Improving breathing

Air filter

Drop in replacement panel filters

Many replacement panel filters are available to drop straight into the standard Impreza air box.

The RamAir is a foam pad filter which is oil impregnated and reusable by washing and retreating. RamAir used to be the most popular and was fitted as part of the Prodrive Performance Pack before 1999. A warning on fitting these as you must get it seated correctly or else it is possible to have it deforming with air being sucked past...not good. Also being oil impregnated if it is over oiled it is possible for the oil to contaminate sensors up stream...again not good.

One alternative finding favour is a foam pad filter manufactured by ITG. Again a drop in replacement it is though much stronger structurally than the RamAir.

Another manufacturer is K&N and this is also slightly better made than the RamAir being reinforced paper/cotton material with a mesh. I have heard reports (unsubstantiated) that these give better intial performance to the RamAir but this drops off after a few months.

Induction kits

Induction kits are available for the Impreza and these replace the whole air box assembly with a pod style filter inside the engine bay. There is an argument that they seem to be of limited benefit on the Impreza Turbo because they suck air from the very hot engine bay. They also have the disadvantage of being very loud as the air gets sucked in! There has also been several reports of the MAF (mass air flow) sensor being contaminated due to the poor filtering offered by some systems and also cases of failure due to vibration. The MAF is located in the section of intake just after the pod and the weight can cause deflections and vibrations enough to break the thick film element of later MY99/00 cars. Ensure that the kit you have has enough bracketting for the pod and supports the MAF and dampens the vibration.

Cold air ducting

Popular in Australia are cold air kits which reroute the air intake from the front of the engine bay to somewhere cooler. It should be noted that the New Age Impreza takes its air directly from the front grill and thus emulates this approach. The use of an induction kit on a New Age is also of limited benefit because of this feature which would then be removed.

Dump Valves/Blow off valves (BOVs)

Firstly you have to ask yourself, "Why do you want an after market unit?" The Impreza is equiped with a satisfactory DV as standard.

The blow off valve does not increase performance. In fact the best performance increase can be gained by not having one altogether, the WRC cars do not run one.


...the main advantage of the blow off valve is to reduce stress upon the turbo blades when you lift off of the accelerator to lets say change gear. The turbine is still spinning and thus pumping gas into the input...but the engine is off power. So what to do with the excess. A blow off valve set to a certain pressure releases this excess. A vent to atmosphere allows it to go to air giving the characteristic pfffft, a recirculating one feeds it back into the input of the turbo thus keeping the system pressurised. When you hit the accelerator the vented system then needs to momentarily build up the pressure again whilst the recirculating system doesn't.

Without a BOV the turbine can actually stall or air is drawn back through the wastegate in the wrong direction. The turbine is spinning but there are no exhaust gases coming out of the engine, so it either stalls or draws airthrough the wastegate from the exhaust side.This is the chirp you hear on the WRC cars. The stress on the bearings is quite substantial and in a completely different direction to that which the turbo is designed. The turbo in the WRC cars has a 360 degree bearing surface to counter this (and the turbo is changed and refurbished per rally if not sooner), the road going turbo does not.

After market BOVs are adjustable, and the pressure at which they blow can be set higher than standard which means that there is more pressure retained at gear change.... BUT how high can you go, what is a safe setting and will it ultimately shorten the life of the turbo?

The other reason an aftermarket BOV is selected is if the system is running more boost than stock and the stock BOV can not operate at the elevated levels.

Finally the main reason I can see a BOV is fitted is because people want that characteristic PFFFT! which probably loses them performance rather than gain it as they are dumping turbine pressure everytime they change gear.

One other negative thing about vent to atmosphere BOVs is that the ECU monitors the air into the system via the MAF. This is in the input before the recirculating pipework reintroduces the gases back into the system. Thus the ECU takes into account this recirculated air in its calculation for fueling, and ultimately over fuels for a VTA-BOV and won't do the turbo much good or any following catalyst in the exhaust.

If after all of the above you would still like to investigate an aftermarket BOV, checkout Scoobysport who supply the Forge designs.


The exhaust on the Impreza consists of four sections.

Rear silencer

Most popular and easiest part to start with is a replacement silencer. This gives a less restricted flow for the turbo output at higher rpm but probably more importantly gives the Impreza a voice!

Listen to a Scoobysport exhaust!

The most popular rear box currently is probably the one manufactured by Scoobysport. This has a single round 2.5 inch exit pipe and is effectively a drop in replacement for the original rear silencer using existing mounting points and brackets. Manufactured in stainless steel this is a very high quality and will probably last longer than the car!


