Although some modifications are made to the body styling it is what is under the bonnet, under the car and under the wheel arches that counts. Upgrading the air filter to a Pipercross foam or Green Cotton or K&N panel filter to provide an uprated air flow can increase performance by a few horsepowers at relatively small cost (£40-50).

One of the most common upgrades that is carried out is replacing the standard exhaust with an aftermarket one such as a Jetstream.  Ranging from a simple upgrade of replacing just the back box to give a nice shiny look and sound and a slight increase in bhp to a complete turbo back performance de-cat system (everything behind the turbo replacing the catalytic converters with a through pipe) to give an increase in performance and sound level.  An alternative to this is to fit a hi flow cat system which will still provide performance increase at just below a decat system but has the added advantage of keeping it street legal*. Replacing the exhaust system with performance upgrade high flow cat and decat can give around 20-25 bhp increase.

The real enthusiast however will be looking to really push up the performance from the standard 218bhp.  (All newer UK spec. STi models now come with a 265bhp engine, a six-speed gearbox, inverted suspension struts, Brembo brakes, front and rear limited-slip differentials, a faster steering rack, satellite tracking and a full body kit with high-rise spoiler). 

Replacing the standard injectors and fitting an uprated fuel pump help to boost performance but at this stage remapping or replacing the ECU and fitting something like a Pipercross, Green cotton or Powertec induction kit & Front Mount InterCooler and ceramic coating of the exhaust headers, up and down pipes and turbo to keep the heat down becomes desirable.

Further performance upgrades involve replacing the turbocharger for a larger aftermarket version, fitting parallel fuel rails to avoid any lean running, equal length exhaust headers and further remapping of the ECU.  With all of this extra power comes additional torque.  The wise upgrader will also fit an uprated aftermarket clutch and maybe even change the gearbox gears to straight cut synchro and have the engine rebuilt with pistons, conrods, crank and shells and even having the block converted from open deck to closed deck or relinered with thick liners.

With all of this exra power comes the need to be able to stop efficiently.  Other common performance upgrades come in the form of uprated suspension and brakes.  Manufacturers such as Eibach and Whiteline produce enhanced performance adjustable suspension in the form of adjustable shock absorbers and anti roll (sway) bars and lowered springs.  Companies such as Brembo, BlackDiamond and Mintex produce a vast array of uprated products from drilled, grooved and combination brake discs to race track large 4 pot brake calipers and race track sport 6 pot professional calipers and brake pads of a varying degree of hardness.

Upgrading of the Subaru suspension is something that can benefit the handling of your now faster Impreza. Uprated Whiteline Anti Roll Bars and drop links is stage one. Many people just fit an uprated 22mm rear anti roll bar and front and rear drop links to improve handling but for a slightly stiffer approach fit the 22mm front anti roll bar and the 24mm rear anti roll bar and front and rear Whiteline alloy drop links.

A move away from standard shock absorbers to something like Kayaba stiffness adjustable shocks and Whiteline or Eibach lowering springs is something that can be undertaken on cars up to 2002. These are a reasonably good replacement fully 'on car' adjustable gas charged shock absorber for high performance cars.

Moving on to coilover suspension. This usually allows ride height adjustment by way of moving a large nut up and down the strut to raise or lower the spring and a separate adjustment for the stiffness of the shock absorber.

Whatever performance upgrade you are thinking of doing to YOUR Subaru we suggest you do it incrementally rather than throw a lot of money at it all in one go.  If you upgrade something and don't like it you can reverse engineer but if you add a number of performance upgrades all in one go and do not like one of them you may have to reverse engineer more than one, such as exhaust and remapping if you decide to reverse the exhaust you may need to have another remap.

*Beware the new MOT requirements that the noise of an exhaust must not be unreasonably louder than a standard exhaust in average condition. Also new legislation regarding fitting catalytic converters to cars registered after 1st March 2001. Cats fitted to these vehicles must be "type approved for the vehicle" for road use, therefore all aftermarket non "type approved" cats fitted to vehicles registered after this date must be for off road use only.

Commonly asked questions.

Will performance upgrades void my warranty?

Warranties are peculiar things with grey areas and be prepared to have a fight on your hands with your main dealer.  Having said that there is such a thing as the Magnuson-Moss Warranty Act.  This basically states that your warranty cannot be deemed void because of an aftermarket part (this would include performance upgrades) unless the aftermarket part caused the problem that the warranty claim is about.  For example, if you fit an aftermarket back box to the exhaust and your shock absorber failed then the manufacturer should not deny your warranty claim because a back box should not affect the suspension.  However if you fitted an aftermarket high flow performance exhaust and a remapped ECU and your turbo failed the manufacturer could claim that the extra stresses caused by the remap and high flow of air had caused the failure.  The grey areas come in the form of possible links such as if you fitted the above exhaust and ECU and then you had shock absorber failure the manufacturer may try to claim that the additional performance of the car put additional stresses on the suspension.  The law looks at what is reasonable to expect and while the law may be on your side, the dealer service department may or may not be.


What can I do to my Subaru Outback  

The answer I am afraid is that there is very little that you can do to an non turbo car to make it go much better than standard.


What can I do to my early Subaru Legacy T

There is only so far you can go with these early Legacys before you blow the engine.  How much power did you want to go to?  The early turbo Legacy UK was 200 bhp and the early Impreza UK was 210 bhp and you have probably achieved this by changing the exhaust (if it is decatted).  Also the Legacy was a heavy car so may seem slow by comparison.

A decat or high flow cat (to keep it street legal and MOT passable) exhaust system and replacement hi-flo panel filter are the first steps to more power.  Next you would be looking at replacing the engine management system with an aftermarket ECU and having it remapped to the car. 

