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Selecting Airboxes

A new filter doesn't have to mean an aftermarket pod

by Julian Edgar

Click on pics to view larger images

At a glance...

  • Shape
  • Plumbing
  • Aerodynamics
  • Noise
  • Filter
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This article was first published in 2008.

At AutoSpeed we’re not big fans of exposed aftermarket pod-type filters. Unless they’re mounted outside the engine bay, it’s hard to stop hot air getting to them, and more importantly, the filtering ability of many aftermarket filters leaves a lot to be desired. So when doing an upgrade on the breathing side of the engine, instead of going for an aftermarket pod, it makes a lot of sense to look at using the airbox from another, more powerful car. That way you get cheap airfilters that you know will catch the crud, you can duct cool air to the box, and you also have a car that doesn’t attract police comment when the bonnet is lifted.

But what do you look for when selecting an airbox?

There are literally thousands of different factory airboxes to pick from. In fact, if you visit a wrecker where you can wander the yard, you’ll see all sorts of airboxes – from huge boxy ones, to flat pancake-shaped designs, to those that are largely cylindrical in shape.

So which are best? We’ll get to that in a moment, but the first question to answer is: which will fit?

Shape

Click for larger image

Many airboxes follow the shape of the inner guard of the donor vehicle – that is, they have a curved bottom (and also often side) so they nestle into place. Unfortunately, though, that makes them suitable for use in a different car only if: (a), the box is going to be placed on the same side of the engine bay, and (b), the shape of the inner guard of the new car is much the same as the donor car. This is one aspect that means a box that looks like it will easily fit oftentimes turns out to be no where near the right shape to fit!

It therefore makes sense when selecting a new airbox to have first measured the available room very carefully.

Note that if you’re prepared to take off a guard liner or undertray whenever you change the filter, the box may be able to be placed inside the inner guard, ahead of a front wheel.

Donor Car Power

It’s certainly not an infallible guide, but keep in mind the power output of the engine from which you’re sourcing the airbox. Obviously, you want it to be as high as possible, but if you’re forced to compromise in filter area or box size, don’t got lower in donor car power than the power you’re hoping to make.

Plumbing

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Once you have found an airbox shape that will fit, the next aspect to look at is the sizing and positioning of the inlet and outlet ducts. Again, these come in a huge variety. Sometimes the inlet is at the bottom and the outlet is at the top – and other times it’s the other way around. Sometimes the inlet and outlet are round tubes, while other times the inlet might be rectangular in shape.

Does the outlet pipe direction match the direction the pipe actually needs to go in to connect to the airflow meter, throttle, turbo or supercharger? Is the inlet pipe going to pick up cold air from a new location or is the inlet going to be from the same location as the standard car? These might seem like kinda obvious questions to answer but make sure that you do so before heading to a wrecker as otherwise you’re sure to be confused when you see the range of airboxes available.

While most people want airbox plumbing that’s as big as possible, it makes more sense to look at the diameter of the bits and pieces that you’re trying to connect the airbox to. For example, in airflow meter cars, what is the diameter of the airflow meter? If you can connect the airbox outlet straight to the airflow meter without a change in diameter, life becomes easier. In many cases the inlet pipe to the airbox is able to be enlarged, so the standard diameter of the inlet pipe on the new airbox isn’t so important.

Click for larger image

This VN Commodore airbox uses a rectangular entry into the lower half of the airbox and a large curved duct on the outlet. It’s also a very ‘boxy’ shape so needs a lot of space to fit it.

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This 300ZX airbox is very flat and compact – you could fit nearly three in the same volume as the above airbox, despite the fact its filter is about the same size.

Integrated Airflow Meters

Some cars have the airflow meter built into the outlet of the airbox. In these cars the sensing element can normally be removed and installed in a new tube of the same internal diameter. This allows you to then run a pod filter or a new airbox. However, be careful – when the airflow meter is so close to the box, it’s likely that the outlet of the original airbox was configured to flow air in a way that suits the airflow meter. Whack the airflow meter into a new tube and then butt it up against an entirely new box and the airflow meter output signal may change, so altering mixtures.

Aerodynamics

Generally, the more curved and smoother flowing the box, the better it will flow.

So specifically, the outlet tube should be integrated into the curved shape of that part of the box, rather than being just a tube butted up against a flat wall. The 300ZX box shown above does this very well – all of the lid of this box ‘flows’ into the outlet. For its size, its performance on the flowbench is very good.

Click for larger image

If the box that you’re looking it doesn’t have flowing curves, carefully look inside the box where the exit tube is. At minimum, it should have a radius’d outlet, or even a separate bellmouth. This EF Falcon airbox uses a long, bellmouthed pick-up tube that flows much better than a similar airbox that lacks this feature.

Noise

Click for larger image

In some applications, intake noise can be louder than exhaust noise. If you need to pass a legal noise requirement, or you simply like a quiet car, look for design aspects that suggest the airbox will be a quiet one. Things to look out for include the use of ribs inside the box to stop panel vibration, damper pads (they often comprise fibreglass packing under metal screens), and tuned-length projections and boxes attached to the inlet or the outlet tubes.

Filter

This one’s a bit tricky because some things are not as they first seem. To pick a filter with best flow it makes sense to measure the area of the filter - but the thickness of the filter is also as important. This is because thicker filters use deeper pleats which – when spread out - add up to more filter area.

These two filters have the same area....

Click for larger image

Click for larger image

....but quite different thicknesses.

So when selecting an airbox, you want to pick one that houses the widest, longest and thickest factory filter you can find. However, at the same time it also makes sense to pick an airbox that uses a fairly common filter – otherwise you might end up paying heaps for a new factory element each time you change the filter. If you pick an airbox that’s come from a popular car, you’ll usually find that its filters are cheap.

Conclusion

Swapping airboxes isn’t quite as simple as it first appears, but if you pick carefully, you’ll have the positives of good flow, good filtration and cheap air filters.

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