A trick borrowed from Porsche Motorsport. This is a variation on the theme, basically allowing more air to the intake. Porsche realized that the stock airbox severely limited air intake to the filter by only having a small bell venturi that drew air into a large airbox with a panel filter. What the Motorsports division realized on the cup cars (which had to have the stock intake) was that if they drilled holes into the box, they greatly increased the allowable mass air flow. This made 5-10 horsepower on the 3.6L aircooled flat six.
The MINI has an intake in the cowling (the area between the bonnet and the windshield) that has one small intake hole into the lower portion (the dirty air side) of the airbox. If you replace the factory element with the aftermarket filters available (any of them will allow this modification to provide benefit), you can greatly increase the flow to the filter by modifying the factory plastic backing plate that separates the engine compartment from this "dead space" area directly adjacent to the factory cowl fresh air inlet.
I can already hear some of the questions, and I'll try to preempt as many as possible, but let me know if you have others.
1. The backing plate is not structural, so the modification has no safety ramifications in the event of an impact.
2. The dead space is not critical as far as temperature is concerned. The transfer of engine compartment heat to this area is negligible anyway.
3. I washed and waxed the car right after completing this mod, and no extra water was in the engine compartment - most of it comes from the intercooler inlet.
4. There is a small increase in supercharger whine (above that caused by the intake change) but nothing intrusive at all.
5. Be certain to follow the instructions and remove the partition (backing plate) completely before modifying, as there are brake lines directly behind the plate.
Time required
about an hour - less if you are familiar with the filter set-up.
Tools required
small standard screwdriver
phillips head screw driver
10mm socket with extension
8mm socket
#25 torx
15mm wrench
2" holesaw
hand drill or drill press (drill press makes it a bit easier)
new hose clamp - same size as the one that came with your filter kit
Step 1: Locate the airbox and become familiar with it's surrounding attachments, as well as the partition itself and its attachment points.
You can see the partition (the black piece) behind the airbox.
Step 2: Remove the weatherstripping. It just pulls off and pushes right back on.
The stripping is removed in this picture and laid on the cowl. It came off of the plate we'll be modifying right in front of the brake reservoir. You can also see the stock small inlet at the bottom of the airbox in this shot.
Step 3: Remove the ECU cover. The ECU is attached to the lower portion of the airbox, and we'll be removing it to gain access to the lower attachments of the partition.
Step 4: Remove the ECU plugs. Look at the ends of the plugs. They have a pull tab that disconnects them form the ECU. Pull the tab all the way out and it will do most of the work for you.
The plug is removed in this shot. You can see the pull tab which is in the extended position in this picture. You also will pull the grommet that keeps the ECU plug attached to the partition.
This is what the ECU will look like with both plugs removed.
Step 5: Remove the battery terminal from the air intake. This differs slightly from model to model depending on which you have, but it is painfully obvious how to do it when you locate the terminal.
Here the terminal has been removed from the air intake heat shield.
Step 6: Remove the passenger side airbox bolts using the 8mm socket (or 10mm if you have the Pipercross). There are two, a top and a lower. These also attach the partition we are working with to the center partition.
Here, the heat shield has already been removed, but the bolt holes are clearly visible to the left.
Step 7: Remove the partition plastic screw in the upper driver side corner. This is a phillips head screw. Note: the early production models do not have this screw, but the partition is bossed for it.
There is a screwdriver in place in this picture.
Step 8: Remove the front 10mm bolt on the lower airbox.
This picture shows the bolt being removed. It is directly in front of the fuse panel.
Step 9: Remove the clamp that holds the "snorkel" to the intake tube for the lower airbox. The best way to do this is with the small standard screwdriver. Put the blade into the crimp of the clamp and pop it off. It will then just pull apart and come right off.
Step 10: Remove the #25 torx bolts that hold the forward part of your aftermarket intake down to the lower airbox. With the Pipercross and Rogue/MINI-Madness filter, these hold the forward part of the heat shield down. For the BMP, these are the forward mounting bolts.
Step 11: Remove the #25 torx bolts that hold the intercooler cover down. The rear bolts are longer for the reinstalling phase.
Here's the intercooler without the cover - pretty cool!
Step 12: Pull the airbox stock "dead space" intake hole grommet through the hole and pull the lower airbox back slightly. This will remove it from the lower mounts shown here:
Step 13: Remove the lower portion of the airbox by pulling up and out, using caution for the wire bundle that was attached to the same forward 10mm airbox bolt. If you move this wire bundle out of the way, then hold the intake hose out of the way, the box should pull straight out.
Step 14: This step is only for rally light equipped cars. You have to remove the hot wire to the fuse panel by removing the fuse cover and unbolting the 10mm bolt. Then fish the wire through the partition (disconnect the battery first).
Step 15: Remove the lower airbox mounts by unscrewing the base 15mm nuts under the rubber mounts.
Step 16: The part all of this work was for! We can finally pull out the partition. It just wiggles right out. Here's what it looks like once out:
Step 17: Now the mod. You will take the 2" holesaw and make six holes in the partition above the line of the lower airbox cover, so the intake air is available to the filter. This is easier with a drill press, but you can do it with a hand held as well - just be steady. Here is the finished product:
Now reverse the steps for the reintstall of the partition and lower airbox. This is when you need the new hose clamp - for the "snorkel" to intake hose connection. Here's what the completed project looks like in the car - subtle, but trick!
The disassembly car has the Pipercross intake, while the finished project car shows the MINI-Madness intake.
If anyone has any questions, feel free to call me at the shop. It may benefit others if you post your question on the thread, too.
Randy Webb
Owner, Webb Motorsports
randy@webbmotorsports.com
720-841-1002
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MINI-Motorsport
Yet another of your great step-by-step write-ups! 
DrkS/DrkS "S", Anthracite Headliner, LSD.
