There is an effect called scavenging where the exhaust pulse runs down the pipe, hits a restriction, and bounces back to the exhaust port, which helps evacuate the cylinder, and thus help suck in more mix on the intake stroke. The exhaust system needs to be tuned, i.e. lengthened or shortened, so the whole thing works in the most efficient way. I don't know if Triumph has engineered their pipes to use this effect, but it's something to consider when chopping up your stock pipes. You might get more sound, but you could actually be hurting the power.
I suspect the Thunderpipes use this method as a basis for their claims of increased horsepower.
John
