The best amount of backpressure for maximum horsepower is zero. It is possible that for some engines the best amount of backpressure for maximum low end torque is not zero. I've seen dyno graphs where low end torque was reduced by reducing backpressure, I've seen dyno graphs where low end torque was increaded by doing the same thing. I've even seen one exhaust test of several systems where the first one improved both high and low RPM power, then a second less restrictive exhaust improved high end power but the low end dropped to a point in between the first two graphs.
David Vizard claims that beyond 2.2 cfm of muffler flow per horsepower, you hit a definite point of diminishing returns. The Thrust Magnum Glasspack my turbo Dart will use flows about 500 cfm as tested by Car Craft, so it should be good for a mild turbo build. Perversely, it appears that the effects of exhaust pulses can occasionally cause more back pressure once pipe diameter is increased beyond a certain point (probably 2" to 2.5" for most slants). I've heard reports (but never seen the charts) where increasing pipes too much cost horsepower. However, most of the explanations for why this was so tended to defy several laws of physics and fluid dynamics, sometimes blatantly and sometimes subtly. Turbo motors are less likel to be affected by too large an exhaust pipe diameter, although I'd hate to try snaking a 3" exhaust pipe around my Dart's suspension and shift linkage.
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