The bottom one is notched so it only goes onto the throttle body one way. The second one just goes on 'to fit'.

 

Interesting. I don't recall that. Seem to recall it needed to be tweaked a bit to get the whole thing aligned.

 

I have the same problem I think, the upper boot has a weird bend, but I can't figure out how should it be

 

I just did this yesterday on my car and the best way I found was install the boots loose by not tighten the clamps down. Next install the air intake with the MAF. After you get the perfect alignment and fitment by adjusting the boots tighten the clamps down. In the past I just used to put them on and tighten the clamps right away. Then when installing the boot between the MAF I would fight a bit to get it closer. Now it goes on smoother.

 

PURRFECT!

 

Another option is using the one-piece boot from an E83 X3.
B25: 13543412291
B30: 13543412292

 

Positioning it incorrectly can put quite a strain on the IACV tobe and, I would think, lead to premature failure.

 

When I replaced the intake boot on mine (it was torn), I tried to orient the boot so it had as little stress as possible. Stressing the rubber will cause it to fail. So I turned the upper boot until it naturally "aimed" the MAF at the hole in the air cleaner box. The F connector on top is tilted over, until it almost touches the engine. It doesn't seem to hurt anything.

 

^ This.

I found using a little silicone free lube like inox or even soapy water helped. If you slip the lower boot on with the tab at 6 o'clock, the turn the boot anti clockwise a little bit, you'll feel the tab drop into the notch and then it will resist turning either way after that. The top boot seems to find its happy place if you install it with the airbox and MAF in, then rotate it to the point where it's all relaxed, not stretching. The vacuum "F" fitting lays over toward the motor a bit, almost at 45 deg it seems.

 

Great discussion! @BaliDawg is there any downside to using the one piece X3 version? Is it tougher to get the lower hose clamps tightened with the whole boot in the way? This seems like a clearly superior solution, so I'm trying to figure out why BMW went to all the trouble to make it two parts and add the center joint etc. Or maybe since the X3 was later, they realized it didn't need to be two pieces?

 

I'm trying to figure out why BMW went to all the trouble to make it two parts and add the center joint etc.

Having 2 boots is easier to remove/install the DISA -- just loosen the middle clamp and turn the top boot away . With one boot you have to reach way down to the TB and unclamp it.

 

I think the later reasoning. I've been dying to try the one boot mod.

 

I wish I had known about it, clearly a better way if it fits properly.