Prodrive in response to the popularity of the aftermarket exhaust sales have release a WR silencer with an oval tail pipe (shown above). It should be noted that the WR pipe with a round tail pipe is only officially available as part of the Prodrive Performance Package. Similarly the round P1 tail pipe silencer seen below is only made available to P1 owners!

Silence!  Silence!  Silence!

For an exhaustive review of the above back box see my P1 page.

Centre resonator section

The purpose of the centre section is to reduce boom from harmonic pulses of exhaust gases and as such is termed a resonator box. The section which connects to the turbo down pipe also includes a flexible joint to take into account vibration and movement from the engine when under power. On the UK/European turbo this section also houses a catalyst. It has been reported that the Japanese specification WRX/STi also included a catalyst in this section prior to 1998, changing over mid MY98. Note that the P1 does NOT have a catalyst in this section.

Removing the catalyst from a UK turbo and exchanging the section for an STi resonator will improve gas flow. There are several other alternative centre sections available which vary in complexity right down to replacing this section with a straight through piece of pipe! Obviously sound output will vary considerably. Ensure though that there is some form of flexible joint at the downpipe end or fatigue will quickly set in.

Down pipe

The down pipe connects to the turbo charger and takes the exhaust gases from here, around the back of the engine and down to the centre section. The turbo has two outputs, from the turbo itelf and from the waste gate. On the standard downpipe the output of the waste gate is straight into the wall of the flange plate which mounts to the turbo which is not ideal for gas flow in that it is very restrictive and rejoines the main gas flow at 90 degrees to the output from the turbo vanes thus causing turbulence. By blending in the waste gate flow further down the pipe there is less effect on the turbo.

Gas flow is further restricted in the standard system because of the way in which the pipes are bent during mass production. This leads to creasing and bore restriction. An aftermarket down pipe will be mandrel bent, meaning that during bending a soft inner piece of metal is inserted to stop the width reduction.

The downpipe also includes a catalyst section very near the turbo, and thus gets hot very quick to work most efficiently. Unfortunately this is not good for gas flow, and most aftermarket down pipe replacement systems will replace the catalyst with one further down the pipe with one of a higher flow (sometimes called a Sports Cat) or maybe remove totally the catalyst. It is upto you as to whether running without a catalyst is breaking any local laws.

Shown here is the full three part exhaust system offered by Power Engineering and is also available as seperate sections.

Power Engineering back box The rear section silencer.
Power Engineering centre The Centre section.
Power Engineering downpipe Power Engineering back box Power Engineering back box The Downpipe.

Exhaust headers

For the ultimate solution the exhaust headers from the engine to the turbo are then looked at. The standard ones on the Impreza are not equal length and do restrict gas flow but are fairly good so unless you really need to change these then I would not recommend it. It should be noted that the WRC cars run equal length headers in the search for the ultimate power output. Also the characteristic Impreza growel is totally lost when the modification is carried out!

Although not popular presently with the Impreza Turbo this modification might become more so with the introduction of the New Age WRX. One of the changes introduced with the WRX is a third catalyst pre turbo. Removing this improves driveability of the WRX no end, but unfortunately means the car will no longer pass emmisions tests.

Exhausts conclusions

A warning should be made about extremely big bore systems. A turbo charger works at its best efficiency with no back pressure at all to stop it spinning up. Some manufacturers warn of over spin but in all intensive purposes you would only ever achieve this scenario in running with no exhaust! A problem arises though with exhausts that are too big. Off boost, at low rpm, cruising etc the engine is basically a normally aspirated engine. Acceleration from low rpm will be severely affected as the engine running in NA mode can not achieve the power fast enough to spin up the turbo. By gradually increasing the back pressure by causing a restriction in the exhaust will improve things. Of course there will be the situation that you end up with too much and it then throttles the turbo! A compromise between the two is required.

The legality of a catless system will depend on your local laws. There are cars in the UK which visit their local tuning shop before and after the MOT to have their original downpipe fitted for the test. Stop/check road side emmisions testing has now started in Cambridge, Oxford and Milton Keynes with on the spot fines for cars not meeting the standards. So you have been warned!

The New Age WRX with its pre turbo catalyst is reported to be able to pass the emmisions tests even if the downpipe and centre resonator catalyst are removed. Whether this will still hold true in 3 years or so time when the catalyst has aged is unknown.

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Page originally created 23rd September, last updated 4th November 2002
Copyright © 2001/2 Steve Breen. Copyright © of photographs remain with source. Used here by permission and may NOT be copied for any purpose.