compression numbers

[samsnead]

Hello,

Just had a PPI done on a motor with 178,000 miles and the compression was 145-150 in all cylinders. I know that oil consumption would be the best indicator, but are these numbers good? my last car had 160-165 and it seemed quite strong.

Thanks

[samsnead]

anybody?

[Eagledriver]

The main thing you are looking for is all the cylinders are the same. These numbers indicate the valves and rings are still sealing well on all cylinders. The compression ratio has alot to due with the numbers you see. A 10 to 1 compression ratio should give you 147 PSI if you have no leakage and 100 percent volumetric efficiency at sea level on a standard day.

-Andy

[2.7RACER]

Eagledriver,

Your reply got me thinking. If a 10:1 compression engine really only developed 147psi at sea level on a standard day with 100% volumetric efficiency; how does it explain compression readings significantly higher from most healthy engines, even at 5,000 ft elevations as found here in Albuquerque.

The answer didn't immediately come to mind.

Then slowly a little light came on.

What about the "heat of compression"

As we all know as a gas is compressed the temperature of that gas increases.
After all this is how a Diesel functions, at higher compressions than a gas engine.
Diesels run 16:1 compression ratios and higher.
As a result of this compression the air temperatures run over 700 degrees F.
High enough to cause the ignition of the diesel fuel as it is injected into the cylinder.
The thing is Air temperatures can increase at a rate 1.4 times faster than the reduction in volume at compression.
This results in higher PSI figures than what would be realized by the simple 10 to 1 compression.
30% to 40% higher than simply multiplying 14.7 by whatever the compression ratio may be.
Designers of air compressors face this 30 to 40% loss of efficiency routinely.
For those of us measuring the pressure developed inside a cylinder with a compression gauge we are seeing the hot air pressure.
This the result of "heat of compression"

Thanks to this question, I finally studied the issue and now have a better understanding of why.

This is of course a rather complicated subject involving thermodynamics of which I know very little, maybe even less than very little. But this is my understanding.

I welcome any further clarification from those that do have the credentials.