Fogging Your Airbox (1998 318ti)

[seanhearn]

I just wanted to give a small walkthrough to the folks who have thought about fogging your airbox and havent quite gotten to that point because of any particular reason. The following modification was done on my 1998 1.9L 318ti.

But before we get started, here's some info that you need to know about your cars intake and fuel injection system

The fuel injection in the Z3s is a mass air fuel metering system. What this means is your car directly measures the weight of the air being draw into your engine and injects an appropriate weight of fuel (in the ratio of 14.7 parts air to each part of fuel) which it then ignites through your ignition system. Under cruising conditions and light acceleration your cars computer (DME) uses feedback from the oxygen (O2) sensors to fine tune the mixture.

The DME measures the weight of the air using a sensor called the Hot Film Meter. (HFM) The HFM is basically a film through which a current is passed to heat it. If you were to look at the inside of your HFM you will see that the film is very small and it actually only samples a small fraction of the air that passes through the meter. By measuring the temperature change of the film (and knowing the intake air temperature) the DME is able to calculate the mass (weight) of air that is flowing over the film. It can then calculate the total mass of air flowing through the HFM. This works if the airflow through the meter is smooth and evenly distributed, in other words laminar flow. The DME then uses this to determine how much fuel needs to be injected to keep the AFR at 14.7/1, also called Lambda=1. An AFR of 14.7/1 results in the most complete combustion of your fuel which gives you the least emissions. If an engine is running rich OR lean (more or less fuel to air) the engines emissions will increase. At full throttle the DME runs the engine richer for more power.

Now a key point... if you increase the airflow into your engine, the DME will measure it using the HFM (assuming smooth airflow) and will increase the fuel injected to compensate. The DME must do this or your engine would run leaner which would increase emissions. If you increase the amount of air, to keep the ratio between air and fuel at 14.7/1, the DME must add more fuel. If you increase airflow and burn more fuel you have increased your power output. ---Shawn Fogg (www.logun.org/fog.htm)

First, just take a look at your airbox and start thinking about all the things you need to take off to get the airbox out and by itself. In this picture... #1 and #2 are where two nuts are that you have to take off. #3 is something you may have to wrench out to get in to get to #2.

Image #1

After I opened my airbox, I noticed some kind of baffling material that seemed to be either heat dampening material or sound dampening material. I dont know what the case may be in your car, but in mine it seemed to have "rotted" out. So i took all that out. Only the part above the air filter was ok. Take a look at this picture.

Image #2

And this is my airbox with all that stuff taken out.

Image #3

Ok, so once your airbox is out. You need to think about positioning for the tubing that will run from the air duct that leads to the driver side tire (or brakes). When I was doing this i saw that there was a horn in my way. So i relocated it. See these photos.

Image #4

Image #5

Image #6

Ok, next is to locate the air duct that is behind the grille to the left of your driver side fog light or fog light cover. You can get to it and take it out if you take off some protective plastic peices underneath the front of the car. Make sure you disconnect the sensor on it before you go yanking away. Here is a pic.

Image #7

Now you will have to cut a hole in it. I used 90 degree angled peice of PVC pipe that you can find at any hardware store. I also painted it black. Take a look.

Image #8

Now take the tubing you bought (3-4 inch dryer duct tubing, or anything that works for you). Yeah take that tubing and put a tightening clamp on it, then place the air duct back into place. Be sure to pull the tubing through like in this picture, but, keep in my that the airbox should be completely out at this point so you have a lot of space to work with.

Image #9

Ok. Now comes the modification on the actual airbox itself. You need to cut a hole in it and add another corner PVC peice to connect to the tube. In this picture the #2 hole is the factory hole that was there when you took the box out. The #1 hole is the PVC peice.

Image #10

After that, cut your tubing to the right length, clamp on both ends, put every sensor, cap, plastic peice back on and then you're done.

Image #11

Image #12

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After I fogged my airbox i could feel a great deal of difference in the way the car drove. There is no longer that "plateau" or power. The power is constantly there to the redline. Its great, and i suggest anyone doing this performance mod.


Thanks. -Sean H.



***THINGS TO KEEP IN MIND***

There are risks in doing this modification. Getting too much moisture in the engine could cause hydrolock. Also be sure to connect every sensor back where you took it off.

[robcarync]

that was a pretty good tutorial! i know i did mine a little differently (didnt actually connect it to the brake duct, just had it down by the foglight), but i like the tutorial...lots of pictures make it easy to follow!

[Tyler]

Yes very nicely done,it looks somple with all the pictures.

Only thing i didn't like is that he is a Yankees fan..

[Blackie]

Yeah, that's a very nice write up! I will do this mod in a couple of weeks. You posted the tutorial in perfect timing. Thanks!

[robcarync]

i noticed you cut the whole in the airbox on a different side...

did you use anything to plug up the stock hole so it only sucks air from the new hole, or does it suck air from both?

and i like the idea of using PVC pipe for connectors....very nice!

i found that the originial tutorial for the 'fogged' airbox was a bit vague and didnt have too many pictures...i was able to get it done...but i had to really read into what they were saying...

this tutorial is REALLY nice, pictures every step of the way! i like!

[seanhearn]

Yes, i reconnected everything back to stock, which means i have 2 holes in my air box. It can breathe from the stock inlet also the new one.

I haven't driven in the rain yet and i'm a little nervous about hydro lock. If one of the guys on this forum that has a little experience in hydro lock and how easily it can happen, would you please post a response. Thank.

Oh by the way, i can feel a great amount of difference when i drive my 318ti now. I don't exactly get the gas milage i was getting before the mod. lol. (due to me constantly testing out the new feel)

[robcarync]

i am no authority on hydro lock, but i would guess that with two intake pipes, your fogged pipe wont have enough suction to really suck up enough water to cause problems...even if the fogged hose was submerged in water, it would still just suck air from the stock hose to releive the pressure...and if both hoses are submerged...then you must have driven into a lake

but even so...not submerged...id guess that neither hose has enough suction to really take in enough water to cause trouble

but then again, i am no authority on hydro lock....someone correct me if im wrong

[Tyler]

With two tubes at 2 different heights aren't you gonna get cold air and warm air going into ur engine? With one pipe there would be more suction which means more air to go in.

[rendoll911]

damn looks like the pictures are down

was gonna use that as my guide too

[myblueTI]

Very nice job. It is very well detailed and the pictures demonstrate all of the things that you need to understand. Very nice tutorial.

[m44ic]

correct me if Im wrong but if you are going through all the trouble to fog the airbox why doesnt everyone just get a couple inches of 3' pipe, clamps and a cone filter and just make a full blown CAI. its less restrictive and has a way better sound.

[myblueTI]

actually i think that by fogging your aibox it makes more power than most of the the cai. I remember seeing a comparison on it a while back ill try to find it

[Dusty]

Sweeet write up... and simple. I want to do this mod but the pictures aren't working.

[jflip2002]

Not to sound dumb or like a newbie, but what the hell is fogging an airbox

[rendoll911]

this is fogging an airbox