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Old 06-05-2008, 06:19 PM   #1 (permalink)
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SGII MPG Calculation

Does anyone know how the SGII calculates instant MPG? No injector pulse information runs through the OBDII so I know they dont use that. My best guess is using the fuel/air ratio and the engine rpm but I cant figure out an equation that would work.

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Old 06-05-2008, 06:25 PM   #2 (permalink)
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Here are a couple approximate ways to calculate fuel flow. http://en.wikipedia.org/wiki/Fuel_in...rom_pulsewidth

I should consider (Fuel flow rate) ≈ (throttle position) (rpm) (cylinders) for my bike.

Nobody I know has (or should) reverse engineer the SG, so it is anyones guess exactly how they do it. But there are plenty of threads here and on the net that discuss it and go into varying degrees of accuracy and complexity.
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Old 06-06-2008, 12:14 AM   #3 (permalink)
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i was thinking that it may use gal. per hour, # of cylinders, and speed.
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Old 06-06-2008, 07:15 AM   #4 (permalink)
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I don't know how it calculates MPG, but I am using trip values Xgauges for MAF and AFR and so far it looks more precise than the SGII MPC calculation. I'll know for sure in july when I take a long highway trip to Boston.
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Old 06-06-2008, 10:32 AM   #5 (permalink)
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It uses airflow and calculates fuel flow by stoic ratio. Not the best, especially with varying air/fuel ratios like Tas's TDI, but good enough for standard gas engines. After initial calibration, mine's within 1-2% every time.

This information is from Scangauge, by way of Wayne at Cleanmpg.
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Old 06-06-2008, 11:08 AM   #6 (permalink)
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I don't know the math but I do know that it always is a little low when compared to actual use when calc'd at the fillup. Heaven forbid I should forget to enter the new tank when filling up. It will take 2 tanks to get it straightened out. I really just use it as a yardstick to hunt for the highest numbers, mpg, while driving which in the end give me the best mpg when calc'd per tank. It has worked so far for someone who is tech challenged.

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Old 06-06-2008, 11:31 AM   #7 (permalink)
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Once you get it calibrated, write down the offset on the fillup screen. You can re-enter that later and you're all set. Just do the fillup process, and then do it again, and the 2nd time you can adjust the % offset.

I do this every time I move the unit between cars, which is fairly frequent.
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Old 06-06-2008, 12:16 PM   #8 (permalink)
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It's a heavily guarded secret, kept in the same safe as the Colonel's recipe for extra crispy.
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Old 06-06-2008, 12:33 PM   #9 (permalink)
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Old 06-10-2008, 11:10 PM   #10 (permalink)
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It depends on whether your car has a MAF sensor or not.

MAF Sensor:

MPG = (14.7 * 6.17 * 454 * VSS * 0.621371) / (3600 * MAF / 100)
= 710.7 * VSS / MAF

14.7 - grams of air to 1 gram of gasoline - ideal air/fuel ratio
6.17 - pounds per gallon - density of gasoline
4.54 - grams per pound - conversion
VSS - vehicle speed in kilometers per hour
0.621371 - miles per hour/kilometers per hour - conversion
3600 - seconds per hour - conversion
MAF - mass air flow rate in 100 grams per second
100 - to correct MAF to give grams per second

No MAF (Uses MAP and Absolute Temp to approximate MAF):

IMAP = RPM * MAP / IAT
MAF = (IMAP/120)*(VE/100)*(ED)*(MM)/(R)

MAP - Manifold Absolute Pressure in kPa
IAT - Intake Air Temperature in Kelvin
R - Specific Gas Constant (8.314 JM/K)
MM - Average molecular mass of air (28.97 g/M)
VE - volumetric efficiency measured in percent
ED - Engine Displacement in liters

This method requires tweaking of the VE for accuracy.

I have no idea if these are the methods the SGII uses. The accuracy will be off due to approximations (AFR ~14.7, etc). All the information is from the brilliant mind of Bruce Lightner.

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