression rings are good but the oil control ring (or wiper ring) may be stuck in the groove in the piston.
That is a common issue and can lead to oil consumption as the wiper ring is not cleaning the cylinder wall of oil on the piston downstroke.


The base engine in 1977 was the L48 with a compression ratio of 8.5:1. A compression reading of 150 PSI is reasonable and normal.

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