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I had heard everyone rave about changing the Downpipe. The standard Impreza has two catalytic converters; the main one is in the Downpipe (directly exiting the Turbo) with the secondary one being in the centre section (Impreza exhaust consist of 3 sections from the Turbo back - Downpipe, centre and back box).

The MY99/MY00 UK cars are fitted with a TD04 turbo which has two seperate exits for hot exhaust gases (see right). The main exit is directly in line with the Turbo vanes, but the second is for the wastegate and is offset. This means that the Downpipe has a wide neck to combine gases from the main exit and from the wastegate.

A Downpipe fitted with a catalytic converter provides resistance to the exhaust gas flow of air. By fitting a free-flowing Downpipe, you are removing this added resistance and reducing back pressure which results in the turbo being able to spool-up more quickly. This gives you a better throttle response and the better air-flow provides the extra torque.

There are two designs of Downpipe that deal with combining the wastegate gases with the main exhaust gas flow. One keeps the gases seperate until near the bottom of the Downpipe to minimise disruption to the air-flow close to the Turbo. The second is a simple open-neck design that allows the gases to mix as they exit the turbo.

As you can see, the TSL Downpipe is an open-neck design. Unless a car is heavily modified, the difference in air-flow between each independent exhaust is very negligible. It really comes down to price and sound characteristics that set each of the apart. The Scoobysport exhaust is probably the most well-known system and likewise is fitted on most Impreza's. I wanted something different and heard good reports from people who had fitted the TSL Motorsport system.

I left this job to the experts again and asked my Dealer (A F Noble, Penicuik) to fit the new parts.

The difference was awesome. The car sounded much deeper and the Impreza's flat-4 boxer burble could now be heard. As for performance, it put the grin-factor back into the car. It pulled much harder through the gears and felt like I'd just re-fitted the PPP ECU. I will eventually go for the TSL back box, but at £300 I'm happy to stick with the STI for the near future.

Here is a picture of the Downpipe (top) and centre section (bottom) before they were fitted to my car.

The Impreza is fitted with a metal heatshield which covers the turbo and opening of the Downpipe. With almost 99% of aftermarket Downpipes, the neck on the new Downpipe is somewhat wider and interferes with the fitting of the heatshield. Many drivers leave the heatshield off the car after changing Downpipes, but consensus tell us that this can cause heat soak with the Intercooler and has led to the brake fluid being boiled due to the heat coming off the turbo.

When my Dealer fitted my Downpipe, they simply left the heatshield off, so I had to replace the thing myself. It's more simple to do than I first thought and I can't see why the Dealer didn't bother re-fitting it.

Here are a few pictures of the original heatshield parts as left off of my car. The first picture is the heatshield itself, whereas the second picture is of the heatshield bracket (used to mount the shield to the turbo/Downpipe) and the bottom part of the shield which sits underneath the Downpipe.

Refitting the bottom shield is a lot of hassle as you need to work from underneath the car. It's not essential, so I decided just to leave this off of my car.

So, as you can see in these pictures, the next step is to cut the main heatshield in half. I used a heavy-duty pair of tin-snips to do the job.

OK, once you've trimmed the heatshield, just undo the two Turbo/Downpipe bolts and attach the heatshield bracket to them as shown in the picture. Lastly, just use the screws to re-attach the heatshield cover to the bracket.