Air Filter Enclosure Title
Procedure for fabricating an air filter enclosure for a Toyota Celica GT-Four ST185

A special thanks to Saffo, a GT-Four owner in Australia. The look, and most of the construction, and materials of the air box, is based on his design. To make it easy for those that wish to print this information, a simple text only web page is offered here.

What You Will Need
1/8th Inch Chipboard Piece Of Aluminium Scissors
3/8th Inch Perspex One Long 10mm Bolt & Large Washer Jig Saw
Fiberglass & Clothe or Matting Wood Glue  Screwdriver, Ratchet & Sockets
8 Small Self Taping Stainless Screws Cardboard 3 Inch Aluminium Ducting
Files & Sandpaper Exacto Knife Black Paint & Masking Tape
Two Short Stainless Bolts & Nuts Vacuum Hose Brasso Polish

Note: For my application, there is no Air Flow Meter, so some means of making an adapter will have to be applied if your car uses the stock Air Flow Meter.


Obviously this air box can only be done if your battery has been moved from the stock location. The first step would be to remove everything from the space that is intended for your air box. Once this is done you need to measure the longest distance you have from left to right, and the longest distance you have from back to front of the space for the air box. Cut a piece of the cardboard in a square that is approximetly this size. For example, if your right to left distance was 8" and your back to front distance was 9", you would cut a 9" x 8" square piece of cardboard. Take this piece of cardboard and place it horizontal over the space. Obviously it will not be able to go down into the space, so you next job is to start cutting of pieces (in straight lines) until the cardboard can fit into the space and be rested down on the sheet metal of the car that used to support the battery. If you look at the picture at the top of this page, and note the shape of the clear perspex cover, then that is similar to what your cardboard template should look like. It is important that as you are lowering the cardboard down, you keep it going straight down as vertical as possible. Don't go crazy making to many different angled cuts, the less sides the template has the less work you need to do. As you can see, mine only has seven sides. When your cardboard fits properly, you will need to take off about 1/4" from all the sides. The reason for this will become clear later.

Making The Pieces

Take your template and use it to mark out a piece of the 1/8" chipboard, and then cut the chipboard to the shape. Once this is done, check your fitment in the space for the air filter box. If it's fits fine, you have completed the base of your air box. It's now time to do the sides which are relatively easy to do. If your box has, for example, seven sides then you will need to make all seven sides the same height, but the width will be determined by the length of the individual sides of the base.  You have to make sure that you don't make the sides to tall, otherwise the hood may not be able to close. The box when finished, should still have a small space between it and the hood. You can get an idea of how to make the sides by looking at the diagram to the left. The diagram shows the seven side pieces, as well as showing how they would be arranged on the base. The only part that you have to pay attention to, is the edges of the sides that meet with each other. Because the pieces are being fitted at various angles you will have to file the edges of the sides so they fit almost flush with one another. Getting them absolutely flush is not necessary, as the glue can fill imperfections. Once all of the sides are finished and are fitting together well on the base, it's time to move on to the construction of the air box.


The first part of construction is simple, just pick one of the sides, and glue it onto the base. The trick here is make sure that the side is as close as possible to a 90º angle from the base, ie the side should be vertical. To accomplish this, use either a set square, or an object that is known to be square to make sure the side is vertical, and then allow the glue to dry. Pick your next piece and glue it to the base in the same way. Don't forget that this time, you have to put glue on the edge of the side you are now applying so that it can bond with the edge of the side you previously glued to the base. When the glue is dry, repeat the previous step with all the sides, until you have completed the box. At this stage you can use some sandpaper, or use a file, to smooth out any areas where glue may have squeezed out. The next step is to add some small blocks of wood that will be used for the self taping screws that will hold the cover on. The length, width and height of these blocks are determined by the size of the self taping screws you are using for the cover. Once you have made your blocks, you can glue them on at the various points around the top of the box (refer to the diagram on the right). When the glue is dry, drill a hole in each block for your self taping screws (remember when drilling a hole for a self taping screw, use a bit the thickness of the shaft inside the threads, not as thick as the threads). At this stage it's a good idea to mark and drill the hole in the box that will secure it the flooring, where the battery used to sit. Take a 10mm bolt (you will need a long bolt), and screw into the existing hole in the floor until the head of the bolt is just a little higher than the flooring. Now place your air box in position, and press down firmly on the base. When you remove the air box you should be able to see a mark left in the wood from the bolt head. Drill a hole for the 10mm bolt in that location.