After this and you need to be considering replacing the fuel injectors and turbo (and probably the inlet manifold to enable a front feed turbo to fit) and having the map tweaked. 

Next you would be looking at rebuilding the engine because it was not strong enough to take the pounding.

The questions are, how far do you want to go and how deep are your pockets?


How does a turbo work? 

The following diagram created by Matt Randolf simply explains this.  Click the image to enlarge.  Simply put, when you accelerate you allow more air and fuel to flow through the engine.  This in turn produces more exhaust output.  The turbo has two parts and is connected to the exhaust (exhaust turbine) and the air intake (compressor wheel).  The exhaust output travels through the up pipe to and through the turbo spinning a small turbine (a sort of fan in reverse, the airflow turns the blades not as in a fan where the blades create the air flow).  The exhaust gases travel on through the downpipe and out through the rest of the exhaust.  The turbine that has been caused to spin by the exhaust gases is fixed on a shaft which has a compressor wheel (like a fan) which is forced to spin by the turning shaft.  This compressor wheel is connected to the air intake and the spin causes it to suck more air through and into the engine.  This, in turn, produces more air flow and therefore faster exhaust gases to flow out of the exhaust port and into the exhaust side of the turbo thus spinning it faster and therefore turning the air intake compressor wheel faster which sucks more air..............    



What is a blow off valve?

A "dump valve" or "blow-off valve" (BOV) and a "compressor bypass valve" (CBV) do the same thing, which is to release compressed air in the intake tract when the throttle closes, so as to prevent high pressure from smacking back into the compressor wheel of the turbo. If the valve vents the relief air to atmosphere, it's a blow-off valve or dump valve,  if it sends the air back to the intake, it's a compressor bypass valve.  To explain this more, when the turbo is working it is blowing air at high pressure into the engine.  If you ease off the accelerator then you are closing the duct through which this air enters the engine.  This causes this high pressure to bounce back along the route it came from and smacks into the turbo thus stopping the compressor wheel from turning.  This is turbo stall.  When you accelerate again the turbo has to get back up to speed again (spool up).  A blow off valve will allow this excess pressure to be vented to the engine compartment thus reducing turbo stall and helping with spool up.  A bypass valve vents this excess pressure into the air intake on the other side of the turbo and this not only relieves the pressure on the compressed side of the turbo but the increase in pressure on the air intake side helps to keep the turbo turning.


The Subaru Impreza Exhaust explained.

The diagram above (click for larger version) shows the Newage (2001-) style exhaust system. THe up pipe contains a precatalytic converter often called the front cat. STi versions do not have a catalytic converter in the up pipe. The earlier cars and jap imports were very similar but usually had a three piece exhaust after the turbo, the down pipe (2) and mid pipe (2) (sometimes called a link pipe) were made as one piece with only one catalytic converter and the rear catalytic converter was in the rear exhaust pipe (3) often called the centre pipe just to confuse people. On the newage vehicles the rear exhaust pipe (3) (centre section) contained a resonator not a catalytic converter.

The exhaust gases lead the engine via the exhaust ports of the heads and enter the left and right exhaust maniold (often called headers). The left hand exhaust gases having to travel through the front exhaust pipe (called a cross tube) before combining with the exhaust gases in the right manifold. It is this difference in distance that the exhaust gases have to travel that gives the Subaru flat four engine it's distinctive 'burble'. It is a sort of blub,BLUB,blub,BLUB,blub,BLUB.

After combining the exhaust gases travel up the up pipe to the turbo where if the exhaust gases are moving fast enough (usually at 2800-3300rpm depending on the turbo) the gases will turn the turbine. The turbine is attached to a shaft which is also attached to the compressor wheel so the faster the turbine turns the faster the compressor wheel turns and forces more air into the engine which casues more and faster exhaust gases.

After passing through the turbo the exhasut gasses enter the downpipe where the catalytic converter processes the gasses turning harmful carbon monoxide, nitrogen Oxide and Hydrocarbon into less harmful carbon dioxide, Nitrogen and steam (water). If your engine is running to rich or too lean then there will be soo much pollutants that the catalytic converter will not be able to cope and your car will fail an emissions test. There are sensors in the exhaust to minotor the emissions (see next article). After passing through the down pipe the gases pass through a second catalytic converter and then through a resonator chamber which helps to quieten the exhaust and then through the muffler (silencer, backbox) which also helps to quiten the noise from the exhaust.

THe exhaust on a car provides many restrictions on the airflow. Starting with the manifold (headers) at each joint the hole in one part must match exactly with the next part and due to manufacturing processes the holes may not match up by as much as 2 or 3mm and the indides of the pipes may be very rough. THis all helps to slow the gas flow. Porting is a process where the rough and uneven sufaces are smoothed out and the exit hole of each section is opened up to ensure the gas flows smoothly from one section to another.

The catalytic converters are another restriction. The STI does not have the restriction of the up piep cat and many Subaru owners change the up pipe and link pipe to remove the cats and either replace the down pipe with a cat less version (not legal and will fail an MOT) or change it for a 100 cell high flow cat pipe. This is a down pipe which contains a pecial catalytic converter that allows the air to pass through it at a high rate. The difference in horsepower between a decat down pipe and a 100 cell high flow cat down pipe is around 4 or 5 bhp points and it is street legal and will pass an MOT but has to get hot first. A word of warning, beware of cheap high flow cats. THere are a number of high flow cats on the market that do not last very long and struggle to get through an MOT emissions test.

Next time - Exhaust sensors.