Quote:
Originally Posted by BMWeber
you don't know. it all depends on the bends of the exhaust and the rectrictions of the muffler/resonators.
the standard formula is 2.2 CFM per hp= no loss from back pressure. straight pipe flows 115 cfm per square inch. 3" straight pipe is good enough for approximately 490 hp. now two 2.5" straight is good for like 820hp. but then you need to take length and number of bends into account on top of the muffler/resonator restriction. Also the duel exhaust is longer and has more bends. there is no way we could know unless we flow tested the two exhausts. but if you could have a 3" exhaust with minimal bends and a shorter length it could easily flow as well as a dual 2.5". do we know the 135i exhaust isn't bigger than 3"?
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From the photos the 135i, the exhaust doesn't look like the pipes get BIGGER from the headers back. So it's safe to assume that the 135i is the same size dia. Though, thats not stopping BMW from suprizing us.
BTW, the CFM you state is in perfect laminar flow. one 45degree bend will reduce those figures considerably.
A simple observation will sugest the 335i's exhaust will have less turbulance, thus a less restrictive ahaust.
-Garrett