duels on stock 225

These are very square-bodied cars that tend to create low-pressure areas behind them. With a straight-out-the-back tailpipe you're pulling exhaust along with you, which is (at best) unpleasant. With a down-discharge tailpipe, the exhaust hits the slipstream and doesn't get pulled along.

This is related to the reason why station wagons have side-(and down-)discharge tailpipes; the rear body shape of the wagon tends to create a BIG low-pressure area right behind it, so it really drags exhaust along, which is extra-dangerous because of the openable tailgate glass.