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....
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