I reopened this, after reading the linky to the explanation. There's some things I wanted to add, and even contrast (no surprize to veterans here).
I agree with the basics of the explanation, except that he forgot about gas velocities. It's all cute and clever to make comments about vacuum cleaners on the backside, but the reality is you can't run well without SOME kind of exhaust pipe.
Let me explain...
Yes, in the BASIC sense, the best exhaust is no exhaust, but the reality is a bit different... part of what speeds the exhaust along is conduit velocity. Remember where he mentions the narrow runners in the manifold? There's a reason for it...
I've always said it: port velocity plays a part! Smaller diameters result in higher fluid velocities (remember that a gas is a fluid of sorts). So too small an exhaust pipe means more restriction, but too big takes away the exhaust velocity. See, exhaust gas is HOT, and as such, wants to *expand*. Since it can't expand as easilly backwards towards the turbo, it goes the easiest route, which is OUT. The heat literally helps it along through the piping.
So you have to have SOME kind of pipe. It can be much smaller than standard, and yes, you CAN indeed route them out through the front fender, which is what most drag and some road course cars do. Others vent it out the side middle of the car.
There's something else to consider here. BOOST CREEP. Ever seen a small turbo'd car with too big of an exhaust? It hits max boost (that the boost controller is set to), and then starts to creep past it slowly... I saw a Mitsu Eclipse 1G with the 14B turbo (between the size of a T25 and a T28) and 3" exhaust, and that sucker spooled out of control, absolutely beyond the ability of the boost controller to stop it. So you have to be careful!
Exhaust theory basics are great, but the bottom line is that those of you running a T25 or smaller, a 2.5" is plenty. Bart ran a 3" with a T25, but he also had a massively huge intercooler and a custom tubular O2 housing among other things to offsent the massive flow on a T25.
3" is generally accepted perfection for most T3 and T3/T4 applications, with 4" reseved for larger setups.
It's that simple. Read around and see results. Theory is nice, but results are the proof in the pudding.