13 button obc question

[Dodobrown]

I just converted my broken 9 button over to a 13 button and I'm really happy with all the extras that I gained but one thing that kinda sucks in now I can see my average mpg and let me tell you 17.9 (says the obc) is not sweet. I almost prefer my broken 9 button lol.

So I'm wondering how accurate is the thing? I do not have the proper coding plug in it currently so that may have some effect and I do have an ix which I drive some what aggressively.

Any one ix or not experience that low of mpg or am I having issues else where?

[spdracrm3]

i did the same in my IX and the donor 13 button was my E30 M3 racecar which has 2.3l 4cyl and 15" wheels but a 4:10diff ,as opposed to my IX which has 2.5l 6cyl and 15" wheels and 3.91 diffs ,i didnt have the IX coding plug so left the M3 one in there and was surprised how accurate it is , i have done a couple long trips and did the manual calculation and it was within 1-2mpg as i remember. so i think it depends what car you got your 13button and coding plug as to how cross compatible it will be.

i have the german document that lists the coding plug for each bmw model and their numbers /options

[ForcedFirebird]

The OBC reads directly from the ECU. Only way it won't be accurate is with an injector change.

[spdracrm3]

The OBC reads directly from the ECU. Only way it won't be accurate is with an injector change.

true but it needs to know all the other factors to caclulate MPG accurately, it reads rpm and injection MS from ECU to determine quantity fuel injected then calculates based on wheel/tires/diff ratio to get MPG reading(and determine quantity of fuel left in tank for miles remaining (needs to know tank size)). the 325i/ix/e is batch fire of 3 inj each whereas the M3 is batch fire 2cyl each and different sized injectors , and 318i/325e yet again will have different size injectors /wheels/diff ratio's ,which is why there are so many different coding plugs....23 to be exact

[nando]

it doesn't read RPM. RPM is irrelevant. it reads the injector pulses and vehicle speed input (it doesn't care about diff ratio either). Unless your tires are the wrong size or you change out injectors, it will be accurate.

batch fire doesn't matter either. it only reads off one injector pulse and each injector only pulses once per cycle. the coding plug does matter because it contains the tank size, injector fuel mass quantity, and # of cylinders.

[ForcedFirebird]

Diff ratio doesn't matter as the MPH is read at the output, otherwise, your MPH would be off with a ratio change. The tire height between the m3 is .100" as a non-M (205/55-15 vs 19/65-14), so that difference isn't any more than regular tire wear. You are right about the tanks, but the ECU sends the econogauge and OBC based on injector pulse. That's why when I write a chip for the ECU using larger injectors (changing the injector constant), the MPG goes up since they are shorter/less pulses. And yes, not too many people are aware that the m20 is semi-sequential. This is why the old theory that batch fire injectors need to be about 1lb/hr per 100whp, yet a healthy m20 makes more WHP then the injector suggests, and can support even more power. When I converted my old car from an e to an i, the mileage stayed accurate, and my cluster didn't have a square coding plug.

[ForcedFirebird]

it doesn't read RPM....

Got ninja'd by he r3v OG :(

[nando]

diff ratio doesn't affect the VSS reading. all E30s and other BMWs of a similar vintage use the same 9-tooth signal for the speedometer. the only thing that will effect it is tire size.

BTW the speedo reading on the OBC is more accurate than the one on the cluster. the cluster purposefully reads high so you don't get tickets. :p

[ForcedFirebird]

diff ratio doesn't affect the VSS reading. all E30s and other BMWs of a similar vintage use the same 9-tooth signal for the speedometer. the only thing that will effect it is tire size.

BTW the speedo reading on the OBC is more accurate than the one on the cluster. the cluster purposefully reads high so you don't get tickets. :p

Right, that's what I said, isn't it? ;) Also if you calculate the height of different tire sizes offered on the e30, they are within .100" of each other.

Yes, the cluster reads about 10% over, there are unmanned radars on a lot of the speed limit signs here, and as many e30's as I have had/driven, I agree with that 100%.

[Dodobrown]

So the coding plug that's currently in the obc is out of a 87 or 88 325e would my mpg be high or low based on this combo. I also have the correct plug but I haven't had time to change it out.

Also I do plan on changing the injectors over to m50s very soon will that render the mpg function useless?

[nando]

325e's have a different injector size and also often smaller tire sizes than an ix so yes it will be inaccurate.

you can make it work with an M50 by getting a circuit conditioner from seattlecircuit.com

[Dodobrown]

Im not sure we're on the same page. I wanna use m50 injectors not an m50 motor.

[ForcedFirebird]

You don't need different injectors. Many guys are making well over 160hp at the wheels with a stock DME/injectors in the spec series. Just changing the injectors to larger ones do nothing, specially if the tune doesn't match.

[Dodobrown]

I'm unfortunately only looking for efficiency not power gains I live in center city philadelphia so I do all city driving. gas prices are kicking my ass. Making more power will only exacerbate the issue. One day when the ix is a toy and not a daily I can care less about being responsible.

[ForcedFirebird]

Then find some 14 or 15lb multi-pintle injectors. Your mileage will suffer if you put larger injectors in.