ive built many different types of manifolds , mostly over the gearbox type . i use 1" 1/2 pipe on long headers and any thing upto 1"5/8 for high powered car running short hearders as karlos g says its about velocity and ive got provern manifolds at well over 500+bhp with unequal lenght headers (which are not the best) and a cone style collector pot using 1" 1/2 pipe . now ive moved the game upto the next level with the use of race merge collectors which are the ultimate in terms of flow design. the only limiting factor of fitting a 4-1 merge collector is room in the engine bay.
heres a few of mine and on some well known cars.

old style manifold , unequal lenght headers and cone design made 522bhp



next stage equal lenght headers but still with cone design collector



another one i built



next was equal lenght headers with the race 4-1 merge collector
and a very well known passionford engine bay "luca"











another one





and another one that did well at central day fitted to a white mk6 fiesta zetec turbo





cheers paul