Not everyone can use a race header in their setup, for the folks who cannot and wish to remain emissions legal you can use a hi flow cat.
Hi Flow catalytic converters
There are truly only 2 hi flow cats on the market and they are:
Random technology hi flow cat
Focus central hi flow cat
A high flow cat can make very good gains when used along with a full length exhaust. A car using an aftermarket shorty, flexpipe and cat back can gain anywhere from 5 - 8 whp an as much wtq when adding a hi flow cat. A shorty header and hi flow cat combo also allow the engine to produce usable low rpm torque, this is good for daily drivers.
Hi flow cats can be pricey, a RT cat pipe goes for around $ 300 and the FC cat for around $ 250
Cat Eliminator pipe/Off Road pipe
Only one company makes an off road or cat eliminator pipe for the zetec equipped focus, that is focus central.
Focus central cat eliminator pipe
The off road pipe for the zetec is quick and easy to swap, so it’s a good part for someone who wants to remove their stock cat for a day of racing. They are also fairly cheap in price.
An off road pipe on your focus will make it very loud. The gains when using an off road pipe are ok but nothing to brag about.
If you have a question about one of these parts or another related exhaust part, feel free to PM me or e-mail me at firstname.lastname@example.org