Well this is the foundation of the program i released earlier, haven't seen it posted but maybe it's around somewhere. Anywho used it in another topic figured it might be useful enough to have around:

How to Google Your Compression:

A) go to google, or use their searchbar addon
B) enter the following into the search bar, replacing "COMPNUM" with a 2 digit single decimal (5.5 is used for this example) and "RPM" with your recorded RPM during the rotary test (189 is used for this example):

(COMPNUM*98.0665/1000+(-0.514*ln(RPM)+2.838030912))*1000/98.0665
which would become (with 5.5 comp and 189 RPM):
(5.5*98.0665/1000+(-0.514*ln(189)+2.838030912))*1000/98.0665

C) Google will yield a calculation result, in this example it determines the answer as:
(5.5*98.0665/1000+(-0.514*ln(189)+2.838030912))*1000/98.0665 = 6.9660760428826

D) Take that number, round it to one decimal place. You wind up with 6.9/7.0. What this does is convert it to the spec RPM of 250. so at 250rpm your compression number is 6.9/7.0, which is dead on the spec. Anything at or below 6.9 @ 250 rpm is below spec aka bad and/or failing. This is for kgf/cm2.

here's the formula's for psi and kPa:

kPa conversion (aka the actual standard, and therefore easier to calculate) [min fail spec: 680kPa@250rpm]:
(COMPNUM/1000 + (-0.514 *ln(RPM) + 2.838030912))*1000
simpler, barely less accurate: (COMPNUM/1000 + (-0.514 *ln(RPM) + 2.838))*1000

psi conversion [min fail spec: 98.6psi@250rpm]:
(COMPNUM*6.894757/1000+(-0.514*LN(RPM)+2.838030912))*1000/6.894757
simpler, barely less accurate: (COMPNUM*6.894757/1000+(-0.514*LN(RPM)+2.838))*1000/6.894757


Enjoy,
kevin.

side note: replace 2.838030912 with 2.838 for practically unnoticeable change in result other then easier to type/remember.

new options (using 5.5 and 189 again):
++encalc.com/#expr=(5.5*98.0665/1000+(-0.514*ln(189)+2.838030912))*1000/98.0665

Good info! How would that work for PSI? I've been wanting to figure out something like this for correcting the readings from my compression tester.
-John

teknics:

Your expression: (COMPNUM*98.0665/1000+(-0.514*ln(RPM)+2.8411))*1000/98.0665

Simplifies to: COMPNUM + (28.971 - 5.241* ln(RPM))

Don't understand option


TwistedRotors:

To convert to psi, multiply above by 14.194, or equivalently, with COMPNUM in psi calculate

PSI at 250 rpm = COMPNUM + (411.2 - 74.39* ln(RPM))

teknics:
Your expression: (COMPNUM*98.0665/1000+(-0.514*ln(RPM)+2.8411))*1000/98.0665
Simplifies to: COMPNUM + (28.971 - 5.241* ln(RPM))
Don't understand option

actually
6.9 + (28.97100 - (5.24100 * ln(250))) = 6.933023329484
rounds to: 6.93301, off 'exact spec' by .03301

and
(6.9*98.0665/1000+(-0.514*ln(250)+2.8411))*1000/98.0665 = 6.9312959901577
rounds to: 6.93129, off 'exact' spec by .03129)
(what i originally posted, i have gone back and corrected the formula due to a slight miscalculation, 2.8411=2.838030912 now)

*NEW FORMULA*
(6.9*98.0665/1000+(-0.514*ln(250)+2.838030912))*1000/98.0665 = 6.9000000022312
simpler, barely less accurate: (6.9*98.0665/1000+(-0.514*ln(250)+2.838))*1000/98.0665 = 6.89

So the original formula is more accurate by .00172, perhaps with my updated formula yours may be dead on or at .00001 difference as well.

