"Scientific Design of Exhaust & Intake Systems," Third Edition 1972, by Philip H smith and John C. Morrison. See it at: http://www.bentleypublishers.com/product.htm?code=G309
This is must reading for anyone who wants to better understand exhaust systems. I borrowed a copy from my Ford-engineer neighbor and liked it so much I bought my own copy to study and mark up.

 

After your reading....What do you think is the best solution for the I6? I plan on replacing the twin cats on my rig with a single high flo cat and do not know whether to get a 2.5 inch or 3 inch exhaust and performance muffler.

WHaynes

93 F150 4.9L 4x4
4" ProComp StII Lift
BFG 33in 4.10's
Detroit TrueTrac's

 

Beware, after one reading I am no expert. The book talks about a stepped pipe working well on their test engine. A larger diameter cat-back system would provide a sort of stepped pipe. The book notes some experiments where the stepped pipe providing a bore with 50% greater cross section (like a 2" pipe into a 2.5" cat-back) worked well and another experiment where they found a cross section 2 times larger (like 2" into 2.8") was not enough, 2.5 times larger (like 2" into 3.2") was good, and 3 times larger (like 2" into 3.5") was not necessary. There is a lot of other neat, more exotic stuff in the book like interference pipes to assist in scavenging and use of three exhaust manifolds (pairing cylinders 1&6, 2&5, and 3&4) on inline 6 to totally isolate each exhaust stroke from the others with the manifolds joining later into one exhaust pipe. I plan to put a 3" cat-back on my '95 F150 300 I6 when my exhaust goes. I do this not so much based on this book as based on my Ford-engineer neighbor who said the 3" system on my truck "can't hurt and might help." Since its cost is not a whole lot more than a stock replacement exhaust system, what have I got to lose? Further, enough folks on this site seem to have had good results with their 3" cat-backs.

 

I should add that I am extremely satisfied with the way my 300 I6 runs (it's totally stock)with the stock exhaust. This thing may not be that fast accelerating, but the high torque sure makes if feel good to accelerate, not like my wife's Aerostar 3.0liter which probably accelerates just as fast but dosen't feel good because the little V6 is winding up so much.