An Easy Airbox Modification

The induction system on injection minis are quite restrictive as a result of strict emission standards and the requirement to meet them. Both the SPi and MPi engines use the same basic airbox, the only difference being that the SPi model has a myriad of attachment holes for sensors and such. The throat opening is also a bit larger to allow for the larger throttle body. Looking at the picture of the stock MPi airbox, you'll notice the tall fence at the lip that goes over the throttle body. Also, you can see the large diameter "posts" (3) that house the mounting screws. The idea is to eliminate as much of these that block the flow of air into the throttle body.

A quick and easy modification that will allow our Minis to breathe easier is to take a Dremel tool or a similar portable grinder and cut or grind away the lip and the posts to allow more air to flow into the throttle body. I estimate that cutting and grinding away the lip and posts added about 20% volumetric efficiency. I haven't measured anything on a dyno but I can certainly feel the difference in the low end torque and acceleration. The intake sounds "throatier"; adding a little growl when you step on the gas. Besides, it's a good way to spend an otherwise boring rainy or snowy afternoon.

Steps:

  1. Unscrew the 3 attachment screws at the top of the airbox.
  2. Lift the box carefully straight up to free it from the throttle body.
  3. On the SPi box, disconnect the vacuum lines to the sensors that are attached to the bottom of the box
  4. Now with the airbox out, undo the 4 clips that hold the 2 halves of the airbox together.
  5. Set aside the filter, and if it's a bit dirty, spend the extra $ and spring for a K&N replacement element. (it DOES make a difference)
  6. Cut and grind the lip and the posts as close as you dare. Be careful with that tool, especially around the sensors on the SPi box.You can sand the ground and cut parts smooth with #150 sandpaper.
  7. Blow out all bits and debris with an airhose or vacuum cleaner.
  8. Wipe the insides of the box with a damp rag and reassemble in the reverse order that you took it apart.
  9. Re-attach the assembled airbox to the throttle body BUT tighten the screws only until the top of the airbox starts to deform or cave in, then back out 1/2 turn. The post and sleeve that supports the 2 halves are no longer there, remember?

If you want to take this mod a bit farther, buy a $20 roll of Thermotec adhesive-backed heat barrier tape from Summit or any speedshop, and line the outside bottom of the airbox to shield it from the manifold heat. On the MPi, you can discard the airbox intake tube and attach a 3" dia. X 18" flexible hose, tape it securely to the airbox opening, then route it to the grille for even cooler "ram air" induction.

I found this mod, particularly on the MPi, really adds punch to the low end; but at higher RPMs (above 4500), runs out of steam, like it's getting too much air and the ECU can't supply enough fuel to the mixture. Well, that's another modification to think about...

By Ray Maramara