Easy way to correct your comp. numbers for rpm
#1
Certified Mazda Tech
Thread Starter
Easy way to correct your comp. numbers for rpm
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):
which would become (with 5.5 comp and 189 RPM):
C) Google will yield a calculation result, in this example it determines the answer as:
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]:
psi conversion [min fail spec: 98.6psi@250rpm]:
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
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
(5.5*98.0665/1000+(-0.514*ln(189)+2.838030912))*1000/98.0665
(5.5*98.0665/1000+(-0.514*ln(189)+2.838030912))*1000/98.0665 = 6.9660760428826
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
simpler, barely less accurate: (COMPNUM/1000 + (-0.514 *ln(RPM) + 2.838))*1000
(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
simpler, barely less accurate: (COMPNUM*6.894757/1000+(-0.514*LN(RPM)+2.838))*1000/6.894757
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
Last edited by teknics; 04-24-2009 at 06:18 PM. Reason: (extra parantheses used, and new options, equation made considering wrong spec, adjust to proper 6.9 spec)
#2
Compression Tester Guy
Join Date: Nov 2006
Location: Houston, TX
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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
-John
#3
Registered
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))
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))
Last edited by Delmeister; 04-23-2009 at 07:11 AM. Reason: ask about option
#4
Certified Mazda Tech
Thread Starter
6.9 + (28.97100 - (5.24100 * ln(250))) = 6.933023329484
rounds to: 6.93301, off 'exact spec' by .03301
rounds to: 6.93301, off 'exact spec' by .03301
(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
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
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
simpler, barely less accurate: (COMPNUM/1000 + (-0.514 *ln(RPM) + 2.838))*1000
(680/1000 + (-0.514 *ln(250) + 2.838030912))*1000 = 680.0000002188
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
simpler, barely less accurate: (COMPNUM*6.894757/1000+(-0.514*LN(RPM)+2.838))*1000/6.894757
(98.6*6.894757/1000+(-0.514*LN(250)+2.838030912))*1000/6.894757 = 98.600000031734
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.
Last edited by teknics; 04-24-2009 at 06:12 PM.
#5
Bigus Rotus
iTrader: (3)
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.
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.
Last edited by Nemesis8; 04-26-2009 at 09:00 PM.
#6
Certified Mazda Tech
Thread Starter
kevin.
Last edited by teknics; 04-24-2009 at 06:13 PM.
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