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DIY: Do It Yourself
Post here to share or improve your wrench turning skills! All BMW E46 DIY tips, tales, and projects discussed inside. Learn to work on your car and know the right BMW parts you will need!

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Old 05-26-2008, 06:06 PM   #1
Oz Striker
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Airbox Mod: Additional/3rd Cold Air Feed

I know this has been done before, but i thought i would post my pics of the install. My understanding (after reading S2K forums) is that a 10F drop in the intake air temp can result in a 2% increase in bhp.

I went on a trackday today after fitting the 3rd CA feed and was all ready to have my bro monitor intake air temps (in the passenger seat) with the additional feed as well as with it capped off..............BUT guess what??.......... my OBD Elmscan box gave up the ghost, cheap ebay junk!!

Ended up doing 4 stints of 5 laps, and my EBC Redstuff fast road pads performed well, with OEM discs.

So heres the pics of the mod. Note i have a Rep M3 bumper and have the M3 brake ducts fitted which have the cutout you see in the 1st pic in the roof of the brake duct.




heres the cutout from above with the airbox removed (F denotes the front of the car)




Same view but with a 50mm trumpet in the cutout:




Metal hosing with jubilee clip to tightly connect on to the lower end trumpet:





Hosing ceonnected on to lower trumpet:






50mm hole cutout in front lower portion of airbox (i tried 1 other location on a spare box i had):




Hosing pulled up though the hole, it fits in so tightly that there was no need to use the 2nd trumpet in the airbox, just a jubilee clip:




Adding a CA feed that is so exposed to the front of the car, means insects all manor of fine debris can enter the airbox, so I got hold of some fine mesh/net type material (from my mum!) that would stop fine debris but still allow decent airflow. The jubilee clip helps keep the mesh in place and also acts as a safety stopping the hose from falling down (even though its fits tightly):










A pic looking down at the hose coming up again, but now the airbox fitted back in place (standard 2nd CA feed can be seen on the left).




And lastly from the bottom and top again, with the airbox fitted.







costs involved 10 for the 50mm pipercross cold air feed kit, and also for me an additional 10-15, for an airbox off ebay to experiment with.

Just need to pick up a decent OBD interface box so i can do a more quantative test at a racetrack. The car certainly seems to rev more easily, but that could also be due to the remap i had done last week. Will post my OBD reading results when i have them, prob end of july.
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Old 05-28-2008, 11:51 AM   #2
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Looks great. I've been picking up some pieces to do the same.
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Old 05-28-2008, 04:59 PM   #3
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very good idea, and you can't even hydrolock your engine because of the top air inlet. Might actually try it
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Old 05-28-2008, 05:11 PM   #4
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verrrrry cool
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Old 05-29-2008, 08:23 AM   #5
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Just a thought...

Suppose you are driving at high speed, then the air entering the brake duct will have greater speed than the air in your tube, causing low pressure (because the tube is vertical to the direction of the air), basically sucking air out of the tube. Combined with the air coming from the upper air inlet, this would push air out from the tube rather than through the air filter.

What do you think? On the other hand, if the tube inlet was in parallel with the incoming air, the result would be much different.
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Old 05-31-2008, 09:31 AM   #6
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What supplier or merchant did you purchase the 50 mm trumpets from?

This kit looks similar to the Dinan set up...
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Old 05-31-2008, 09:39 AM   #7
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Quote:
Originally Posted by paraklas View Post
Just a thought...

Suppose you are driving at high speed, then the air entering the brake duct will have greater speed than the air in your tube, causing low pressure (because the tube is vertical to the direction of the air), basically sucking air out of the tube. Combined with the air coming from the upper air inlet, this would push air out from the tube rather than through the air filter.

What do you think? On the other hand, if the tube inlet was in parallel with the incoming air, the result would be much different.
omg that makes a lot of sense. whoa
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Old 05-31-2008, 02:39 PM   #8
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Originally Posted by paraklas View Post
Just a thought...

Suppose you are driving at high speed, then the air entering the brake duct will have greater speed than the air in your tube, causing low pressure (because the tube is vertical to the direction of the air), basically sucking air out of the tube. Combined with the air coming from the upper air inlet, this would push air out from the tube rather than through the air filter.

What do you think? On the other hand, if the tube inlet was in parallel with the incoming air, the result would be much different.

A very good point my friend, but one that was solved over at BMWland forums, when someone else did this mod, and somebody posed the same Q.
Here was the explanation:
The actual effect is when an airflow is passed at speed across the open end of a pipe. The movement of the air across this open pipe will cause a drop in pressure, causing suction in the pipe. However this will only work when the other end of the pipe is it at atmospheric pressure, i.e positive pressure, in your case it's not, it's at a negative pressure as the motor is drawing air in through this pipe/duct at the same time.

