I can see that with a exhaust that's built using a pipe bender there will be restrictions that reduce flow in comparison to a mandrel bent equivalent but on what scale would this be and what would the power loss be expected to be? I realise this is quite a broad question.

Is there a guide as to the diameter reduction on the pipe, 2.5" for example?

I've got some bender-bent water pipes, and the constriction seems to occur in the form of corrugation on the inside of the bend. This is where the gas velocity round the bend will actually be lowest anyway, so I'd expect maybe 1%.

 

More generally, it's best to keep the pipes as straight as possible and use the widest radius bends you can.

 

For example, Jason's mongrel presently runs a side-exit so it only needs 2 bends, one down to backwards and one 45 degree.

And when you're going over the rear axle it's a different story, bends here there and everywhere! Thanks for the info.

Maybe on a massive power car! But on a road going fabia Vrs for example I really cannot see it making a difference at all

it's not just ridges it's the slimming down----horses for courses but if it matters go +20% over mandrel bent