Cheapest Modification Ever?

Having looked carefully at various induction modifications and read up on the 'false' chamber put in the air box I decided to experiment with my own modification. Basically, I removed the false chamber and plugged the hole with an aluminum piece; joining the offset rubber hose (originally fitted to the chamber) to the plug. From what I can see this is pretty much what the Fabspeed, DesignTec etc options do and charge you about £200 for the privilege! My understanding is that the chamber is put there to reduce induction noise thus comply with the EU. The net result is that the induction noise is increased (sounds very exciting at full throttle) and feels like it is a tad more responsive; probably because the car can breath properly. I should say at this point that despite being a qualified engineer I am no mechanic so if my car is about to blow up as a consequence of this mod then please let me (and Fabspeed etc) know!! I haven't changed the air filter bye the way. Mark

Any pics for this mod... I'm always up for mods that actually work...

Yes please - chop, chop!

Okay; I'll see what I can do. It means I'll need to take the air box off as you can't see any difference from the outside, which is a good thing I think.

Having checked various technical and sales websites it would appear this mod is still active. My understanding still is that the chamber is a noise reduction and and air restriction feature to meet with EU regulations. Fabspeed and others (see photo) effectively replace this 'muffler' with a blanking plate and then add a brightly coloured hose!

Another photo of the before and after parts. Basically I retained the original black hose and created my own blanking plate that I plugged into the small offset hose so that no air is drawn through this section. Eveything else (MAF) is retained.

On RennTech there's quite a lot about this mod. Amongst the many there's this one:- http://www.smiley.net/orangecap.htm Also, link to the firm which markets the plugs mentioned:- http://www.niagaraplastics.com/flash.htm - part no. 273-1P

Although this plugs the hose they don't mention removal of the 'muffler' silencer or chamber (take your pick) which is what fabspeed et al do and what I've done (for free). Basically the 'plug' I made fills both the hose and the hole left in the air box without filling the gap left by the chamber. When you do this you really do get a more urgent feel and quite a 'wail' from the engine at full throttle. I appreciate, however, that this is a bit of a boy racer thing but then aren't we al to some extent?

This is a bit reminiscent of the 993 airbox mods, where you cut a bl**dy great hole in the airbox - to make it exactly like the Porsche Motorsound air box. It seems to me like this is purely a sound mod - nothing wrong with that I hasten to add - just don't expect to add 10-15 bhp! The box that you are cutting out is a resonance chamber. There could be several purposes for this as outlined by Craig Lord on Renntech: http://www.renntech.org/forums/index.php?s=&showtopic=1518&view=findpost&p=6688 . There is also a muffler in the lower part of the airbox, which is what (I think) was for noise reduction to comply with regulations. Fabspeed still make their "cold induction kit" ( http://www.fabspeed.com/996_GT3.html ) and I think the backfire issue Steve (skyline1) refers to is more on the air-cooled engines. There are some good opinions of the modification in this thread from Renntech: http://www.renntech.org/forums/index.php?s=&showtopic=9437&view=findpost&p=46088 . Particularly interesting is the note from PCGB Forum member Ian Wallwork who has tried this mod on both 3.4 and 3.6 engines, and got better results on the 3.4. There is also a very good DIY with photos here: http://www.renntech.org/forums/index.php?s=&showtopic=6422&view=findpost&p=31870 . Mark has retained the original hose, but removed the resonance box and plugged the hole with an aluminium plate - am I correct Mark? Personally, I definitely wouldn't fit an oiled filter like the K&N, but this modification retains the original filter. I wish I knew the exact purpose of the resonance chamber - if it is purely to reduce noise then it would be worth thinking about. Just my 2p worth.

I thought I might clarify my post on the Renntech site regarding the airbox mods. On first acquaintance, it seems like a good idea to remove the silencer inside the airbox - it looks like it must impede airflow. On this basis, I tried the Fabspeed kit on my old 3.4 whilst on standard exhausts and was very pleasantly surprised. It produces an intoxicating 'yowl' at high rpms with no apparent downsides. No discernable increase in power though. Odd but what the heck? I later added Fabspeed exhausts and these complimented the airbox mod well for an awesome sounding car. Once I changed the car for the 3.6, on went the Fabspeed airbox mod. Not what I expected at all. Instead of a melodious howl, there was a discordant drone and the throttle response became uneven. This persisted even after resetting the DME. Off came the mod. I then fitted PSEs to the car. Superb! So I just had to try the PSEs + Fabspeed airbox to see if they worked together. The combination was even worse than with the standard exhaust! The drone from the airbox clashed completely with the sound of the PSE - the car sounded terrible. The Fabspeed mods have been ditched ever since. I have a suspicion that the airbox silencer does more in a 3.6 than just keep the intake quiet. There are so manyy variables in this engine - cam timing, cam phasing, resonance valving in the manifold etc. I think removing the intake silencer box on a 3.6 causes unknown problems with this complex resonance system unless other items are modified to suit. I wouldn't have a clue what these would be. This got me thinking about the design of the airbox in general. One of the big problems of the rear engine layout is high intake temps. Basic Physics; the higher the temp, the lower the air density and the lower the power output. Yet if you look carefully at the airbox, Porsche has drilled several dozen holes in the base on the drivers side. This is very strange as extremely hot air will be drawn in through these holes - they are immediately above the catalytic converters. So why has Porsche drilled all these holes that exacerbate the intake temp problem? I believe the answer is that the most restrictive part of the airbox isn't airflow around the intake silencer, it's the x-section area of the intake snorkel - it appears smaller than the circular intake for the engine in the airbox lid. So I tried a mod that blocked off the hot-air holes to reduce intake temp (in theory improving charge density, hence power) yet the engine felt strangled at the top of the rpm range. Open the hot-air holes again and power was back to normal - totally against logic unless the snorkel can't flow enough air! A further check to attempt to 'prove' my theory was to examine a 3.4 airbox and guess what? There are less additional hot-air holes. Less airflow is required for 300BHP than 320BHP and this is exactly how the airboxes are made. My point regarding all the above? If the snorkel is the biggest point of restriction, modding the intake silencer will have no effect at all on power, just sound. As most kits address the silencer, not the snorkel, it explains why most give little or no power increase. I think a dual snorkel airbox with no hot-air holes or silencer would give big airflow and reduce intake temps spectacularly. Now I have a hunch that if any enterprising engineer out there can make one of these we really will see some power increases....

