Engine compression vs. power

[bysior]

What is the relation between measured pressure at compresion test and actual power of an engine?
Let’s say that a new engine had a certain compression as measured during the test. And now, the same engine, after years of running is showing compression 25% lower. Does it mean that its power dropped by 25% as well? Is there any known relation how these two parameters change with engine wear?

 

[jetmech1]

Short answer is no, it is not a direct linear progression. There are several speed books that deal with it. There are published charts that estimate what the losses are. Several other factors effect it too. Seems to effect mpg more than HP, if I remember correctly. Can anyone find one of those charts??

 

[Shannow]

In theory, the compression ratio gives the theoretical thermal efficiency of the engine...

approximate efficiency is 1-1/(CR^0.3)

That's theoretical ideal cycle, and assumes complete cylinder filling, and doesn't account for funny cam timing.

 

[Scott_Tucker]

In general, as cranking compression goes down, so does power output. However, there are too many factors in play to make that a hard and fast rule. I know of no published charts that show this relationship.

 

[mechtech2]

Cranking compression stays about the same during the life of an engine.
It may go up from carbon deposits.

With a 25% [huge] loss, you have severe problems.

 

[Propflux01]

A leakdown test would be more linear, as it is calculated using a percentage, so if your having 20% leakage in your cylinders, in theory, your losing 20% of your engines output.

 

[Scott_Tucker]

Originally Posted By: Tim H.
A leakdown test would be more linear, as it is calculated using a percentage, so if your having 20% leakage in your cylinders, in theory, your losing 20% of your engines output.


I agree with this.

 

[Shannow]

I agree in principal that leak down test is a good thing, but as to the percentages meaning "20% leakage", it's not how they work.

IIRC, the leakdown test is comparing the engine leakdown to a 0.040" orifice, with 100psi supply pressure.

The low pressure side between the engine and the orifice reading of 80 psi gives you the "20% loss", but that's not a measure of how much "stuff" gets past the rings as a percentage of what's in the cylinder...it's an arbitrary, but useful measure.

 

[mechtech2]

Also, a leakdown test is at low pressures, not the high firing cyl pressures that force the rings to seal tighter. [gas goes behind and pushes them out - that's how they work].