Airbox Flap - Facts

[select]

Sorry, to everyone who is tired of this topic, but i had to post this.
To answer the age old question what the airbox flap is actually for.

This is a internal BMW Document which describes how the airbox & sport button work.
Hence why i don't want to post it in the public section. :gayfight:

Basicly it confirms my thoughts that the flap is there simply to reduce noise at a certain speed.
It however states that the Sportbutton sharpens up the throttle and increases engine power, however that is just because the flap opens earlier.

So, if you have the flap removed, the engine just gets all the air it needs at all times. It might even increase power further, but that is speculation on my part. As i have said earlier, BMW actually calls the flap "noise flap" internally and as this document proves, the 2nd hose behind the bumper is called "insulating hose".




TL;DR I was right :blalalala:

[Trawler]

:thumbs:

[DazBlackCSL]

Always prefer the flaps out :-D

First mod I did :thumbs:

[select]

Always prefer the flaps out :-D

First mod I did :thumbs:

Yep.. been driving without it for over 3 years. Thats how it was meant to be, if engineers at bmw had their way. :thumbs:

[NZ_M3]

Actually this has already been debunked by the guys in the USA doing csl conversions. The internal document is wrong.

It is NOT a noise flap. Albeit it functions as one to a certain extent.

The DME has a specific "flap" fueling map to account for when the flap opens or closes, this is added to the main alpha n map when actuated.

The CSL also has its own alpha N fueling map that is far more defined than the stock alpha n map, which some tuners incorrectly tunes instead of the csl one.

I have tested this on a csl converted M3 that I own and if you don't change the flap opening to idle the fueling is all wrong if you run no flap.

[select]

The DME has a specific "flap" fueling map to account for when the flap opens or closes, this is added to the main alpha n map when actuated.

Hi Tom,

That is no surprise because when the flap is closed, it restricts airflow (thus reducing noise) so, engine needs less fuel. When it is open, the engine needs more fuel.
That is also where the "power increase" myth from the sport button comes from.
Sportbutton does increase power but only because the flap opens earlier, if it weren't restricting airflow you wouldn't need the sportbutton.

Why else would it be there if not for noise emissions?

[NZ_M3]

Hi Tom,

That is no surprise because when the flap is closed, it restricts airflow (thus reducing noise) so, engine needs less fuel. When it is open, the engine needs more fuel.
That is also where the "power increase" myth from the sport button comes from.
Sportbutton does increase power but only because the flap opens earlier, if it weren't restricting airflow you wouldn't need the sportbutton.

Why else would it be there if not for noise emissions?

You are talking about completely different maps there.

There's throttle mapping for sports on and off, it just varies the the speed the e-throttle opens nothing else. It doesn't increase power as there are no "sport mode" fueling map.

[select]

It seems you didn't understand what i wanted to say.
I know what the sportbutton does, i was meant to say that it, contrary to popular belief, doesn't increase power actually. It opens the flap earlier, therefore increasing airflow and thats what is actually increasing engine power. I think we are on the same side here, i just didnt make myself clear.

Regarding the different fuel maps, what do you think about my assumption that they are there to simply adhere to the state of the flap. (Whether it is open or not). Thats also why the flap has its own potentiometer.

I think it might help when you understand how Noise Emissions on Cars are measured/regulated in the EU.

The Procedure is as follows:
The Car drives by the recording device with 3/4 of the max RPM and gives full throttle at a certain distance from the device.

3/4 of 8000RPM = 6000RPM, exactly the reason why the flap opens at that RPM in non sportmode.

The Regulations as to how cars are measured are fairly old, so they didn't account for the trickery like the sportbutton. Same Story with the exhaust buttons on newer cars. But they changed that, so newer cars will probably get a lot quieter.

[Mike RT4]

I believe that what Tom is eluding to is that although it might just be for noise as you state, it still has different fuelling maps to take into account it's existence, so removing it will cause a hesitation, as the car will be fuelling as if it is still in place (unless you get it remapped).

[select]

Ah, i see what you mean..

I agree that if you remove the flap it probably has an effect on driveability.
I never intended to state the opposite. Sorry for the confusion. :bigcry:
However, that does not change the fact that the sole reason for the flap being there in the first place is to reduce noise emissions.
The internal document is not wrong.

If the sportbutton throttle map weren't so twitchy i would just simply use that.


EDIT:
I believe the throttle maps are the same on M3 & CSL.
Thats what the table for the M3 looks like.
It makes me wonder how a true 1:1 linear map would feel like?


EDIT: #2
Found a graph for easier comparison.

[CSLManual]

I ran without my flap for a while, didn't like the hesitation so put it back. I've always pressed Sport before setting off anywhere though, the normal map feels a little sluggish.

If I could have the car mapped properly without the flap though, that sounds like a great idea. Where can I get that done?

[select]

I'd imagine that its just a matter of overwriting the closed-flap-map with the open-flap-map? Maybe try Evolve, they seem to know their stuff.

[GregorFuk]

I found the car more susceptible to kangarooing with the flap removed. Pulling from low revs in second gear was where it was most apparent. As much as I love the noise the car is smoother with the flap so that’s how I prefer to run the car.

Incidentally. I think a lot of people went ‘flapless’ because the actuator motors frequently broke and were expensive to replace. Times have moved on however and the same actuator is now used in multiple BMW models for various different things. As a result the actuators regularly appear on eBay for little over a tenner.

[GregorFuk]

BTW. NEVER, I repeat NEVER, dare to suggest that the flap may impact the car's performance / drivability over on Cutters. You'll get shouted down as a fool by the non CSL owning masses.

[select]

Yes low revs and then accelerating hard makes it buckle a bit, not sure however if its solely because the flap has been removed.
But i just drop a gear or 2 down before heavily accelerating. Problem solved.
The SMG isn't exactly silky smooth in the low rpm either, so it fits together :smokin:

Primarily i wanted to dispel the myths about the flap so everybody can make a educated decision whether he wants it removed or not.

BTW. NEVER, I repeat NEVER, dare to suggest that the flap may impact the car's performance / drivability over on Cutters. You'll get shouted down as a fool by the non CSL owning masses.

Don't know much about cutters but there seems to be a war going on between the register and them? I don't mind, as long as we are winning. :hahaha: