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A few things to keep in mind when criticizing factory exhaust systems... They design the things to be really quiet so they satisfy all types of potential buyers.
They also have to pass emissions in a lot of different countries. A more restrictive exhaust can often light off the cat much quicker. Cold cats don't do anything for your emissions! Some countries have more restrictive cold start requirements so the vehicle can pass without the company having to redesign their exhaust system for each market.
I really wanted to design some exhaust systems for fuel economy but it turns out it's really hard and really expensive to leave emissions unchanged. Sometimes just 50 degrees of cat difference can have a 200% effect on emissions output.
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