Remember that an internal combustion engine is really just an air pump. In the Spring 2006 issue of MC2 Magazine Matt Richter explains the Ideal Gas Law in simple terms. Now you’re thinking Zzzzzzzzz, but it’s really very interesting.
The stock Cooper S is able to get a 50 hp boost over the Cooper by virtue of the supercharger. By increasing the pressure of the air entering the combustion chamber, the engine is able to produce more power with the same volume of engine displacement. The increase in pressure is accompanied by an increase in temperature — hence the intercooler. The intercooler drops most of the temperature gain while retaining the benefit of the boost. Make the intake path more free flowing and increase the pressure gain some more.
The first step is to replace the stock filter element with a cold air intake (CAI). Make the pulley smaller, the supercharger spins faster (produces a bit more heat) and the pressure increases some more. The next step is to increase the amount of air available to the filter. That’s where today’s mod comes in. This works if your CAI leaves the stock cowl in place. And it’s free.
The idea is quite simple: Modify the cowl behind the air filter to draw air from the cowl vent below the windshield. The panel behind the intake is not structural, but it still makes sense to remove it first before making the modification so you don’t mess up your brake lines which are directly behind the panel. In my case, I first bought a panel to modify from MINI (part number ) so I’ll still have my original piece should I ever need to swap back. (OK, so not really free, but still cheap.)
I drilled six holes with a hand drill above the center line in the panel. Randy Webb has posted very clear instructions here. The procedure takes about 30 minutes. You remove the cover to the intercooler; your CAI; disconnect the ECU; pull out the lower air box; and pull out the panel. Replace the panel with your “holey” one and reverse the order above. At low RPMs you will not notice a change in the intake noise, but above 5,000 RPMs there is a slight increase in the supercharger scream. I actually enjoy that sound. Here’s what it looks like when it’s all back together.
Update: Jan 2015. Randy has long shut down his MINI site so I pulled the instructions from the internet archive. No photos, but simple enough to follow along.