[Daniel M. Dreifus]

Redline complete fuel system cleaner used regularly - keep it up perpetually, will do more for you than a performance muffler and will add more than a horsepower or two.
Larger diameter exhaust pipe will reduce exhaust velocity and actually hurt low end torque.
Also make sure your air filter, spark plugs and wires are in excellent condition, and that tire pressure is correct - with stock 14" tires try 30/26, F/R.