The FC3S turbo uses two stock catalytic convertors.  The first, "pre-cat" is used purely for cold-start emissions; once the engine (and exhaust/main cat) is warmed up to normal operating temperatures, the pre-cat does little to aid emission other than ending up as an exhaust restriction.

A popular upgrade for the exhaust is to replace this pre-cat with a downpipe.  With this downpipe, you keep your main catalytic convertor intact and thus the majority of emissions still working.

In fact, we recommend replacing the pre-cat with a downpipe, especially if you have very high mileage pushing 100,000 miles on the odometer.  The stock pre-cat has a notorious reputation of failing; once the pre-cat fails, this normally plugs and kills the main cat at the same time.  Again, replacing your pre-cat with a downpipe will have no bearing on emissions once everything is warmed up; you'll pass any emissions test, short of a visual test, with no problem.

Popular 2.5" downpipes which are drop-in replacements for the stock pre-cat are made by Racing Beat and MindTrain.  HKS offers a 3" downpipe that replaces the stock pre-cat.  Racing Beat also offers a 3" downpipe, but it does not fit with the stock main cat.

Downpipes can gain you up to 2psi of boost.

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