Arnie and Baldy are right. A close friend of mine has one of the top pull tractors in the country and spends not hours but days on his dyno. He sometimes takes my humble advice although the student is now much smarter than the teacher. Here is a description of exhaust function in very common and hopefully easy to understand terms. To visualize scavaging think of the old suction guns used to suck up oil out of the old cannister type filters, when you pulled up the piston in the barrel of the gun it created a vacuum that sucked up the oil. In the exhaust tube on an engine the piston is replaced by a pulse(ball) of exhaust and the faster it moves through the tube the greater the vacuum is behind it. At this point when the intake valve opens and this vacuum from the exhaust helps pull(scavage) the burned gasses from the cylinder and pull in the new air/fuel mixture. Now, if you increase the size of the tube the ball doesn't fit as tight and loses some to much of it's suction so the scavaging effect drops. If the engine can run at a very high rpm then the large tube gives less restriction and though each pulse has less draw there are so many more of them moving at much higher velocity that there is a net gain in power. In a low rpm engine big exhaust tubes will kill low end. I know I've used "vacuum" with license but it is easier to visualize. Now if all this back to school stuff hasn't put you to sleep you can go on to the next thread.

Evan