The BFI is a Big Intake for getting air to the engine. It’s pretty simple to do, no special tools needed. Different people have made them with different materials, I suggest start with an easy one, then you can make it better later once you know what you are doing.
Materials needed, 20mm foam, preferably the type with a foil coating on one side, it comes in a 1 metre roll, you only need 1/3 metre or less. I also used some coreflute plastic, the stuff advertising signs are often made from.
First, remove the drivers side headlight. See here
Next, remove the inlet duct which is held in by 2 screws and a bolt. Slide the duct forwards out of the air box and remove.
Remove the air filter, then the 4 nuts that hold the air box to the AFM (metal box), and the 2 bolts mounting the air box to the car. carefully lift the air box out.
Cut the front of the air box out. Drill a starter hole in each corner, then cut using a jig saw or even a new hacksaw blade held with a handkerchief (this is really high tech here). You will need to use the hacksaw blade around the bottom anyway as a jig saw won’t handle going around the duct outlet. Tidy the hole with a file, removing any rough edges.
Clean the air box out thoroughly, you don’t want little plastic filings going through the engine. Refit the air box.
Cut a piece of coreflute or foam to form a side wall between the box and the engine bay and fit it in place. Mine just fits nicely and doesn’t need anything to hold it in place. Cut and pack a small strip of foam to seal under the air box, and another two seal between the air box and the side of the car - I use a double thickness for that one.
Now cut a piece of foam to make a lid that seals across the front of the airbox, the side panel and the front lip where the bonnet seals. I cut my foam with the foil side out, then painted the top lid black to make it look less obvious. Fit the lid, it should stay in place by being a firm fit.
You have now finished the first chamber of the BFI, which removes the restrictive duct. If you have a ‘31 or ‘32 you may want to construct the 2nd and 3rd chambers to get more air to the air box, as I have a UZZ30 which has no clutter below the headlight, I haven’t yet bothered.