If you have a large cam overlap, then lowering the back pressure to far can cause the incomming fuel charge to exit through the still open exhaust valve rather than filling the chamber rather than just scaveging the exhaust out. Too little flow however will mean the incomming fuel charge is blocked by the escaping exhaust

For each overlap/engine/fuelling/revs combination there is a perfect exhaust flow that allows the exhaust gas to exit and the fresh charge to fill the chamber. Its a bit of a suck it and see situation for each engine

Its not back pressure as such either, its reflected pulses.

"Modern Engine Tuning" and "Modern Exhaust and inlet manifold design" (both really good books) describe situations when increasing and decreasing flow can help, in reality there is a narrow band that is good for your engine

In general an increase in "backpressure" can raise the fuel charge (up to a point) and increase torque, whereas a decrease tends to aid gas flow (again up to a point)

So it is wrong to say that a particular pressure, or raising or lowering it is good, as every car will behave differently and you would need a dyno and a variable length/backpressure zorst to test

By the way F1 cars have variable length exhausts that can be altered for fast (speed) or tight (torque) circuits

Yamaha experimented for a while with a variable exhaust length gizmo that changed with the rpm on their 2 strokes

Mad Mole
Visit Mad Moles Mondeo Madness
2L Zetec, Cams, Stage 2 head, Valve work, Powerflow, Unichip, AEM Ram, K&N, Ecotek, 75mm Lower etc.
192 BHP, Faster than a speeding Duratec!