Back pressure is just BS, there is no reason for it. If the guys designing the engine wanted back pressure, they would have plugged the exhaust ports.
Give me a good reason for having any back pressure, or any reason for that matter because I sure can't think of any.
Headers are designed to pull the exhaust out as fast as possible. The exhaust pulse from one cylinder pulls the next one inline out with it due to a vacuum made by speeding exhaust gas. This is why cars with headers run lean, they actually pull fuel out of the cylinders when the valves overlap due to cam timing. Open exhaust manifolds do the same just, just not on the same scale as headers.
Here is a good read on exhaust pipe sizing. If you will note, a 1 1/2" pipe diameter can support approx. 78 hp. A 1 5/8" can support approx. 98 hp, both with in range of a stock 235 engine.
http://www.exhaustvideos.com/faq/how-to-calculate-muffler-size-pipe-diameter/ Joe
