[BigSwede]

Thats not back-pressure...

Back pressure is something you get when there's not enough room for the exhaust gasses to flow freely. For instance, on a turbo car you get back-pressure because the turbo is obstructing the free flow, you can even get a negative pressure differential over the engine i.e. higher pressure in the exhaust manifold then the intake manifold and thats why you should be a bit restrictive running "wild" cams with lots of overlap in a turbo engine with a small turbine.