There have been many arguments about how much hp a 2.5" exhaust system will flow before 3" is necessary.
some say 600-700, some say 1800.
The main consensus on tnett seems to be that 3" is too large and will cool/slow down exhaust flow, and that the best system should be a full 2.5" system keeping the same pipe size from downpipes to exhaust tips.
I found an interesting article about an exhaust system made by magnaflow for a project mustang"the guys had built a really trick system consisting of 2½-inch tubing from the header collectors into the X-pipe, 3-inch out of the X flaring into 3½-inch tubing running for about 26 inches before necking back down into a pair of 3-inch mufflers with turn-downs. "
this seems like a lot of transitions but it actually made 12 hp more than a full 2.5" system. It also made 2 more horsepower than open headers. I understand that they kept the exhaust 2.5" leaving the headers to keep speed up but afterwards they went up to 3 1/2" on a car thats not even making 350 hp with great results.
"The theory behind this design is that it will keep the velocity of the exhaust gasses moving quickly through the headers into the X-pipe to maximize the scavenging effect, while the larger-diameter tubing ahead of the mufflers prevents the gasses from stacking up as they pass through the mufflers to avoid excessive backpressure in the system."
La-Z-Link
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Food for thought about 2.5" and 3" exhaust pipe sizing - 
