Conventional wisdom says exhaust backpressure is bad. If you want to maximize horsepower, the thinking goes, you have to minimize backpressure. Consider the ultimate low-restriction exhaust system: A Top Fuel dragster like the one you see above. Each cylinder gets its own exhaust pipe, which, at roughly three feet long, only serves to steer the exhaust flow up and away from the engine—in the process, using the force of the exhaust to create a little extra downforce to boost traction.
But there's a conflicting theory that you'll hear: Too little backpressure is a bad thing. According to this line of thinking, if your exhaust system is too free-flowing, it can actually decrease your power output. Though if you ask a forum poster to explain this, you'll likely end up with an even foggier understanding of the science.
Thankfully, is here to set the record straight. As it turns out, backpressure is simply bad for power output. But the too little backpressure argument? That's a whole different matter.
Basically, when people claim that a certain amount of backpressure is beneficial, they're mixing up backpressure and scavenging. The latter is a phenomenon where the movement of gases through the exhaust system creates a partial vacuum that can actually suck the exhaust out of the cylinder. A properly-designed exhaust system maximizes this effect across a wide RPM range, efficiently evacuating spent exhaust gases using a precisely tuned and shaped exhaust system.
We could go on, but we'd rather let Fenske explain the fine details, as he's imminently more qualified for the task.