Backpressure
Some people say a motor needs a bit of backpressure to preserve low-end torque, while others argue that the less backpressure you have the better. "The fact is, an engine does not need backpressure," Scott says. "What it needs is scavenging, which is directly related to exhaust velocity. A common misconception is that you'll lose some low-end torque by stepping up to a larger after-cat system. What's really happening is that the larger-diameter piping is moving the rpm range where the scavenging takes place higher up the powerband. Since exhaust velocity increases with rpm, you're not losing power but moving it higher up in the rpm band instead."
Resonators Vs. Mufflers
"Resonators and mufflers are similar, but not quite the same. Consequently, there is a time and place for both. By definition, mufflers and resonators are both sound-attenuation devices, so they're one in the same. However, by practice, a resonator is perceived as something round and compact, while a muffler is perceived as being larger and more substantial. Resonators reduce exhaust noise by 4-6 decibels, while mufflers reduce noise by more than 20 decibels, so the applications in which they are used can be very different. Resonators are small and light, so they're popular in racing classes that require some type of muffler. On the other hand, for street applications, using a resonator without a muffler typically emits more noise than people care for. If someone is having a problem with resonance, a practice becoming more popular is installing one resonator or multiple resonators in addition to a muffler, which can reduce resonance enough to where it's no longer noticeable. Our Bullet mufflers are used very often for these types of applications."
Mandrel Bends
It's almost universally accepted that mandrel bends are superior to crimped bends, but just how much better are mandrel bends over a very well-done crimp job? Typically, you'll see a 20 percent reduction in the inside diameter of a crimped pipe compared to a mandrel-bent pipe. "If you're mandrel-bending a 2.25-inch exhaust, it will flow as well as 2.5-inch inch crimped bend," Scott explains. "If someone wants to run 2.25-inch pipes with crimped piping, then they should step up to 2.5-inch tubing. A mandrel bend is as close as you can get in flow to a straight pipe."
Case Size
The physical dimension of a hot rodder's muffler choice is often dictated by the amount of space available beneath a car. However, the size of the muffler case can also affect how well it flows, so there's more to consider than just packaging issues. On a straight-through muffler-one that does not use internal baffles or deflectors-the size of the muffler case will not affect flow. In a chambered muffler, case size can affect flow depending on how the reflectors are arranged. "In general terms, the shorter the muffler the better the flow, and sound is greatly impacted by case dimensions and by shape as well," Scott says. "Although oval mufflers have a tendency to resonate, they are typically quieter than round designs. Also, a longer case usually results in a quieter exhaust."