Exhaust flow talk

[NoHachi]

You are still suffering to much from the myth...don't think backpressure..think in terms of flow and resistance..resistance costs hp...full stop. It does it at high rpm as well as low rpm..resistance is the enemy.

Knowing that, 99% of the exhaust parameters have been decided..the ideal system is a trumpet right after the header..(search around, I think that they are even mentionned in Bell, along with a picture of them on a drag VWbug), but that would leae the neighbours deaf and you in jail. THATS why we have exhaust, we need to dampen noise, and the best way of doing that, is adding as much pipe under the car as we can (exactly because of your observation that the gasflow loses energy as heat as it progresses through the exhaust). Now, the header has a diameter, pipes come in constant diameters, so what to do? Keep weight to a minimum (small diameter)(weight is the no1 performance gain from aftermarket exhausts!), maximise power.
In practice that means that you keep the entire exhaust path at the diameter with which it exits the header..inc the perforated tube in the fartcan. The further back you get, the lower the velocity, the less trouble you have from discontinuations. Thats why fartcans, when done poorly, dont gain you any hp, but also cost you almost no power or torque..show me a dyno that proves me wrong. A fartcan done well (inner diameter equal to the rest of the system) gains you power, it reduces resistance along the entire revband (and you pay for this through police tickets and eventually a hearing impairment.)