Compression test anomaly?

[QuadManiac]

I just ran a compression test on my son's 99 Blazer V6, alas #4 is running 1/2 pressure relative to the rest. Not rings, oil was no help. Time to perform a leakdown test and then likely pull the head.

But that isn't my question:

Bad cylinder was 90lbs... all others were around 175 - 180. Very nice... but then I started thinking... Manual says the compression ratio is 9.8:1. I know atmospheric pressure is about 14.5 psi. HOW do i get 180 psi by compressing 14.5 by a factor of 9.8??? Seems to me it should be a lot closer to 140 or so. Think it's just a poorly calibrated compression gauge? I actually tried 2, and they agreed to within 2 psi.

Any comments? Am I overlooking something? Every time the intake valves open and the piston goes through an intake stroke, the pressure in the cylinder is gonna become atmospheric's 14.5 lbs or LESS (due to a bit of flow loss through all the intake components). You can't get something from nothing, so what gives???

 

[SS MAYFLOAT]

Re: Compression test anomaly?

Did you have the throttle body butterfly opened up allowing proper air flow into the cylinder when doing your test? With it closed, it can restrict the incoming air not giving a good result. Just a thought

 

[QuadManiac]

Re: Compression test anomaly?

True, but that would reduce the initial pressure in the cylinder, making the compressed pressure lower. I had the butterfly open.

 

[BoatBuoy]

Re: Compression test anomaly?

Quad, if it was a properly calibrated pressure guage, it wouldn't read zero to begin with. It would read whatever the atmospheric pressure is where you are. Secondly, you know the displacement but you don't know the terminal volume of the combustion chamber at the top of the stroke. Both these will affect your STP calculations.

Look up compression ratio at Wikipedia for the formula.

 

[SS MAYFLOAT]

Re: Compression test anomaly?

I'm not a figure head, but those figures are for per inch, now what is the surface are of the piston? Will the figures work out?

 

[QuadManiac]

Re: Compression test anomaly?

Quad, if it was a properly calibrated pressure guage, it wouldn't read zero to begin with. It would read whatever the atmospheric pressure is where you are. Secondly, you know the displacement but you don't know the terminal volume of the combustion chamber at the top of the stroke. Both these will affect your STP calculations.

Look up compression ratio at Wikipedia for the formula.

By virtue of the compression ratio, I do know that at TDC there is ~1/10th the volume in the cylinder that there was at BDC. (Manual states compression ratio is 9.8:1).. and i understand the difference between gauge pressure and absolute pressure... but I think that assuming guage (differential) pressure would make the reading even less...

Not worth a huge effort at figuring out why, just a curiosity that I thought someone would have an immediate, logical answer to.

The Wikipedia description follows my premise exactly, EXCEPT, they state that the pressure can be more than atmospheric pressure X compression ratio, "depending on cam timing". I can see this for an engine at high RPM, where the valve opens early and closes late, taking advantage of momentum in the air/fuel mixture in the intake plenum... but no way at cranking speed.... Still not an answer.

Oh well. Last possible thought, if my piston and head is heavily carboned, (it does have 95,000 miles) then the compression ratio will have increased.

Thanks for the answers.