So effectively the massive suction force of the engine overcomes any downward movement in the pipe. But i guess i could angle it slightly more towards the front of the car.

Also AFAIK the M3 has the same set up with the intake tube pointing staright down, so again if BMW are happy with it this way then so am i.

The 2 50mm trumpets came as a pipercross kit with with the hosing and jubilee clips for 10 off ebay. The trumpets were bright blue, so i painted them black, heres the kit on their website:
http://www.thepipercrossshop.co.uk/accessories.asp
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Old 05-31-2008, 06:50 PM   #9
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However this will only work when the other end of the pipe is it at atmospheric pressure, i.e positive pressure, in your case it's not, it's at a negative pressure as the motor is drawing air in through this pipe/duct at the same time.
I hardly doubt the sucking force of the engine with filter attached could cause a drop in pressure inside the airbox.

Besides, you are trying to force more air in the airbox, so you need air coming in the airbox from 2 paths. Of course, all this theory is for high speeds, at city driving there will be no difference
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Old 07-24-2008, 02:21 PM   #10
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Thanks for the link. I ordered mine yesterday. Woot.
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Old 07-24-2008, 02:41 PM   #11
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dinan uses this same setup but recommends that you insert the new tube through the side of the brake duct rather than on top (vertical)
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Old 07-24-2008, 03:40 PM   #12
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Good to know.
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Old 07-25-2008, 05:48 PM   #13
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Originally Posted by paraklas View Post
Just a thought...

Suppose you are driving at high speed, then the air entering the brake duct will have greater speed than the air in your tube, causing low pressure (because the tube is vertical to the direction of the air), basically sucking air out of the tube. Combined with the air coming from the upper air inlet, this would push air out from the tube rather than through the air filter.

What do you think? On the other hand, if the tube inlet was in parallel with the incoming air, the result would be much different.
In order for there to be a pressure drop at the pipe/duct junction, there would have to be an equal to, or smaller cross section in the brake duct in relation to the pipe. You have to either have a steep venturi (narrowing of the duct at the pipe junction) or the duct would need a smaller volume cross section (at the junction) than the pipe. This is why it doesn't suffer from any reversal of airflow and this set-up works as designed, providing another path for cool air to enter the intake.

Angling the pipe inlet would indeed create more of a "ram-air" type effect, but could supply too much air volume. Too much positive pressure could push air around the filter seal and introduce more dirt into the engine. Or the air volume would bleed out from the OE intake points and set-up a resonance wave out of phase with the intake pulses and could "starve" the airbox.

The OE airbox volume is large enough, and fed with more air than the engine requires (positive pressure). If it didn't, you would see the airbox lid getting sucked in (negative pressure). These factors are designed into the intake system.
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Old 07-25-2008, 06:17 PM   #14
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just to let everyone know this works with a car without hid headlights because there is more room behind the headlight to run the pipe of a larger diameter.

I've done this to my 325i with no hid, and last weekend I did it for my brothers 328ci with hid, there was barely any room behind the headlight, so I had to use a smaller diameter hose (2'' dia) radiator hose and it fit perfectly, his gas mileage when up from 25mpg to 31mpg. then again the opening of the hose was positioned next to the fog light, thus creating a velocity stack.

heres the link to the diy that i put together for my 325 non hid, i actually modified the inside of the intake box to use an aem cone filter.

http://forum.e46fanatics.com/showthread.php?t=589170
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Old 07-25-2008, 08:36 PM   #15
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6 mpg more, that's a 25% increase? over what period of time did you measure this gain?
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Old 07-25-2008, 08:41 PM   #16
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6 mpg more, that's a 25% increase? over what period of time did you measure this gain? Sounds too good to be true.
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Old 07-28-2008, 11:24 AM   #17
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6 mpg more, that's a 25% increase? over what period of time did you measure this gain? Sounds too good to be true.
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Old 07-28-2008, 05:03 PM   #18
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6 mpg more, that's a 25% increase? over what period of time did you measure this gain? Sounds too good to be true.
it was from oceanside to las vegas and back
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Old 07-28-2008, 06:26 PM   #19
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it was from oceanside to las vegas and back
So you measured you MPG from oceanside to Las Vegas, then you did the mod and drove back measuring your MPG again?

Where is "oceanside" mileage-wise from Las Vegas?
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Old 07-29-2008, 07:39 AM   #20
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So you measured you MPG from oceanside to Las Vegas, then you did the mod and drove back measuring your MPG again?

Where is "oceanside" mileage-wise from Las Vegas?
correct, from oceanside to vegas was 25-26mpg, did the mod then got 31-32 mpg on the way back going 75mph. i too am shocked as well as jealous because its my brothers car, my 325 only gets 20-23mpg in las vegas.
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