An impressive response Ian, most informative. As you will have gathered mine is a 3.4 and I agree that the sound with the chamber removed is particularly enjoyable and just feels right for the car. I suppose the point of my original post was that I did my mod for free (effectively) and it was this fact that I thought was most worth noting. Having said this I've enjoyed the discussion and feel much better informed. Interestingly, I'm having some new exhaust silencers fitted next week and tips. I will be interested to hear how the relative sounds compliment or clash with each other. The good thing about my air box mod is that it will be free and easy to reverse if not. I'm having a pair of Dansk exhausts fitted with their large oval tips. Should sound and look good - I hope. I have heard the Dansk boxes before and they are not too loud (louder than standard) just nicely tuned. Finally, why is sound so important? Well, after smell, hearing is our most evocative sense in so much that it stirs emotions and fixes and reminds us of memories; providing us with life's soundtrack if you like. I should also say on this, however, like with most things, some people place too much emphasis on quantity rather than quality. Loud is definitely not always better. I'm hoping my car will sound more like a symphony than a thrash band!!!

Ian - some really interesting observations there, thank you. So it looks like it only really suits the 3.4. Shame. Mark - I hope I didn't sound negative about this mod. I quite fancy giving the 'orange plug' method a try, to see what it sounds like. I have a 'sports' exhaust, so I'd like to see what effect it has before going the whole hog.

Richard - I certainly felt no negativity from what you had to say. I really enjoy the discovery associated with owning "˜classic' cars, something I missed with the BMW M3, which for me lacked soul. I also like to hear from people who know their stuff and I guess you are in this category so forgive me if I continue to pick your brains and those others who "˜know'. Anyway "" enough about induction for now. I'll post again when I've had the exhausts fitted, particularly as I'm using a small independent guy who when I went to his workshop had wall to wall 911s on the go so I'm hoping I've struck gold.

Now that you aren't driving a BMW people will let you out at junctions. [ ] Let us know how it sounds in conjunction with the exhaust. Who's the indy, by the way?

I'm using Addspeed in Horsham. So far I've had a very positive response from them. I'll keep you informed.

I carried out a similar modification on my 3.6 C4S airbox well over 18 months ago and have had no problems with it. To me, the chamber is a resonance chamber...and removing it from the TB side has no ill effects. Also, removing the chamber (I cut it all away leaving the orignal mounting lugs and plugged the outlet with a rubber 'foot' designed to go under a piece of furniture sitting on a wooden floor, which was suitably wedged in from the airbox side, glues and sealed...thus leaving the outlet 'tube' sticking through the airbox lid for the standard inlet hose to connect to) gives a larger amount of air within the filtered side of the airbox for when you open the throttle fully and the engine gulps in the air. Sounds great...and has given me no problems.

Great minds and all that (till our cars blow up I suppose). I had considered cutting the chamber but thought better of it in case it all went wrong. Interesting that it appears to works on the 3.6. It does make a good sound and now that I have a pair of Dansk exhausts fitted I'm mighty impressed with the overall sound package.

I bought a second airbox before hacking about...just in case! Mine works well and gives the PSE an added back-beat to it's soundtrack. I like it...a lot!

Regarding my thoughts on a dual-snorkel airbox with no hot air holes or silencers (above) just look what Porsche has fitted to the 997S Power Kit! Perhaps I really was on the right track? http://www.porsche.com/uk/accessoriesandservice/personalisation/tequipment/911-997/sport/?focus=leistungssteigerung911997&hideintro=true

Oooo. Looks amazing! Surely they must owe you a few quid for thinking that one up?

[ ][ ][ ][ ][ ] ! I wonder if the carbon airbox could be adapted to fit a 996 and if it would show any gains?

The carbon airbox looks the business to me. What do they do with the engine cover? There must be a mod to accept the other snorkel. I seem to recall someone on the forum doing a 996-997 body conversion - it would be interesting to know if the engine cover from the 997 is the same shape as the 996. Edit: Looks like the 997 spoiler already has cutouts for dual snorkels:

To add my 2d's worth. I got the fabspeed mod for the tiptronic in the US a couple of years back and fitted it to my 3.4 Tip. The kit doesn't include the new red pipe as this is excluded for the Tip. The removal of the soundbox and replacement with a blanking thing does make the car sound better and IMHO, there is a power increase - it certainly feels quicker. The removal of the round filter thing and replacement with a thin round piece of plastic seemed utterly pointless and I think has now been discontinued by Fabspeed as the plastic filter/gauze is supposed to measure airflow or something. I have ordered some Naiagar plastic samples as I am in the US next week (hopefully will get 8) so if anyone wants one just email me at [email removed] or call me on 07710 835 837 and I will send you one. I think this is a nice quick and cheap mod which makes a bit of a difference. Views?

Got some plugs from Niagara and have plugged the auxiliary pipe as described. Difference in performance? Can't say there is. Difference to sound? Yes, not massive, but a pleasantly satisfying improvement.