Boy this topic gets kicked around alot. It is all personal prefrence to looks and cost. My 2002 peterbilt c12 435 horse runs a 4 inch downpipe until under the cab. As far as exhaust being only as good as the smallest point, (in refrence to actual turbo outlet) i partly agree with but as the exhaust cools, it also looses velocity which is helped by the free flowing pipe all the way out being the same size. The only way you could see if one was slower to spool than the other is a controled enviroment as a dyno and datalogger to actually know exactly waht rpms spoolup happens. Tim