Equation for Converting Measured Compression Values in psig to Compression Ratios?

[Mark F.]

This is related to another post that I did not want to interfere with, but is there an equation to convert measured (fully compressed) psig readings on a cylinder to the compression ratio (CR) as a fraction (e.g., 10.25 to 1; 11:1 etc.).

Is the measured value in psig divided by 14.7 (corrected for altitude)? thx, Mark

[James G.]

Re: Equation for Converting Measured Compression Values in psig to Compression Ratios

IF the intake and exhaust profiles do not overlap... maybe?
On an engine with a HP cam which has intake and exhaust overlap, compression is bled off.
The intake valve opens prior to the exhaust valve closing, taking advantage of the velocity of the exhaust charge beginning to pull the intake charge into the runner prior to the piston descending on the intake stroke - this is why runner cross sections/volume are critical in relation to velocity of the incoming gases or burned mixture.

[Mark F.]

Re: Equation for Converting Measured Compression Values in psig to Compression Ratios

James,
Thank you - mine is a '67 400HP, which does have overlap as you say...I guess what I'm saying is I don't see bleed-off, I see higher numbers than expected.

Here's the numbers I have...this was a "hot test" - ran the engine up to running temperature and then did the compression tests. Should I have done CRs cold?

Obviously I don't like cylinder #4 because it's out of whack compared to the others - that's why I did it twice to be sure

CRs seem way too high for a 400 - s/b ~10.25:1 (and I do not believe anything was modified by previous owners).

Cylinder & (order tested)	Measured psi	Calculated Compression Ratio (measured/14.7)
1 (5)	180	12.2
2 (1)	170	11.6
3 (6)	180	12.2
4 (2 & 9)	120	8.2
5 (7)	171	11.6
6 (3)	176	12.0
7 (8)	172	11.7
8 (4)	173	11.8

[James G.]

Re: Equation for Converting Measured Compression Values in psig to Compression Ratios

I know this question has been asked many times - compression pressure tests to determine compression ratio, however I do not believe I have ever seen an answer as definitive as the one you have hypothesized.

FWIW The numbers you have provided do not appear out of line with what I would expect, it's not a solid lifter high overlap.

That being said the low one concerns me, add an oz or so of oil to the low one and perform the test again... if it comes up, you have ring issues, if not probably valve issue- I am assuming you made sure that the valves are opening and closing on the low cylinder.

[Mark F.]

Re: Equation for Converting Measured Compression Values in psig to Compression Ratios

James,

Thanks...and yes, I agree I should have oiled the #4 and done a third measurement at that time to see exactly what you say...I did this test a long time ago and only brought it up as a result of me seeing the other post.

The car runs strong - even with the low #4...no unusual oil burning or consumption...next steps would be as you suggest...

Also, I was still wondering about the automotive engineering math, too...but now that I think about what you say about overlap - doesn't seem like it's possible to know how much is going out vs how much is coming in at the same time during the overlap...it just still seems odd to me that my calcs have it coming at a much higher CR for a 400hp 10.25:1 rather than a lower CR...anyhow, thanks for your input and advice!...Mark

[Mark F.]

Re: Equation for Converting Measured Compression Values in psig to Compression Ratios

correction on what I said - regardless of overlap, nothing would be coming IN on the compression stroke - it would all be going OUT.

[Duke W.]

Re: Equation for Converting Measured Compression Values in psig to Compression Ratios

First point: Overlap has little or nothing to do with cranking compression pressure.

Second point: There is no "equation" or reliable method of computing compression ratio from cranking compression pressure.

A BIG factor in cranking compression is the point that the inlet valve closes. For a given static compression ratio, the later the valve closes, the lower the cranking compression pressure.

That's why the '63 340/360 HP engines' nominal OEM specified cranking compression pressure is 150 psi versus 160 for the 250/300 HP engines despite the latter's 0.75 lower advertised compression ratio, and the reason is that the Duntov cam exhaust valve closing is much later than the much shorter duration cam used in those engines that closes the inlet valve much earlier, which means less inlet stroke reversion at cranking speed than the Duntov cam.

Duke

[Mark F.]

Re: Equation for Converting Measured Compression Values in psig to Compression Ratios

Thank you, Duke

I've learned something today that I have wondered about for a long time - I thought for sure there was some huge quadratic one could use to get the answer I was searching for - but now I understand.

I guess my take-away from this is that although cranking compression pressure is a physical measurement that can be obtained, it is only an indirect indicator of potential "engine health" and/or one that can be used to diagnose other potential internal problems.

thanks much - Mark

[Duke W.]

Re: Equation for Converting Measured Compression Values in psig to Compression Ratios

That's why the '63 340/360 HP engines' nominal OEM specified cranking compression pressure is 150 psi versus 160 for the 250/300 HP engines despite the latter's 0.75 lower advertised compression ratio, and the reason is that the Duntov cam exhaust valve closing is much later than the much shorter duration cam used in those engines that closes the inlet valve much earlier, which means less inlet stroke reversion at cranking speed than the Duntov cam.

Duke

Oops! I meant INLET valve:

That's why the '63 340/360 HP engines' nominal OEM specified cranking compression pressure is 150 psi versus 160 for the 250/300 HP engines despite the latter's 0.75 lower advertised compression ratio, and the reason is that the Duntov cam inlet valve closing is much later than the much shorter duration cam used in those engines that closes the inlet valve much earlier, which means less inlet stroke reversion at cranking speed than the Duntov cam.

Duke