Preparing  For The Ducting

If you have a look at the area inside the engine by the fender wall, you will see a 2" hole. This hole leads to the stock intake resonator box. You will no longer require the resonator, so you will need to remove it. To remove it, jack up the front left hand side of the car, place a jack stand under the car, remove the wheel, remove the fender liner, and then remove the resonator box under the fender. You will need to enlarge the 2" inch hole to a 3" hole. Unless you are very skilled at this sort of body work, I would highly recommend you get someone in the automotive body repair business to enlarge the hole. If you plan on attempting it yourself, you will need a small air saw. Once your 3" hole has been cut, you need to prepare a rubber insulator so that the tubing you run through doesn't get cut or worn out by the metal edge of the hole. Find a suitable size hose, I found that vacuum hose worked on mine, and measure a length of vacuum hose to fit on the inner circumference of the 3" hole, then add about 1/2", and cut it. Take an Exacto Knife and cut a slice down the length of the hose, and then slip the vacuum hose onto the inside of the 3" inch hole (refer to diagram on the left). It will be too long so mark the point where the two ends should meet, and cut that piece off. Once the vacuum hose is fitted, test fit your 3" ducting to see if it can pass through the hole. If the hole is too small you will need to remove the vacuum hose and start filing out the hole with a round file. Once you have filed what you think is enough, install the vacuum hose, and test fit again. Keep repeating until the ducting fits snugly through the hole. You don't want it too tight, but you also don't want it fitting slack. You will find that at the end of this procedure your vacuum hose is now a little short, and not meeting at the ends, so you will have to make a new one.

Fiberglassing the Air Box

This is where I answer the question a few might have been asking along the way, "Is this guy nuts, wood won't last in an engine bay enviroment, will it?" Indeed it won't, but the box you have built is just a "skeleton". You need to now coat the box inside and out with a layer of fiberglass, hence the reason for the 1/4 inch taken off the base template, is too allow space for the increase in thickness created by the fiberglass coating. I do not intend to go into any detailed information on the fiberglassing process. I chose to have mine done, rather than doing it myself. I can, and have worked with fiberglass before, but I don't like it, not to mention the itching it causes.

Finishing Up The Air Box

Note: From this stage on I recommend you wear a long sleeve shirt, long pants/trousers, shoes, rubber gloves, and a filter for your nose and mouth. This will dramatically reduce the chances that you suffer days of itching, and possible respiratory problems due the microscopic particles of glass in the fiberglass.

Let's first deal with that hole you drilled for the 10mm bolt that secures the base of the air box to the flooring for the battery. Obviously, it's no longer a hole, because it's covered with fiberglass. However you should still be able to see the hole through the fiberglass, so drill it again, but use a slightly larger drill bit this time. This will allow you to have some play when finally installing the box. Once that is done, you need to cut a 3" inch hole in the side of the air box by the fender wall so the 3" inch ducting can enter it. Again, as described in the "Preparing For The Ducting" section, you will want to use vacuum hose, or a slightly larger hose, if the vacuum hose isn't big enough to fit, to place on the inner circumference of the hole. Once this is done test fit your ducting. Again it should fit snug so that it can be pushed into the hole but doesn't come easily. Once the ducting fits snug in both the hole in the fender wall and the hole in the air box, then it will stay in place without having to secure it any other way.

I no longer need to use the Air Flow Meter (called AFM from here on) because the Autronic engine management system uses speed density to calculate load, and doesn't require it. This means that all I had to do was cut another 3" inch hole where the intake for the turbo could enter. If you are still using the AFM, you will need to find a way to build and adapter to accept the AFM. One possible solution, if you already have a performance air filter adapter, would be to remove the four studs in the AFM, and replace them with longer studs, then cut a hole to the square size of the opening on the AFM in the air box. You will need to make sure the surface of  the box, on either side of the hole, is very smooth, and then get some gasket material, and make two gaskets for either side of the wood. Then all you need to do is bolt the AFM and adapter onto the box, with a gasket between the AFM and box, and a gasket between the box and the adapter (refer to diagram). With this set up you will need to purchase a piece of hose that has a section designed to flex (usually looks like a bulge in the hose at one end), otherwise you might break the air box, as it will be mounted solid to the chassis, when the engine rocks back and forth under power and throttle release.