Using 6.9kgf/cm2 @ 250rpm for the example since 6.9kgf/cm2 is the exact minimum fail spec and 250 is chosen because this calculations purpose is to convert the RPM to 250 and adjust the compression to 250, therefore using 250 as the rpm input makes rpm a non-factor and therefore the result should be as close to the input compression as possible.

Honestly tho, I highly appreciate the simplification of the formula. Close enough to be the same and easier to calculate altho you'll most likely still need a calculator to figure ln(RPM). The formula i posted is actually setup like that for 2 reasons, first is easier swapping to different measurements (kpa, psi, kgf/cm2), just remember to multiply the comp by 1/1000, 6.9/1000 or 98.1/1000 and then multiply by inverse division. and it's plug and play essentially. Secondly it's to show you the seperations of the compression conversion and the RPM conversion (example 680kPa = 98.63 psi, 98.63psi*6.894757 = 680.02kpa is essentially the initial part of the calculation, then they show the rpm conversion seperately and it's "correction factor" is added to the original comp number to get the comp @ 250rpm.

kPa conversion (aka the actual standard, and therefore easier to calculate):
(COMPNUM/1000 + (-0.514 *ln(RPM) + 2.838030912))*1000
simpler, barely less accurate: (COMPNUM/1000 + (-0.514 *ln(RPM) + 2.838))*1000

conversion to exact minimum fail spec (680kpa@ 250rpm, since 250 is the spec RPM that this equation converts to it will essentially become a non-factor, therefore should effect the 680kpa input as minimally as possible)
(680/1000 + (-0.514 *ln(250) + 2.838030912))*1000 = 680.0000002188
We now have the kPa converted with basically the least work. Realistically if you substitute 2.838030912 with 2.838, the result is "as close as you need to be" (it would then = 679.97 essentially), so really for ease of memory you can stick with 2.838 which is slightly less accurate, if you wanna be balls-on accurate you bust out the 2.838030912

psi conversion:
(COMPNUM*6.894757/1000+(-0.514*LN(RPM)+2.838030912))*1000/6.894757
simpler, barely less accurate: (COMPNUM*6.894757/1000+(-0.514*LN(RPM)+2.838))*1000/6.894757


conversion to exact minimum fail spec (98.6psi @ 250rpm, since 250 is the spec RPM that this equation converts to it will essentially become a non-factor, therefore should effect the 98.6psi input as minimally as possible)

(98.6*6.894757/1000+(-0.514*LN(250)+2.838030912))*1000/6.894757 = 98.600000031734

We now have the compression converted with basically the least work. Realistically if you substitute 2.838030912 with 2.838, the result is "as close as you need to be" (it would then = 98.59 essentially), so really for ease of memory you can stick with 2.838 which is slightly less accurate, if you wanna be balls-on accurate you bust out the 2.838030912

kPa is in all reality the standard measurement of compression used (we only use kgf/cm2 because it moves the decimal point over two points making an easier/lower number, allows tolerances to be easy to set etc)

Again, I do appreciate the breakdown, you can breakdown the above two as well if you'd like to help make it more streamlined, which is always a good thing, since theyre mainly written in my style as being easier to show how the formula works (aka the comp and rpm conversions seperately)

kevin.

So, now that you made changes to your formula, my numbers went down a bit.

Rotor 1 @ 285 RPM
6.8, 6.7, 6.7

Rotor 2 @ 294 RPM
6.8, 6.9, 7.1

Rotor 1 Corrected to 250 RPM
6.0, 5.9, 5.9

Rotor 2 Corrected to 250 RPM
5.9, 6.0, 6.2

I did not have time to recheck compression last night, but I am willing to bet they are allot better, at least it feels better. My car fires right up cold or hot within a couple of seconds.

So, now that you made changes to your formula, my numbers went down a bit.

I did not have time to recheck compression last night, but I am willing to bet they are allot better, at least it feels better. My car firs right up cold or hot within a couple of seconds.

got it even more dead on now, even psi.

kevin.