Determining compression ratio?

Porsche-O-Phile · Porsche-O-Phile's Garage

Anyone know of a definitive way of determining what c/r an engine is other than ASS-U-ME ing that the stock internals are still in place and conform to the book spec?

I can certainly do a compression test but there would be some error introduced due to heating of the air in the cylinder as it is compressed. Is there a conversion table or something available that accounts for this and that I can use to reasonably see if my c/r is stock or something else? TIA.

911pcars · 07-01-2008, 08:09 AM

It's a measure of the volume relationship of the combustion chamber/cylinder between TDC and BDC. To be most accurate, the combustion chamber volume is measured with liquid. The cylinder volume is measured along with the thickness of the head gasket.

I believe NASCAR uses a rig to measure cylinder volume in an assembled engine as part of their tech inspection, but I'm not aware of the details.

A compression test provides only a very rough indicator. Higher-than-normal compression pressure indicates a higher CR. However, variables can affect this reading. They include carbon build up, cranking speed and camshaft timing.

Sherwood

afterburn 549 · 07-01-2008, 08:20 AM

The Ez way.
C/R is all about volume and squish , nothing to do with comp readings.
Step 1- Cyl @ TDC, fill to bottom of spark plug hole, (Atf with some dilute r works well . ) Now that amount is your "one"
Record the amount, do not take it out, go to BDC add as many ones it takes to fill to bottom of plug hole again, now you have the amounts plus one as one was already in there .
So for example one first then 7 would be 8 to 1 as one was in there from the 1 st read.
This will get you as close as most will ever need...not counting area taken by spark plug

911pcars · 07-01-2008, 08:57 AM

Compression pressure is related to CR. While CP doesn't indicate the CR, the relationship is not mutually exclusive.

Measuring cylinder volume in the manner you describe is fine in theory and possible on an engine stand. However, with an installed engine, there is a matter of positioning to insert the test fluid. The spark plug opening should be at the top or at least in a manner to ensure the cylinder contains just liquid and no air. That means rotating the horizontally opposed engine approx. 90º, not an easy task to perform with a car attached.

Porsche-O-Phile didn't indicate the location of said engine, but asked if a compression test would reveal the CR. That tells me the engine is attached to a gearbox, starter motor and battery, parts not typically included in an engine stand.

Sherwood

Porsche-O-Phile · Porsche-O-Phile's Garage

Thanks guys - the engine in question is still in the car and is (according to serial #) a '74 "S" spec engine. I have no idea what internals are in it or even if it's ever been opened, hence my question.

I imagine I can suck out the ATF and any residual will burn off without too much difficulty. Good thought.

My thought on a compression test was the same kind of idea just using air as the measuring medium instead of a liquid. Take the peak #, divide by 1 ATM and that should yield c/r (except for the aforementioned issue with compressive heating).

911pcars · 07-01-2008, 01:34 PM

"Thanks guys - the engine in question is still in the car and is (according to serial #) a '74 "S" spec engine. I have no idea what internals are in it or even if it's ever been opened, hence my question. "

Jeff,
An indirect, but more valid method to determine what's inside would be to remove a valve cover and inspect/measure one of the cam lobes. If it's an S cam or equivalent, rest assured, the resultant pistons will be compatible with the lift and duration specs. Record your findings.

One step higher up the ladder would be to remove the intake manifold and measure the diameter of the port. Compare with 2.7S engine specs of that era. Record your findings.

Hope that's enough to verify what you have.

Sherwood