factory restriction

[maherbaz]

I haven't fogged mine yet, but I am seriously considering it. I was going to this weekend while I had my car in my dad's garage (new bilsteins!). But I started thinking about possible reasons why bmw may have restricted the air flow. Go to the fogged airbox website:

http://www.logun.org/fog.htm

There is a chart of airflow for different RPMs. From 2.6-4K, the airflow remains unchanged. I spend a lot of time in this range. After 4K, airflow begins to increase slowly, but it does increase. Thats good. More power when you stomp it.

But it also begins increasing as RPMs drop below 2.6, and it looks like it is increasing pretty fast (much faster than it does on the high end). The chart cuts off everything before 2.2, but if we assume the numbers continue in the same manner, we can see the spread will be quite large as we approach idle speeds. I live in the city and have lots of traffic lights and I drive an automatic, so I spend a lot of time in the 0.8-2K range. But if my engine is sucking a lot more air at these ranges (when im not even trying for performance) that means it is using much more fuel.

So could fuel economy for daily driving situations be an additional reason for the restricted air flow (besides sound emissions)? I love the idea of getting more pull at higher RPMs, but in all reality I don't spend a lot of time above 4K; I might wind it up a couple times a day. I hate the idea of wasting all my gas idling at a stoplight.

Sorry to bring up the topic of fogging again, but I'm bored at work and enjoy thinking through things like this.

[ScnStlr]

Good observation. I think your right in your assesment from at least what I can see. I tried recently a CAI and I hated it (noise was way too much for my taste) and finally removed everything and reinstalled the old air box with a new K&N fitler.
I didn't notice any improvement in preformance but I am like you in my dirving. I also did not notice any gas mileage improvements or reductions.
I'm sticking with the normal air box. unless you are racing i don't think it's worth the effort or time.