If you have a medium wild cam and very low back pressure, the exhaust will
carry some of the intake charge out and thus reduce torque at low speeds,
so that in that particular case you can enhance driveability with a little
increase in backpressure with the loss of high-end performance. This would
not be a problem with a "C" cam but might be with a "D" cam. Certainly
at the center of the torque range, lowest back pressure is best, but driving
around town it might be otherwise. In any case you don't want much backpressure.
--
George Downs, The "original" Walrus3, Bartlesville, Oklahoma