Subject: OBC Mileage correction factor is limited to 1250 - and, most formulas are wrong Author: erwin : member since November, 2007 : 885 posts Posted on: 2008-02-08 10:34:21

The correct OBC mileage correction factor formula is:

new factor = old factor * (OBC mileage / actual mileage)

the instructions I found (on this board and elsewhere) had it as actual/obc.

I followed the wrong formula, came up with a factor of 774 (12/15.5)*1000, and inputted that into the OBC. This made the OBC read even higher, away from the truth.

So I reversed the ratio, and inputted 1292 into the OBC. Once again, the mileage went higher ! WTF !! I went into the OBC again, and found the factor was still at 774. This time I paid closer attention, and when I pressed S/R after inputting 1292, the display went back to 774. It refused my input.

A little trial-and-error led to the problem: the OBC will not accept any values higher than 1250.

I'll use this value for a few tank fulls and see how close it is, not that I have much choice.

This limitation seems to imply that anyone with 24lb injectors will never get accurate mileage readings from their OBC.

Something interesting: as soon as I changed the factor from 774 to 1250, the mileage display instantly went from 18.x to 12.x mpg. I was expecting the new factor to come into play for mileage calcs going forward, and would see the reading steadily drop from 18. Instead, the OBC seems to apply the correction factor when it displays the number, not when it calculates and stores the accumulated mileage readings. So, it must only be storing the accumulated injector pulses (raw data), and not calculated values. Which is, of course, the proper way to do it !!

Erwin 89 750iL Lowered (a little)

Engine Wolf chips 19 lb, 4-hole (Design 3) fuel injectors PowerFlow Intake Air Filters Royal Purple 20W-50 In process: Custom Cold Air Boxes Interior Phone removed, replaced with cut-to-length dash wood trim on hinge E38 Shifter Knob M-Tech 2 M-technic Steering Wheel - air bag deleted E38 Self-Dimming Mirror 2B Installed: E38 shifter surround plate and boot Audio BMW Business CD Player "CD43" Head Unit Refurbished factory amp w/equalizer Bass blockers (300hz/4ohm) on stock front kick panel speakers Rear deck speakers: Alpine SPS-13C2 5.25" 2 way 175 Watt with bass blockers Subwoofers: 2 JBL GTO804 8" 4-ohm subs Subwoofer install: custom sealed enclosure mounted behind ski bag opening Subwoofer amp: Alpine MRP-M450 400w MONO CD changer relocated to driver's door bottom pocket actuated by bio- mechanical arm controlled by semi-intelligent nut behind the wheel Suspension H&R Lowering Springs (1/2 coil removed from fronts) Koni adjustable front shocks Stock rear shocks (self leveling) Racing Dynamics Stress Bar Stock front sway bar Dinan rear sway bar - 21mm M5 aluminum lower control arms Front tires: 245-40-18 Goodyear Eagle F1 GS-D3 on 8.5" EuroTech rims Rear tires: 275-40-18 Michelin Pilot Sport PS1 on 9.5" EuroTech rims Exterior 25% Side tint 5% Rear tint BMW Mud flaps - rears trimmed to tire width Rear fender lips rolled Smoked side markers Zymol wax Way too many paint chips touched up Brakes Front Rotors: 850CSi, 324 X 30 mm, cross-drilled and slotted Rear Rotors: stock size, cross-drilled and slotted Pads: Hawk Performance Ceramic (may switch to HPS) Stainless steel lines Racing Blue brake fluid

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    • OBC Mileage correction factor is limited to 1250 - and, most formulas are wrong - erwin 2008-02-08 10:34:21 (210 views)
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