It's all to do with the time it takes for an exhaust pulse to be reflected back to the exhaust valve.. all sorts of things enter into the equation.. like speed of the pulse.. engine frquency.. valve timing .. header pipe length diameter.. etc
the idea is to 'tune' the pipes length to resonate at whatever engine frequency that you have chosen through choice of cam profiles.. and other parameters..
Upshot is.. that it is beneficial for all of the exhaust ports to 'see' exactly the same conditions .. 1 and 4 can be linked as can 2 and 3.. unfortunatley because of the layout of an inline four cylinder engine.. this requires some fancy plumbing or a compromise.. usually a bit of both.. its not just a question of them being all of equal length.. but also of them being the correct length..(that's the hard bit)..which requires calculation and then testing..
So you could end up buying a system with equal length headers/downpipes that is not tuned anywhere near to the optimum.. it just looks nice.. buyer beware..
EDIT.. sorry just realised that you have a six.. same principles apply
Gettin' to old for this sh*t
Last edited by seadart; 25-08-11 at 18:09.