I was going through and i saw your post....i myself am changing out the old system so i went searching and this is what i have found.... Correction to what this post underneath - i believe you can install hi-flow cats AS LONG as they have all the correct government exemption markings on them. Now the real question is....should you replace all four or can you get away with just two of them??
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Can I replace the OEM catalytic converter on my car with a "high flow" catalytic converter?
Update: 1/1/09 - New catalytic converters sold or installed in CA must be designed to meet the following requirements:
Must be an OE replacement only - no "high flow" or performance cats
Must be granted an ARB exemption
No used or rebuilt used cats can be installed
Briefly, this means that you are now required to obtain a new ARB-exempt catalytic converter for your specific vehicle if you require a replacement. The street use of aftermarket performance "high flow" units and aftermarket mid-pipes with non-exempt cats is not legal in CA. For more information on this legislation, visit: Aftermarket Catalytic Converters
Both CARB and the EPA do not allow the replacement of a catalytic converter with non-exempt "high flow" performance replacement unit. In fact, it is technically not legal to even replace a converter unless it has been proven by technician to have failed, and then it can only be replaced with an exempt OEM equivalent unit.
Due to the hundreds of vehicle models produced over the years, it is virtually impossible to determine by glancing under a car whether the converter is the original unit, a legal OEM replacement, or a non-legal aftermarket "high flow" performance unit. However, changing regulations require imprinting the exemption number, manufacturer code, and manufacture date, making it easier for technicians to determine the origin of the product and the intended application.
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Hope it helps. I've heard of shops getting hit for 25,000$ fines and a
suspension of their license if they, "hollow" out your cats or, "tamper," with smog stuff.
I don't know if this answered your question about having to replace all four of the cats but taking a quick look on the c.a.r.b. and a.r.b. websites leads me to believe you should have them on it before smog/after smog. Especially in california. The too fast too furious days of san diego/rancho bernardo have only intesified the resolve of cops to **** with everything that even sounds/looks modded.