Modifying Your Air Induction System

Air box modification (Similar to forced induction)


You try this at your own risk, and neither the author or the club accept responsibility for you carrying out this work on your own vehicle!

Ever wanted to increase the response of your throttle, and possibly increase the performance of your car by a few bhp? Here’s a simple tip, which needs the minimum of tools, requires very little mechanical knowledge (I managed to do it!), and costs loss than £50 (typically). (Try Dialynx Performance for the filter) This procedure was carried out on a series 3 90 quattro, 10 valve, with a KV engine. This will work on all fuel injected petrol engines. If in doubt, seek technical assistance first!


more responsive throttle, induction noise


slightly higher fuel consumption (not always), induction noise. (Take your pick- if you like the sound of the 5 cylinder engine, you’ll love this!) This modification is similar to fitting a “ram air” induction system.


A free- flow air filter, such as those manufactured by K&N, Ramair, Jetex,etc.

A drill and drill bit (approximately 1″ in diameter)

Socket set (to remove air box)

The basic principal is to get as much free-flowing (cool) air into the fuel injection system as possible.

Please Click the photo’s to enlarge the image.


Engine shotTake a look at the picture on the right (this is a stock shot from a 20 valve engine)

Familiarise yourself with the layout- most 5 cylinder fuel injections have a similar layout. On the left side, you can see the air filter box, which is in front and below the throttle body and metering unit.

To the right of this is the intake manifold, and directly below that is the exhaust manifold. If you refer to the two diagrams below, FIG.A shows this layout. You need to remove the clips that hold the top of the air filter box in place. there is 1 at the airbox & metering unit.rear, under the intake (black rubber), two on the manifold side, 1 at the front, and 1 on the left side (B). (This may vary from 1 model to another). Unclip these, and carefully remove the top of the air filter/ metering head unit. DO NOT BANG THIS UNIT. Remove all the pipes from the bottom of the air filter box which attach to the front of the car. There is a bolt on the right hand side of the box, which attaches it to the main body of the vehicle. Remove this, and you should then be able to remove the bottom half of the air box assembly, with the air filter intact, looking similar to FIG.B. Remove your old air filter. This is where the drill comes in handy. Take a look at the two pictures, and compare to your own. Mid-way down the air box, you will see a ridge that the filter sits on. ALL the holes you are about to drill MUST be below the filter line. (Otherwise fitting the filter is pointless, and could result in damage.) Do NOT drill any holes on side D- this will have a negative effect, and warm air will be drawn into the fuel injection system. You need to make side A look like swiss cheese below the filter line! Obviously leave enough material to hold the filter in place! The same needs to happen on side B. This allows the fuel injection system to draw in as much cold air as possible. Only a few holes need to be drilled in side C- this is so that air flows through the box. If air only enters the box, and you have too much pressure on the plate in the metering unit, this could be damaged- and you could end up with an expensive Airbox exploded viewrepair bill! After you have drilled your holes, remove any swarf that may be left. Fit your new free flow airfilter in the position your old one was. These should come pre-oiled, and should be maintenance free for approximately 50,000 miles, although these vary from different manufacturers. Refit the airbox assembly, tightening the location bolt, and then carefully refit the top of the unit, again taking care NOT to bang the unit- if you look underneath, you should be able to see the metering head air sensor plate. Re-clip all the clips that fit it in place, and ensure all the piping is connected. Check all the fittings again, to be sure! Now comes the test! Start your engine, and let it idle for a few minutes, to check that everything works ok, and the engine idles as it did before. If you are happy with the engine, etc., and that everything is in place as it was before, take the car out for a spin- and see if you can notice the difference. I did this with the editorial 90, and quite a few have heard it since. Many think that the noise it makes is generated by the exhaust, but it is in fact the induction. (Note, BIG grin factor!)- The sound has been noted to sounding similar to a V8, although not quite. A while after I carried out the conversion, a Scorpion stainless steel exhaust system was fitted, and noticeable gains and noise were expected. To be honest, I didn’t get either! The downside is, on my part, the fuel consumption, which has dropped through the floor, but then again, I love the sound of a Five on song, so the right foot is usually buried in the carpet!