Quote:
Originally Posted by cmiovino
As far as the cat, as others have said here, just use either the stock front pipe, or get a high flow aftermarket cat. The stock front pipes are actually quite good and swapping them doesn't net much of a gain. The cat also keeps rasp down.
As far as "back pressure", what you really want in an NA exhaust is the ability for the exhaust gas to exit as freely and quickly as possible. This isn't just going with the largest pipe available.
Typically, stock exhausts are a hair small, but they're generally close to the right size because manufactures are looking for efficiency to balance power, fuel economy, etc. Usually speaking, going a bit larger helps power. Let's say from 2.25in inner diameter to 2.5".
The idea behind reducing "back pressure" as people say is just fluid dynamics. You want that exhaust pipe "the right size" to allow for the gasses to exit quickly. Not too big, not too small. And that is highly dependent on the engine size (IE V8 vs a boxer four cylinder like these), forced induction or NA, etc.
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And age of the engine design! Many old engines with parts just sort of floating around in them needed back pressure to help things open and close. Modern engines with overhead cams, hydraulic lifters, etc just don't have the same requirements.
The Twins already come pretty much optimized for power in stock form. What traps people is thinking they are like Mom's old Civic with it's cast exhaust manifold, tiny exhaust and restrictive muffler that can see huge improvement by opening them up. Not to say that there can not be gains by increasing flow but they just simply will not be a great as those to be had on a car not designed for performance in the first place.