Other than the physical size of the 6 speed gearbox and possibly its cost is there a reason why Morgan would have chosen to use a 5 speed gearbox instead of the 6 speed for the M3W? After all the S&S engine behaves very much like a turbo diesel, loads of torque but a limited rev band and that usually results in a 6 speed box these days. Is it related to the strength of the gearboxes maybe? Could the 6 speed handle the torque peaks?
