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Building your own airbox

A simple to make, high-flow design

by Julian Edgar

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At a glance...

  • High flow, compact design
  • Easily made
  • Can be upscaled or downscaled to suit application
  • Uses cheap, high quality paper filter
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There are lots of reasons why you might want a new airbox. You might have done an engine swap and found that there wasn’t room for the original airbox. Or you’ve added a turbo or supercharger and found that the original airbox no longer suits the revised plumbing. Or you have an engine boasting a lot more than standard power and the original airbox is now posing a major restriction.

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If you need a new airbox, you can just use the airbox from another car - in many cases, that’s a simple, cheap and effective approach. In fact, shown here is a Holden airbox being used with a Peugeot diesel.

But often existing airboxes are not the correct shape to fit in the space, or use small filters, or have inlets and outlets in the wrong places.

In that situation, it’s best to make your own airbox.

Designs

Any custom-made airbox needs to satisfy these design characteristics:

  • Uses a commonly available filter (why make life hard and expensive for yourself?)

  • Flows well

  • Filters well

  • Is straightforward to make

  • Has inlets and outlets positioned appropriately for the application

  • Fits in the available space

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Back in Building and Testing an Airbox we built an airbox that used two aftermarket cone-shaped filters. The box was made from aluminium sheet and was pop-riveted together. It used the inner guard to form the floor and one of the walls of the box.

However, that approach would not have been effective in the car for which the new airbox was made. In this case, the airbox had to be positioned over the gearbox in the transverse engine front-wheel drive, and so the box had to seal in isolation, rather than against existing panels.

Initially, airboxes from other cars were examined to see if one would fit in the required space and have inlets and outlets in the right places. But the only factory boxes that would fit were from very small engines and so would not likely have the required flow capacity.

So how to fit a filter with a large area into a small volume? One airfilter shape that would fit in the available space was a long and thin rectangle. But this would have needed in turn a long, thin box that separated into two halves (so that you could change the filter) – and such a design would be pretty difficult to construct. And anyway, a long and thin filter gave a fairly small filter area.

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So after lots of thinking about different options, this design was developed:

  • Cylindrical paper filter element, 290mm long x 118mm in diameter, normally fitted as standard to a Saab 9000

  • Cylindrical airbox, made from 150mm diameter, thin-wall truck steel exhaust pipe

  • Flat end plates, cut from steel sheet

The design was MIG welded together (with just a little bit of brazing as well). If you don’t have any welding gear, it would be easy to take the finished parts to a welder for this process.

Steel (rather than aluminium) was used because of the ready availability of the thin-wall tube – the truck exhaust pipe. But if you have the gear, you could roll the cylinder from aluminium sheet and use aluminium end panels – the end result would be lighter.

Note that TIG’ing aluminium is more difficult and slower than MIG welding steel, and so if you need the services of a TIG welder, you’ll pay more.

Building it

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The starting point was a 300mm length of 150mm diameter, thin-wall truck exhaust pipe. If you live near a truck exhaust workshop, you may be able to pick up a short length as a low-cost or free offcut. If that’s not the case, any major exhaust shop can order in a 1 metre length of the tube. The 150mm tube gives about 15mm air space all around the cylindrical filter.

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A depression was formed in one of the walls of the tube to give the required clearance between the airbox and the starter motor power terminal. This depression was formed in the following way:

  • Discs that were a tight fit in the ends of the tube were cut from chipboard

  • The tube was fully filled with sand, kept in place by the chipboard discs and some temporary tape

  • A 50mm tow ball and a hydraulic press were used to form the depression in the tube wall

Note that both the sand and the chipboard endplates are needed if the tube is not to be crushed.

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At the outlet end, a ring was cut from thin steel plate and then welded into place. This ring was then drilled and nutserts (arrowed) installed. (The nuts could also have been brazed into place prior to the ring being installed.)

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A 60mm hole was cut in the removable lid of the airbox. To aid airflow from the filter into the exit tube, a plastic bellmouth was glued into place. The bellmouth was cut down from a flared port salvaged from an old subwoofer enclosure. In addition to the glue, the bellmouth is held in place by being firmly sandwiched between the filter and the lid.

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A cone adaptor (arrowed) was formed in steel exhaust tube to adapt the 60mm airbox outlet to the 50mm tube that, in this case, runs to the turbo inlet duct. The adapter was formed using a cone-shaped steel mandrel (as it happens, a plumb-bob!) and the hydraulic press.

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In this installation, an unusually long exit tube was welded to the lid. The airbox lid is held in place with three screws that pass into the nutserts installed in the inner ring. A thin rubber gasket sits between the lid and the ring.

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At the other end of the airbox, a steel endplate was welded into place. To locate the filter at this end, three small steel spigots were brazed into place prior to the endplate being attached. The filter will insert fully into the airbox only when the far end is nestling within these locating spigots.

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The air intake comprises a 63mm tube cut at an angle and then welded to the wall of the airbox tube. Prior to welding, an oval-shaped opening to suit the tube was cut in the airbox wall. A plastic flared extension is connected to the intake via a short length of rubber hose. This flare is actually the internal end of the subwoofer vent tube that also provided the bellmouth airbox exit.

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Three mounting brackets were cut from 30 x 4mm steel bar and were welded directly to the airbox wall.

Other points

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You need to ensure that when the airbox is installed, there is enough clearance to (1) access the screws that hold the lid on, and (2) allow you to physically manoeuvre the airfilter in and out of its enclosure.

The airbox can be painted or powder-coated – in this case, before being finished and ready for painting, further plumbing needed to be added to allow for crankcase ventilation and blow-off valve hoses. If getting the airbox powder-coated, sandblasting, undercoating and (only then) powder-coating it will give the most durable finish.

In this application, fairly small inlet and outlet tubes were used. These can of course be much bigger if that’s required.

Finding the filter element

When designing your own airbox, the starting point needs to be the filter that you are going to use.

We believe that paper factory filters provide the best combination of filtering and flow, so this design was always going to use a factory-spec paper filter. But how to find a suitable filter?

This was the process that was followed:

1. Do an image search on the web for ‘airfilters’, looking at lots of different shapes.

2. Find a filter that looks as though it might be suitable in shape, then drill into the website to find the car it is fitted to.

3. Use the website of a filter manufacturer that includes dimensions of the filters in the spec list (we used http://www.rycofilters.com.au) to find precise dimensions. Note that this website also allows you to search for filters within certain size ranges.

4. Buy the filter and build the enclosure to suit.

Conclusion

This airbox design fits a large and readily available filter into a small overall volume. The use of the truck exhaust pipe means that most of the construction work of building the enclosure is already done for you – just add end-caps. Finally, especially if you have access to welding gear, the construction is straightforward.

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