Exhausts

Usually the first thing to be replaced is the restrictive factory exhaust. You will note that tuners do not just replace the muffler with one that has a coffee can sized tip. The benefits are two fold. First, the ability of your engine to expel combusted gases increases. Increased airflow gives your engine the potential to make more power. Remember that the power your engine makes is a function of how much and how efficiently it can combust the air/fuel mixture. The second benefit of an exhaust system is the great sound they generate…in other words, your sports car doesn't sound like the family Ford Mondeo.

There are usually two types of exhaust "systems". The difference lies in where the exhaust system starts.

"Cat-Back"

A "cat-back" exhaust system begins at or rather connects to the factory catalytic converter. The larger than factory tubing then runs to a less restrictive muffler (that is louder and sounds more powerful) before venting exhaust gases to the atmosphere. Exhaust gases still leave the engine and pass through the low cost, restrictive factory catalytic converter.

Turbo-Back Exhaust

A turbo back exhaust, on the other hand, removes the factory catalytic converter and replaces it with either a straight pipe or a high flow catalytic converter. With a higher flow capability, a turbo back exhaust has greater potential for more horsepower. Typically, with all things being equal, on a properly tuned, forced induction vehicle (turbo charged) a "turbo back" exhaust will net 5-10 more horsepower than a "cat-back" exhaust. With reduced backpressure coming off the turbo, a "turbo back" exhaust will theoretically allow the turbo to spool up quicker (i.e. 100-500 rpm). Below is the APS Turbo back exhaust system.

Notice the high flow cat coming down off the engine (top of picture) and the fairly straight shot to the tail pipes. A "turbo-back" exhaust provides you with a good foundation if/when you plan to pump up the ponies even higher than a typical "stage I" upgrade.

Up-Pipe

An up-pipe is a straight through pipe that replaces the catalytic converter before the turbo. Below is a picture of the APS up-pipe.

An up-pipe is good for approx. 9 hp and is a worthwhile upgrade. The downside is you are removing one more of the catalytic converters and will, therefore, need to be cautious about emissions.

Mid-Pipe

A mid-pipe is a straight through pipe that replaces a catalytic converter...typically a mid-pipe is used in association with "cat-back" exhaust or muffler only replacement since a turbo-back exhaust has already replaced the factory catalytic converter (sometime both of the cats...remember the stock WRX has two cats after the turbo) with a high flow catalytic converter.

Down-Pipe

A straight through pipe immediately after the engine that replaces the number two catalytic converter. Used similiar to the Mid-Pipe... Remember that while you you do gain power by removing the cats...unless you use a high-performance 3" catalytic, you will have to replace them for the annual MOT test.