Exhaust pipe length / diameter

["Jack W. Drews" <vinttr4@geneseo.net>]

I'm working on an MG for a friend, am new to the list, and don't
normally work on MG's so am not familiar with MG lore.

My friend has purchased a new 3 - 2 - 1 exhaust header, and we will be
putting an open exhaust pipe on the car, since it will be used as a dual
purpose street- and vintage race car.

My question is, is there a length / diameter for the exhaust pipe that
is optimum for this setup?