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Originally Posted by alroumi  I wonder why the SS arent legal, probably due to being much more free flowing ? |
The SS setup is not street legal because, as it uses long tube headers (much better at producing power BTW), the system relocates the cats from their stock location - that's a big NO NO for street certification. The Kleeman setup uses shorty headers which maintain stock cat location, and therefore it's legality.
The whole cat placement thing has to do with emissions during startup/warmup. The further the cats are from the engine, the longer it takes them to heat up to the temp required for them to function effectively at reducing exhaust emissions. That's why moving them from their stock location results in an exhaust system not being certified for street use.
Any jurisdiction that uses visual inspections as part of an annual survey will fail such a system on the spot, even if the car passes the "sniffer" test.