Cats
Whether you choose to or are forced to run cats, aftermarket units are almost guaranteed to flow better than factory hardware. It's not necessarily due to differences in quality, but rather longevity. An OE converter has a higher volume of precious metals, and therefore larger bricks, since they're designed to last more than 100,000 miles. "In an aftermarket converter, the brick is smaller because federal law requires manufacturers to warranty it for only two years," Scott explains. "The smaller the bricks the better the flow and easier the installation. An aftermarket cat will typically flow 5-15 percent more than a factory cat."
Resonance
"Most people don't mind a loud exhaust but can't stand resonance. Nine out of 10 times, people have resonance problems because they don't have a balance tube in the exhaust system. An H-pipe is good, but an X-pipe is better. If you're still getting resonance with a balance pipe, the next step is using an absorptive muffler instead of a chambered muffler. This may increase the overall sound level, but the resonance will decrease. If that's still not enough, using resonators can reduce sound level enough to where the resonance isn't noticeable. The best way to route tailpipes is to have them exit straight behind the car. Turndowns direct sound waves toward the ground, which will bounce them off the pavement and rattle your floorpan."
X-pipes vs. H-pipes
A balance tube is absolutely essential to a good exhaust system, and you're leaving power and fuel economy on the table without one. Both X-pipes and H-pipes get the job done, so which is better for your car? "An H-pipe is pretty easy to incorporate into an existing exhaust system, and in cars with a tight chassis like G-bodies, an H-pipe is easier to install," Scott explains. "X-pipes are superior to H-pipes because they provide a smother transition from bank to bank for exhaust gases exiting the ports. Due to this reduction in turbulence and sound waves, X-pipes yield increased flow and a reduction in noise."
Header Design
"It's a well-known fact that long-tube headers are superior to shorty headers, but there's more to the equation than just primary length. A shorty is usually just a copy of a factory manifold that happens to flow a little better. Since exhaust velocity coming out of a port can reach 700 mph, it will go through pretty much anything that's close to the port. Long-tubes, on the other hand, are designed with exhaust velocity in mind. With long-tubes, not only do you improve flow, but you're also moving the powerband by changing the scavenging effect of the exhaust. For instance, if a motor has a sweet spot at a certain rpm range, you match primary length to maximize scavenging at that sweet spot and see a significant improvement in performance. In the best-case scenario, every header would be tuned to maximize scavenging for each engine, but that's not realistic from a cost perspective."