At its core, an exhaust system is nothing more than a series of pipes that route toxic fumes from the engine to the back of a car. At least that's what they thought back when outhouses were still in style. Now, as advanced life-forms that take care of business indoors, we've gathered sufficient data that proves otherwise. Despite its perceived simplicity, exhaust-system tuning is a complex art that still baffles some of the top engine builders in the country. Minor variations in collector size, header primary length, and tubing diameter can significantly impact power output for better or for worse. To make sense of it all, we hit up Scott Stutler of DynoMax. In addition to explaining some of the science behind various exhaust theory, he offered some valuable tips on how to select the right components for your car, keep noise in check, and piece together a custom exhaust system that best suits your needs.
Collectors don't look like much, but they play an important role in exhaust tuning. Ideally, the collector diameter should be within 0.5 inch of the rest of the exhaust system. For example, on a 500hp motor with 1.875-inch primaries and a 3-inch exhaust, the collector should measure between 3 and 3.5 inches. "Shorter collectors produce peakier power curves and improved high-rpm performance, while longer collectors produce a broader power curve," Scott explains. "As a general rule, guys with open headers run header extensions to maintain a smooth transition from the four header tubes into the collector. Extensions give another 12-14 inches for exhaust pulses to transition and smooth out."
The virtues of equal-length primaries make for great advertising propaganda and bench-racing discussion, but they're neither necessary nor practical in a street car. The goal with equal-length primaries is to tune the exhaust pulses by experimenting with different primary lengths. If done correctly, the exhaust wave will arrive at the exhaust valve right when it opens to maximize scavenging. "In reality, when you put each type of header on the dyno, the power difference between the two is negligible," Scott says. "In NASCAR or Pro Stock motors, where a 1hp improvement is considered a huge deal, having equal-length primaries is more important. However, you're splitting hairs by comparing headers with equal-length and non-equal-length primaries in a street application."
The two prevailing technologies in mufflers to reduce exhaust noise levels are through the use of chambers or by packing them with fiberglass. Each method has its pros and cons. Fiberglass mufflers work by converting sound waves into heat energy through absorption. "They're less susceptible to resonance than a chambered muffler design and typically flow better as well," Scott explains. "The downside is that the sound level changes as the fiberglass material burns up or melts, at which point it loses all damping ability. On the other hand, chambered designs won't burn out, but they produce a love-it or hate-it tone. We use both sound-damping methods in our Super Turbo muffler, which makes it unique."