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FAQ & How Things Work

The Subaru Impreza Exhaust explained. Cat & Decat

Subaru Exhaust

The diagram above (click it for larger version) shows the Newage (2001-5) style exhaust system. This is a six piece exhaust with two pieces (manifold [also called headers] and up pipe) before the turbo and four pieces (down pipe with catalytic converter, link pipe with catalytic converter, centre pipe with resonator and back box silencer) after the turbo. 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) contains a resonator not a catalytic converter.

The exhaust gases leave 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* (cats) are another restriction in the exhaust. The STI does not have the restriction of the up pipe cat and many Subaru owners change the up pipe and link pipe with straight through decat pipes to remove the restrictive cats and either keep the down pipe cat or replace the down pipe with a cat-less version (not road legal and will fail an MOT) or change it for a 100 cell high flow cat pipe (not legal to carry out this procedure(1) on cars registered after March 2001). A high flow cat down pipe is an exhaust 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 to 10bhp points and may 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 really struggle to get through an MOT emissions test.

*Exhaust Catalytic Converters

The catalytic converter in a car (some cars have more than one and the WRX has three) works by combining noxious gases (carbon monoxide, nitrogen oxide) with other polltants (hydrocarbon) produced by the burning of fuel in the engine. Inside a catalytic converter are metals like platinum, Palladium and Rhodium. At a high temperature these metals are a catalyst which convert the CO, HC and (NO, NO2, & N2O) to produce carbon dioxide(CO2), nitrogen(N2), and water(H2O). Although these precious metals are a catalyst they do eventully become used up although very slowly. A normal catalytic converter should last the life of the car but other factors can cause them to fail premeaturely. **Cheap high flow cats contain less of the precious metals (the catalyst) and therefore can struggle to control emissions and also fail sooner.

Decat - To map or not to map

Removing some or all of the catalytic converters by replacing them with straight through decat pipes enables the exhaust gases to flow more freely through the exhaust which means the turbo can be moved faster by the faster flowing exhaust gases. The risk that this poses is that (as explained in the turbo section) a faster moving turbo will force more air into the engine causing the balance of air and petrol mix flowing into the engine to be upset. Although the engine management system will try to compensate by using the sensors on the car to calculate the changes it is quite possible that the increased airflow will cause this mixture to be lean (too much air and not enough fuel). This can cause the engine to run hotter than intended and can result in engine failure. Therefore if you are intending to remove some or all of the cats and uprating your air filter to a higher flowing air filter it is recommended that you have the engine management system reconfigured (ECU remap) to take account of the increased air flow. You will benefit from a remap in the following ways:
  • Your engine will have the correct air/fuel mix and will not be running the risk of damage due to lean mixture
  • You will be getting the most efficiency from the modifications you have carried out. (Changing the  exhaust  alone  will  increase  the  bhp  but  the  most  gains  will  be made by  having  it properly remapped.  If, for example you gain 15bhp by changing  the  exhaust and air filter then  you  may  be able  to gain  40bhp if you had it  remapped  as well (bhp  figures  are  an example  only).


Previous: - Anti Lag Systems

Next: - 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.


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Exhausts, Catalytic converters, How do cats work. Subaru exhaust. Legacy Exhausts.

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