For enthusiasts of a certain age, combinations like five-liter Fox body Mustangs and dual-chambered Flowmasters go together like peanut butter and chocolate. Pairings like these helped to define the new sound of American muscle as performance cars clawed their way out of the lean times of the late 70s and early 80s, and advances in technology began to usher in an entirely new era of hot rodding.

That beefy sonic signature didn’t come without a bit of compromise, though. Like many of us, your author rocked a Mustang GT with the requisite pulleys-and-Flows back in the day, a car which purportedly made other cars shake while cruising a few lengths behind them out on the road.

Grin-inducing to be sure, but memories of teeth-rattling exhaust resonance also come flooding back in retrospect. The trek from the Bay Area to Southern California (and back) was a regular occurrence at the time, and locking into an agreeable pace in top gear out on the highway meant settling into an RPM range where the exhaust’s bass tones took center stage.

But it probably shouldn’t come as a surprise that hardware which was originally developed for motorsport brings a few caveats with it when you take it to the street. “The original 40 Series mufflers were designed in the 80s in Northern California,” Flowmaster’s Mark Emerson explains. “Back then, a lot of the local circle tracks were coming under fire for noise issues as residential housing was being built up around them. So in order to keep the tracks open and in compliance with the sound restrictions, [Flowmaster founder] Ray Flugger developed the original muffler with a single deflector.”

Flugger’s primary goal was to dial back some of the exhaust volume without negatively affecting engine performance. “It worked,” Emerson notes. “And eventually those components found their way into street cars – it had a different sound compared to a lot of the other mufflers that were out there at the time, and people really responded to that. Suddenly you could make a car or truck sound even healthier than it might be, because of this aggressive exhaust sound.” As a motorsport-derived part, these mufflers were keenly focused on performance in a racing environment, where extended periods of steady-state throttle applications almost never occur. The same can’t be said for a typical street car application, though.

But a lot has changed over the years since then, and technological strides have had a profound impact on Flowmaster’s approach to street car muffler design. Here we’ll get some insight into what exhaust drone is exactly and what causes it, and also take a look at how Flowmaster has developed solutions that address these errant sonic resonances while retaining that burly sound that has helped to make the brand a performance icon.

The 3 Chamber Design (now known as the 50-Series) was Flowmaster's first muffler offering that has sound control as a core focus of the design. It provides the same performance benefits as Flowmaster's more aggressive offerings with a milder tone that is drone-free, making it a solid option for daily driven hot rods.

Flowmaster engineers use spectrum analyzers like this one to evaluate not only what sounds comes out of the pipes, but also how sound waves move through the system. This data helps engineers identify and tune out unwanted frequencies during the development of new exhaust system components.

When Frequencies Attack

First things first: What is “drone” exactly, and what causes it? “At part-throttle RPMs, the sound frequencies and harmonics can kind of bleed out of the system,” Emerson says. “That can cause them to reverberate through the chassis, and in certain applications, that’s not particularly desirable.”

In other words, the exhaust system uses the vehicle’s chassis almost like a tuning fork, amplifying certain sound frequencies when engine RPMs enter specific ranges. Part-throttle driving is typically done at low engine RPMs, where bass frequencies tend to be more prominent. “And that’s why you’d see a lot of folks using products like Dynamat to address it,” he says. “You’re basically damping those frequencies with material like that.”

Emerson tells us that muffler position and a vehicle’s overall design play roles in exhaust drone as well. “I remember when the 2005 Mustang first came out, Ford had moved the mufflers to the back of the car. With previous Mustangs, we had always used 40 or 50-Series Delta Flows in the exhaust kits we produced for the car. But by moving the mufflers to the back of the car, you lose almost all of the tailpipe, so you end up with more potential for drone as a result, and that necessitated some changes. And the layout of the car matters too – SUVs are basically big speaker boxes, for example. That makes them more susceptible to drone.”

Tuned To Taste

As Flowmaster began to develop more street-focused exhaust products, the company developed several different strategies for keeping exhaust resonance at bay. The first of those to come about was the 3 Chamber mufflers (now known as the 50 Series Muffler), which built upon the original Flowmaster design while also addressing those requirements.

It’s an approach that has led to more sophisticated products, like mufflers that are tuned to attenuate those unwanted frequencies. “We started playing around with the resonance chamber inside the muffler. That’s where our three-chambered mufflers like the Super 50 come from. In that design there’s a main flow tube for the exhaust pulse to go through down the center of the muffler, and then off to the sides are one or two tuning tubes in the front chamber that are spaced and sized specifically to trap certain sound frequencies that would normally bleed off as they entered the muffler. These mufflers still offer the same performance as the louder options, the volume is just dialed back a bit, and you don’t have that drone between, say, 1800-2200 RPM, where you would traditionally get it in most vehicles.”

The next technology used was the Delta Flow design. “With the original muffler design, the exhaust pulse is split around a single deflector, and when it comes back together, the sound frequencies cancel themselves out when they collide,” Emerson says. “Designs like the Delta Flow mufflers build on that concept, using multiple deflectors instead of one. And that allows us to size and space out those deflectors in specific ways to cancel out certain sounds.”

Flowmaster’s development strategy also moved beyond just the mufflers themselves, in turn providing more comprehensive solutions. “On a lot of applications we’re able to use off-the-shelf mufflers that we offer,” he says. “But sometimes, if we’re not satisfied with the sound on a particular vehicle, we can go in and design a muffler that’s essentially tuned specifically to that vehicle. That gives us more control over what frequencies are canceled, and what’s emphasized.”

Many modern muscle cars utilize software to control valves inside the exhaust, in turn changing the tone and loudness of the exhaust based on driver inputs or the mode the vehicle is set to. By combining these active elements with Flowmaster’s motorsport-derived muffler designs, late model enthusiasts can get the burly sound they want without having their teeth rattle on the highway.

Old School Sound With Modern Tech

The recent proliferation of active exhaust systems – which use an electronically controlled valve inside the exhaust system to control the character and volume of the exhaust system – have also opened up new possibilities for those that want the best of both worlds in their modern street machines.

“On some of the newer Camaros, Mustangs, Challengers, and Chargers, these active valve systems are equipped from the factory,” Emerson points out. These systems use real-time factors, like throttle position and the drive mode currently selected, to determine how much to open those valves. In turn, it provides something akin to a volume knob for your exhaust system.

“And that allows us to use a more aggressive muffler without the potential drawbacks in terms of drone. You can have the loud, aggressive sound of an American Thunder system when you want it, and then the valves give you the ability to mellow that out a bit when you don’t.”

OEMs have been slower to port this active exhaust technology over to their truck lines, though. But fret not – Flowmaster has a solution here as well. “We recently debuted active valve kits for the new Chevrolet Silverado and the new Ram 1500,” Emerson says. “Those allow the exhaust to run through either our FlowFX mufflers, or you can press a button on the key fob and it will switch over to a race-style, single chamber muffler – almost like a ‘dump’ system. So it can be really aggressive when you want it to be, and whenever you need to calm things down, you can just hit that button to switch over to a nice performance sound over stock that isn’t crazy-loud.”

Emerson adds that Flowmaster also offers universal in-line resonators, which can be used in conjunction with an existing exhaust system to quell interior resonance issues. “Typically these would be put in front of the muffler to tone things down a notch or two.”

He also offers a bit of advice for would-be muffler shoppers. “Really think about your muffler choice before you decide. A lot of people tend to gravitate toward the loud stuff, and that’s cool, but it’s not always the best option for a particular application. If it’s a daily driver or something you’re taking trips with the family in, you may want to factor that into your decision. And you can definitely still get a great sound from one of the mid-range options.”