[Neamer_M2]

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Guys, I just have had my 2017 MPE (unfortunately with the red button) together with akrapovic 300 cpsi dp installed. And yesterday I had an opportunity to see an M2 with full akra exhaust set dp and muffler.

And at this point things started to get interesting because the owner mentioned that he had is engine remapped because after installing it they made measurements which showed that that AFR goes almost lean.

First of all Akra says that dp does work fine with it's stock engine map, the other thing is that I don't believe that computer changed air to fuel ratio after switching cats without showing any CEL information.

Some say you don't need to tune the car after changing dp some say you have to. Right now I'm a bit worried if Im going to have any problems. I don't want to tune my M2 as long as I am perfectly fine with its stock power, the only reason why i changed dp is the sound which is better compared to the mpe with stock dp.

[Dav3]

So far I'm guessing there are at least 10+ active forum members that are running a catted downpipe with no tune and no problems

If I recall correctly akra downpipe is not made by akra and is the generic HJS dp most bmw tuners in Germany offer

A DP makes much more sense with a tune but is not exactly a requirement

You should be fine

[Neamer_M2]

Yes it is HJS 300 cpsi, I believe the difference is aprox. 400 cells less. First what i thought is that he should be checking his intake system if the afr went higher.
And as you said I also remember that dp gives the possibility for the bigger power gains with new map, but no one ever said I have to do it.

But to be honest with the whole system installed, car seems to be a little bit more nervous in the lower range or maybe its just my mind lol.

[kiysersosae]

A lean condition would be absolutely normal in closed loop operation, nothing to worry about. Under full loaf most of these sensors are ignored and the car is using target data to account for air fuel mix.

The cars boost is also lower with a DP, so this would change the AFR.

I think a tune would be a benefit after DP install but not necessary. I ran my 1m with no tune and no cats for 2 years and had no issue, AFRs on the Dyno were spot on.

[SeanWRT]

Originally Posted by kiysersosae

A lean condition would be absolutely normal in closed loop operation, nothing to worry about. Under full loaf most of these sensors are ignored and the car is using target data to account for air fuel mix.

The cars boost is also lower with a DP, so this would change the AFR.

I think a tune would be a benefit after DP install but not necessary. I ran my 1m with no tune and no cats for 2 years and had no issue, AFRs on the Dyno were spot on.

The car will not boost lower with a DP. Boost target is controlled by DME and instead DP makes reaching it easier and faster is all.

To OP:
I have had such AFR doubts and didn't see a consensus in the theories. Below is a lean theory that I like.

With our F platform, it appears AFR is mostly controlled by sensors from intake system and, when in closed loop, the feedback of O2 mass (O2 leftovers) from sensor placed in downpipe. O2 leftover is determined by the air mass that gets into cylinders minus those that gets consumed in combustion.

Less back pressure allows for a more efficient exhaust gas evacuation and thus make a slightly more efficient combustion, which consumes more air. Therefore the sensor detects less air leftovers and reports so to DME, calling for an adjustment to less fuel injection as a STFT.

Nevertheless, we're talking about minor difference (back pressure cause more air gets combusted under certain boost and mass intake air flow). And DME today is so sophisticated to maintain AFR target & other parameters under environment variances. I tend to believe it would make little to no difference.