Originally Posted by ymdd
Sorry if this is a stupid question but what's the difference between the secondary cats and the first cats?
In the N54 engine, the 2 primary cats are part of the downpipes, hence are practically located at 15-20 cm from the turbos. They are the most effective in reducing emissions and the most restrictive in term of exhaust flow, quitting power and torque. Removing the primary cats gan give you aprox 20-30 whp, depending on the chip tuning you run, and less turbo lag. The secondary cats are located aprox in the center of the car, welded to the midpipes. Removing them does NOT improve power and torque nor reduce turbo lag. Probably the emissions increase and it depends on how much severe are the laws where you live to go with this mod or not. For what concern the Z4 35i, I think this is the best mod to have a louder sound and an amplified upshift "TUM" @ very low cost, at least in Italy.