How to smooth an airbox.

"Guide to smoothing your airbox, a great free power mod"

We often talk about smoothing airboxes, and our members refer to it as the TorqueCars airbox mod, or smooth intake mod.

So let's look at this simple DIY mod and the benefits you get from doing it and provide a step by step guide to doing it.

Why would you want to smooth your airbox?

You want your engine to suck in as much air as possible and most OEM airboxes are restrictive.

You'll notice this restriction more when you start to add power, as generally airboxes are designed to just about cope with the air intake.

In reality you won't pick up much of a power increase by just smoothing your airbox, but the intake will sound much better and you are less likely to hit a restriction if you are adding other mods.

Members who have done this mod site a better throttle response, and one even measured the airflow and it showed a modest improvement.

What does the airbox do?

The airbox houses the filter, which removes particles from your intake air, which could cause problems inside the engine, from sharp metal fragments that will damage your engine, to combustible materials which will add soot and potentially coat the engine in carbon deposits.

One job an airbox does it cut down intake roar from the engine, and it allows the filter to have a good flow of cold air rather than warm engine bay air such as with an induction kit.

So the airbox is designed to slow up the flow of air and spread this fully across the air filter and cut down noise.

What are the benefits of smoothing your airbox?

You remove the restriction and the car can breathe more easily.

Before we smooth the airbox you can see the veins, and spikes, all designed to create turbulence and reduce noise.

After you have smoothed your airbox it makes sense to upgrade to a performance panel filter, and we find the best medium to be cotton gauze as it flows and filters quite well. Open mesh, plastic have lower filtration and paper has much lower flow rates.

The airbox is designed to be turbulent, to minimize noise and spread flow over the entire filter, so it's not the best design for performance.

How to smooth your airbox

Tools you will need

  1. Sharp short bladed knife or ideally a very sharp chisel.
  2. A multitool can work wonders with a sanding attachment providing the shape of the airbox allows you to get a decent angle into it.
  3. Sanding block and coarse to fine grade sandpaper
  4. Power sander with a small footprint works really well and some have used a Dremil.

Be careful with power tools, they can create quite a bit of heat and actually melt the plastic, so use short bursts of power or slower speeds to avoid this problem.

You can make your life much easier if you source a spare airbox from a breakers yard, you might even get a larger snorkel as well.

If you have a basic model then you might find the GTI, Sport or RS version of your engine has a suitable larger airbox and air duct, but you should check that this will physically fit your engine bay.

When it comes to your intake filter larger is usually better.

Step by step guide

How to smooth your airbox

  1. Remove the airbox

    Remove the airbox from your car, separate it from the ducting. This may involve loosening a screw or two and popping open a retaining clip. It should be fairly easy, and is no more complex than changing an air filter.

  2. Cut away the fins and veins

    Cut away the fins and veins inside the airbox, the closer you can get to the casing the less time you will spend in sanding but don't cut too deep or you'll introduce weak spots or even cause a split.

  3. Sanding the airbox/strong>

    Once the veins are cut away, sand the inside of the airbox, a power sander makes this job much easier, but you will still have to hand sand the hard to reach areas.

    Use a coarse grade of sandpaper to start with, then progress to a finer grade. You'll see the veins melting away as you go. There is no need to get a highly polished finished although we have seen this done, there is negligible gain from it and you'll spend a lot of time trying to achieve it.

  4. Change the air filter

    Replace the OEM paper filter with a higher flowing panel filter to fully realise the benefits of this mod. You could put the removed one back in but it makes sense to source a performance panel filter made from a cotton gauze material see our article on higher flowing panel filters.


Other intake mods worth considering

The intake snorkel is generally quite small, effectively choking flow into the airbox, most performance models have larger snorkels, so try to source a larger snorkel or fabricate your own.

What effect will this have on the cars performance?

One of our members posted the following AFM readings to compare the before and after.

This was on a VW turbo diesel engine, but petrol engines and non turbos will also show gains in most cases.

  • 3040 Rpm MAF before 74.56 after 81.78
  • 3440 Rpm MAF before 82.81 after 87.08
  • 3800 Rpm MAF before 91.97 after 96.92
  • 4160 Rpm MAF before 98.14 after 107.89
  • 4520 Rpm MAF before 110.22 after 118.56
  • 4880Rpm MAF before 120.08 after 126.81
  • 5280 Rpm MAF before 129.83 after 134.83

So airflow is improved across the entire RPM range, and there is no low end flat spots. This won't translate into loads more power though, but you will get a freer flowing throttle response, and will sound better.

This further improved when the snorkel size was also upgraded.

Drawbacks of smoothing your airbox.

One minor drawback of this mod is that you need to change your filter more regularly, the air will flow through the central part much more than it would in a standard airbox, you will also get a little more induction noise (which most drivers will see as a plus, rather than a drawback).

Are the any benefits to drilling the airbox?

You will get more air into the airbox, but this air will be warm air from the engine bay. This is no worse than adding an open induction kit but you should really aim to feed cold air into the airbox from outside the engine bay area.

So drill a hole or two in the bottom and connect it to some ducting to pull cool air in, otherwise you'll be losing a bit of power from the heat you suck in.

So we recommend smoothing your airbox, placing a larger duct and trying to source a bigger airbox rather than fitting an expensive induction kit.

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2 Responses to “Smoothed Airbox”

  1. Bob says:


  2. Daniel Burns says:

    In my professional opinion I think you did a fantastic job especially on loosing power from letting hot air in the intake. I really appreciate it I didn’t think of this but I’m about to try it on my E90 right now. Thanks for the info

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