Other than an upgraded exhaust system, this seems to be one of the most popular mods (together with replacement induction kits).

I bought the filter from Scoobyworld for £35, a reasonable amount for a very simple mod.

The Impreza comes with a paper panel filter as standard and the ITG replacement is foam based panel filter that uses a green-coloured sticky goo to trap dust particles and foreign objects before clean air is passed into the engine.

For fitting instructions see my Maintenance section.

Note: There has been a large debate in the Scooby community as to whether installing 3rd party air-filters and induction kits can cause the MAF sensor to fail. There have been many reports with certain makes of induction kit causing failures but no one can prove the exact cause of failure. It's believed that poor installation or excessive vibration has caused these and with the pre '01 Impreza sensors being a tad delicate it may just exacerbate the problem.

Personally, I've had no problems to report in the 8 months I've been running mine. If you do replace your standard air filter, then please be aware that a MAF failure is VERY serious and that it can be difficult to prove that the sensor has actually failed. One of the common symptoms though includes a poor idle (to the point of almost cutting out), so keep an eye on the cars running once fitted.

Update 18-12-2002: Given all the hysteria surrounding the MAF failures, I've removed the ITG filter and installed my original OEM paper filter. I haven't found any performance gains and until I find a reliable way of monitoring the MAF, I'll stick with the original.