Airbox Modification

Modifying The Airbox in your Turbo (B21FT)

There are many benefits to modifying your airbox. Since the K-Jet system is mechanical fuel injection, more air means more fuel, and so opening up the airbox will give you more power when your engine needs it. Another benefit is that the airbox takes up less space in the engine compartment.There are several ways to modify your airbox. has a few, which is where I originally started. I wanted to modify it without having to cut the airbox, drill holes in it, or change its shape with fiberglass. Since I’m not building this car for extremely high horsepower, I’m going to keep the original airbox, as it can let in enough air to suit my needs. Some people, however, just get rid of it entirely in favor of a custom intake and filter assembly.

Removing the Airbox

That said, the first thing you must do is remove the airbox from the car. It is located in the front-passenger’s side of the car, behind the headlight in the engine compartment. It is underneath a “medusa” of fuel lines and hoses. To get it out, first remove your coolant reservoir tank (the white plastic tank behind the airbox) by disconnecting it from it’s mounting bracket and then removing the mounting bracket itself (using a large phillips screwdriver).

After this, detach the big rubber air hose coming off the top of the airbox. There is a large metal ring around the bottom of the hose where it attaches to the airbox. Use a screwdriver to remove this hose clamp and gently pull off the rubber air hose and set it aside. You might also want to remove it entirely from the car to free up space.

Next, unclip the four brass-coloured metal clips around the airbox below the tangle of fuel lines. Once they are detached, you should be able to lift off the top part of the airbox. Carefully set this piece aside on a towel. I turned mine upside down and set it on a piece of carpet that I had lain across the top of the engine. Be gentle and don’t force anything and you’ll be fine.

Now your air filter should be exposed, and there will be a T-shaped pipe about 2.5 inches in diameter coming off of it on the driver’s side of the airbox. This entire assembly must be removed. It includes a metal-coated pre-heater hose that runs to the exhaust manifold (near the turbocharger), the T-shaped pipe that attaches to the airbox with two screws, and the plastic pipe that runs from the airbox out to the front of the car through a hole in the sheet metal behind the headlight.

Once your airbox is stripped down, you can detach it from the sheet metal of the car. Directly underneath the airbox on the dirty underbelly of the car are three bolts. Unscrew these (I think they were 12 or 13mm) and then you can pull out the bottom of the airbox.

Now that you have it out, you can choose how to modify it. I recommend the following method:


I went up the road to Diamond Hardware, which is a big chain home improvement store similar to Home Depot, Lowe’s, and the like. After a bit of searching, I bought the following from the “Sewer Pipe” section:

  • (1) Fernco Inc. 2″ Quik Elbow – $11.29 (part #QL200) (made out of hard rubber, comes with hose clamps)
  • (1) 2″ x 1′ piece of ABS black plastic pipe – $0.50 (which I cut to fit in my car)

It turned out that the rubber elbow fit perfectly into the airbox as shown below. I didn’t even need any glue or screws, it was a really snug fit. The ABS pipe fits perfectly into the elbow and is secured with one of the hose clamps that came with the elbow. I sawed it off at about 6″ long (measured at the longest length) and at an angle, so that it would pull air in from the grill.

As you can see by the pictures, the elbow works perfectly and the fit is much better because it is rubber rather than plastic. It is a very hard rubber, but still bends upward enough that it allows the ABS pipe to fit through the hole in the sheet metal behind the headlight. Notice also that the elbow does not get in the way of the air filter.

Installation of the airbox is essentially the opposite of removal.Of course, if you DON’T have a Turbo Volvo, then your airbox is much differerent, and is located on the other side of the engine compartment. However, the principles are the same, and it shouldn’t be too hard to perform the same sort of modification to your N/A car.

Pieces and Assembly

Fit and Finish

Pics in the Car