Making A Bracket For The Air Box

The single 10mm bolt holding the box to the flooring is not enough, so you will need to make a bracket out of aluminium. This is relatively easy to make, and can be seen in the picture at the top of the page. Using a piece of cardboard, cut one edge so that it fits the frame behind the headlight, where two ten bolts are located. Once this is done, mark the location of the two ten bolts on the cardboard, and punch a small hole through the cardboard in the two locations. Next place the air box in position, and screw in the long 10mm bolt (use a large washer, it will offer more area to clamp down on) that fits through the bottom of the air box to secure it the battery flooring. Now take your piece of cardboard and bend it 90º so that the top part fits neatly in the space behind the headlights, and the back part of it touches the side of the air box. Once you that done, you have your template to make your bracket. Take a piece of aluminium and using you cardboard template (straightened back out), mark the shape to be cut, and the position of the two holes to be drilled for the two 10mm bolts, and cut the shape out of the aluminium, then drill the holes. Drill two holes in the other side of the bracket (refer to diagram). Again, using your template as a guide, bend the aluminium at the correct spot by 90º. Next, place some two sided tape on the back of the bracket where it would attach to the side of the air box (but don't block the two holes), and peel the backing off the tape. Loosen the 10mm bolt in the bottom of the air box, and push the air box a little in the direction of the rear of the car. Bolt the bracket onto the frame behind the headlight, and then push the air box toward the front of the car until the side makes contact with the bracket. Now remove the bolts from the bracket holding it to the frame then remove the 10mm bolt in the bottom of the box, and remove the box. The box should come out with the bracket stuck to the side by the two sided tape. Now just mark where to drill the holes in the air box using the holes in the bracket as a template. Remove the bracket, remove the two sided tape, drill the holes in the air box, and you are done. You can now either paint the bracket, or if you want you can sand (start with about 800 and work up to 2000) and then polish it with Brasso.

Making The Air Box Cover

Perspex usually comes with a paper backing on one side. Without the paper backing, it will be difficult to mark out the shape to cut. Take your piece of perspex and lay it flat on a piece of cloth (perspex can get scratched easily, so be careful, so be careful when working with it), with the paper side up. Now turn the air box upside down  and place it on top of the perspex. Trace around the air box, to mark the shape you need to cut out. When cutting perspex you can use a jig saw, but, you have to work in small intervals, or else the perspex will start to cloud from the heat. So start, with the jig saw on the slowest speed, and drill about an inch or two and stop, wait a few minutes for the blade to cool on the saw, and cut another inch or two, and stop. Keep repeating this process until you have cut out your air box cover. There are some small blades designed for cutting tight shapes, I recommend you use one of these blades. At this stage it would be a good idea to check that the top of your air box is flat. Place the air box on a piece of glass and look for spaces. If you find spaces, file the necessary areas and recheck. Once this is done, sand the top edges of the box until they are smooth. Place the perspex cover on top of the air box in the correct position, and then using the holes in the small blocks as a guide, mark the spots where you need to drill holes for the self taping screws. This time you will be using a bit just slightly thicker than the threads of the self taping screws. I would recommend starting the holes with a small hand operated drill, and a small bit, and when you are ready to drill the actual hole, make sure you use a very sharp drill bit, as a dull bit will create heat, and cause you problems with your perspex. Once all the holes are drilled, test the fit of the cover by screwing it onto the air box, but do not tighten the screws right down. If there are any holes that don't line with the holes in the blocks, you can use a round file to enlarge the hole in the direction needed. Next secure the cover to box, by tightening the screws (don't use a lot of force), and sand the edge of the perspex, until it is flat with all the sides.How you choose to finish you cover is up to you. You could do it like I did, and just have a border of black, or paint the entire top black, etc. Remember to paint the inside of the cover, not the outside. The only other thing you need to do after painting it, is too apply an adhesive tape to the edges of the perspex where it makes contact with the edges of the top of the box. This is to stop the cover sticking to the box and pulling off your paint. I used electrical tape, but I have seen a felt material with an adhesive backing that would probably be less likely to cause minor sticking. The last step is to paint your air box, and bolt the bracket onto it.

That's it, your air box is done.

If you would prefer to make an air box, that would be located in the stock air box position, then click here for install instructions for my old air box.

Back To Modification Page

This page was created by Dennis Heath.
If you should wish to ask a question about the GT-Four/All-Trac, you can join the GT-Four Mailing List, where I, and many others with GT-Four's, might have your answer. For information on joining go to

Please note that I am not a mechanic by trade, and that any information offered on this web page is free and without guarantee. Should you choose to perform any of the procedures listed on this site, you will be doing so of your own free will, and I will not be held responsible or liable for any damages that might occur from using information obtained here.