FAQ & How Things Work

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Commonly asked questions.

Will performance upgrades void my warranty?

Warranties are peculiar things with grey areas so 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 is the US federal statute that governs warranties on consumer products and although this is an American law you could use the information contained in it to argue your case in this country. Basically, it 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 subaru performance tuning can I do to my Subaru Outback, Non turbo Forester/Legacy/Impreza?  

The answer I am afraid is that there is very little that you can do to a non turbo car to make it go much better than standard. You could convert it to a tubo car by changing the exhaust, engine, wiring loom, ECU and adding a turbo but the time and expense of doing this would be prohibitively expensive and you would do better by buying a turbo version of the car.


What subaru performance tuning 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 can probably achieved this by changing the exhaust and putting in a high flow air filter.  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), the fuel pump and possibly fuel regulator and having the map tweaked. 

Next you would be looking at rebuilding the engine (and probably gearbox) 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 gas 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 etc.etc.     



So, why doesn't the power keep on climbing? It would, but this would eventually lead to the enging exploding so the ECU has control over a safety valve called a wastegate. When the MAP (manifold pressure sensor) signals to the ECU that the pressure in the inlet manifold has reached a pre-determined point the ECU will open the wastegate allowing the exhaust gas to be vented down the downpipe instead of flowing through the turbine of the turbo thus slowing the turbo down.

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. Blow off valve types


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 centre pipe (3) often called the rear exhaust 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 manifold (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 causes more and faster exhaust gases.

After passing through the turbo the exhaust 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 so 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 monitor 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 quieten 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 insides 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 pipe 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 (not legal on cars registered after March 2001 but at the time of writing will pass an MOT). This is a down pipe which contains a special 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 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.

*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 considered for off road use only.