How does the OBC work?

[Cacatfish]

My MPG gauge in car's OBC is way off now, since I have the M5 motor with the 23lb injectors but still have the OBC reading for 19lb (or whatever stockers are) injectors.
Of course I appreciate the 25.5 average MPG it says IM getting but Id like to see an accurate reading. Do I need to replace my OBC with one from an M5 or 533i or is the conversion done at some other computer or something?
Thanks, Colin

[Tjn182]

Yeah I was curious about this too -- my 30#ers really throw it off -- i'm getting an average of 30mph

[graphite]

i think the OBC may have that information stored on the little chip that is removable from the back of the OBC.. my guess would be that you would need to get one of those chip inserts from an M5 (or the whole OBC).. theres a screw adjustment on the bottom right corner of the OBC but i think that adjusts the fuel tank size..

[Threeshifter]

The OBC has metric, imperial, and us settings. Are you running on the correct units of measurement?

[Cacatfish]

Hmmmm...interesting....I know that the 533i also uses the same 23lb injectors as the M5 so I guess I could just pull an OBC from one of those at the junkyard.
I tried both the metric and US and I get the same results with both (after doing the coversion: 9.2l per 100km or 25.2 mpg)
So then it is the OBC itself that actually does the calcualtions?

[Threeshifter]

I can get 23-24.5 mpg on the highway consistently and I will need a tune-up soon. You might be getting 25.5 mpg. Use your odometer and gallons at the pump to check your OBC.

There is the metric, an English setting, and a US setting. The English setting uses British gallons as opposed to US gallons.

This has some good information:

http://home.att.net/~ebaines/e28_obc.htm

[Cacatfish]

Thanks for the link. Based on fillups and gallons (not exact since odo is broken but based on yahoo maps mileage specs) I am averaging around 18mpg and 22mpg on the last long trip I took. I was really impressed with that actually because I have a 4.10 and was driving her pretty hard a lot of the time.
Colin

[shifty]

The OBC takes an input -- probably injector pulse width -- from the ECU to calculate the average mileage you're getting. Unless you can modify the ECU for your size injectors -- which should solve any fuel problems you would have, too -- you're stuck with the readings you get.