hfc

6MT

Most things to do with modifications on exhaust are illegal in California.

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Andrew

2009 370Z Sport/Touring 6MT sold
2013 Honda Civic Si coupe D/D

Waiz

Everything is illegal

Guard Dad

While I can't quote chapter and verse for you, I do know that California can be nasty about emissions issues if they choose to be. My feeling is that changes on street legal vehicles need to be carb approved or you are putting yourself at risk. So for the moment your bolt-ons are limited (about a 30-40 horsepower gain and more like a 100+ for the supercharger) to things like a Stillen G3 cold air intake, a cat back exhaust, an under drive pulley system and Stillen's recently approved supercharger system. Test pipes, high flow cats, headers (long tube for sure and probably short tube as well), and computer changes (as I understand them, some wiggle room may be present on this one) are all off limits. Having said that, if it looks stock it will probably get you through a roadside visual inspection if the car doesn't smoke or smell. Most shorty headers (maybe 7-10 horsepower) will "probably" go unnoticed as well but removing the cats is a huge red flag issue. The state has very strict rules about removing OEM cats so high flow cats are a no-no too. The one thing about HFC's is if they are actually delivering the required emissions (Note: HFC's are usually not CARB approved) performance they might go unnoticed (but not actually legal) and you might skate through. The thing about HFC's, test pipes, and long tube headers is they often (not always) cause computer and sometimes driveability (not to mention warranty) issues.

My advice is go simple (CARB approved CAI, pulleys and CBE) or go big (Stillen's CARB approved supercharger) but if you go another direction (and I'm NOT suggesting that you do, this is only a theoretical discussion) stealth is paramount or you could be screwed.