Stock Air Filter Replacement

This is a very easy task to complete as the standard airbox assembly is kept and the filter inside is replaced. It involves simply unclipping the top of the airbox and replacing the filter inside. It should take 5 mins to change the filter. So what are the types of filter and which would should you go for? The answer is one of choice. Let's look at the advantages and disadvantages of each:

Paper filters:

The stock filter is made of paper. The filtration characteristics are good but this filter doesn't allow as much air to the engine as 'aftermarket' performance filters (according to some). Paper filters need to be replaced often as dirt can build up and restrict the amount of air getting to the engine. The main advantage of the paper filter is that little or no dirt will enter through and damage the engine/turbo, they are cheap and easy to get hold of.

Cotton filters:

Cotton filters such as K&N allow more air into the engine. This will mean a slight gain in BHP. The filters have less filtration than paper and therefore it may be that some smaller particles will get through, whether this will effect the engine in the longer term is up for debate but many people have used these filters for years with no ill effect. They will last longer than paper filters as the build up of dirt on them will be less if it is kept clean and oiled. This seems to be the filter of choice for many Saab modifiers. You may experience a very very slight increase in cabin noise.

Although many firms will tell you that there is a different part number for 1996-99 cars (33-2166) the better fit is the 33-2663 for years between and including 1994-1999. This filter is slightly bigger by 1/8th of an inch and therefore a snugger fit in the airbox.

Foam filters:

Replacement foam filters (such as ITG) seem to be the choice of many specialist firms, such as Abbott Racing. The filtration is better than cotton but slightly more restrictive in letting air through. It may be an ideal compromise between the two. It will last a great deal longer than the paper element if cleaned and sprayed regularly, which will mean a saving of money in the long run. Again you may experience a very very slight increase in cabin noise and a slight gain in BHP.

Click to buy ITG filter: 9-3 all models inc Viggen & Aero 00'-02'
Click to buy ITG filter: 9-3 & NG900 94'-00' (Trionic 5)

Replacement filters can have a good effect if you modify the stock inlet tube that leads to the airbox and will allow more air to flow into the engine. The intake pipe can be increased from a 2" (51mm) to 3" (76mm) diameter but requires some modification of the airbox (increase the hole in front of it to accept the larger tube).

A suitable replacement pipe is required, some use PVC gutter pipe others aluminium, most have a trumpet end to reduce noise and increase the air entering the pipe. Start by removing the front grill. This is held in by two clips and then is easily lifted out. Remove the front headlamp assembly, signal assembly and the airbox. Measure the existing tube and cut your new enlarged tube so that it is between 1 and 2" longer than the standard item. Enlarge the current hole in the airbox using a suitable tool. The new hole should still be below the filter element. Be sure to check the fit in the vehicle before permenently fitting the new pipe in place. Once this is done and the fit is exact, glue the tube into the hole using adhesive/sealant making sure the water hole in the bottom of the airbox is still functional. Refit the headlamp/signal assemblies and grill.

Remote Cone Filters

Cone filters come in different shapes and sizes, the most usual are conical and cylindrical. They are available in cotton or foam. They will allow approx. 10-15% more air through to the engine than the standard airbox setup. However if the remote filter is placed futher up into the engine bay it will draw in hotter air from the engine and not cooler, denser air like the standard airbox setup. Due to the lack of space the cone is unfortunately easier to fit in this position and it will also be facing away from the air coming into the engine bay. Many people who have opted for remote filters in this position fit sheilding between the engine and the filter to stop the heat from effecting the cone. This should be carried out particularly if you have a foam filter as it does get effected by heat and can crumble (sometimes getting sucked into the engine). A NACA duct fitted into the bonnet of the car is a possible solution as the high pressure air moving over the bonnet will enter into the engine bay and feed the cone. It is important that the cone is not directly under the vent as water could drip down onto it.

Others prefer to point the cone down into where the intake comes in (down inside the bumper) to generate the cooler air, however in this position the element is at the mercy of water and muck coming up from the road and the feed to the engine is not as direct as some would like.