Modified Mustangs & Fords
Shelby Cobra Mustang Exhaust System Upgrade
Exhaust Systems Can Make Or Break Your Mustang Experience
The second area to check involves the exhaust hangers. How loose or tight are they? If you had any fitment problems during the install, you might have had to "adjust" something using tools of low finesse, usually a hammer or a long crowbar. Now, if you moved things too far, you have likely put too much tension on one of the exhaust hangers, which makes it act as a sound transmitter, not an isolator. Still, it has to be really tight to do this, so if you can move the exhaust with your hands near the hangers, that's not the issue.
Why? Oh, Why?
There are some fundamental things going on that create the conditions for this drone to become evident. First, there must be a source, and that-believe it or not-is the firing frequency of the engine. Typically, the range of 1,800 to 2,000 rpm is where your Mustang experience is going to take a turn for the worse. That's also why the drone happens at a different speed when you change the rear axle ratio-it is really driven by engine speed, not road speed.
Now, everything in the world has what is called a "natural frequency." Guitar strings have one, bridges have one, and your eyeballs have one. When you pluck a guitar string, it will vibrate at its natural frequency and produce an audible sound. The Tacoma Narrows Bridge disaster is a famous example of winds causing vibration at the natural frequency of the structure, which eventually led to its collapse. If you want to see some incredible video, search YouTube using the keywords "Tacoma Narrows." Finally, if your body should ever be subjected to vibration with a frequency around 70 cycles per second, you'll find that you can't focus your eyes because they're vibrating in response to the input.
So it is with car bodies. They have a natural frequency, and for most that frequency is in the range of 80 to 120 cycles per second. So how does this relate to the engine speed? Well, 2,000 rpm represents a firing frequency of about 33.3 cycles per second, and that is well out of the natural frequency we just mentioned. But there are also things called harmonics. The third-order harmonic is three times the frequency of the base, so we have an input at about 100 cycles per second (33.3x3). We have a problem, and that is a killer drone.
The first manufacturer to effectively address this issue was Corsa Performance Exhausts, near Cleveland, Ohio. The company developed and patented a technique called "Reflective Sound Cancellation." In theory, it works the same way that sound canceling headphones do. In practice, it's rather more difficult to accomplish, though. While the company's products share a performance-oriented, straight-through design to minimize flow restriction, there is also a specially tuned structure within the muffler that generates its own low-frequency vibrations. These are routed back to the inlet of the muffler and effectively eliminate that third-order firing frequency which triggers the drone.
This is a clever technique and one that results in the best of both worlds. The exhaust system is quiet and drone-free on the highway, when you may want to talk with a passenger or dial up some tunes to help pass the time. When you want the car to roar, it will do that with authority because of the straight-through muffler design. The proof, of course, is in the driving, so when Corsa announced its new Shelby GT500 product, the "Mustang Ultimate Touring System," we stood them to the challenge. This axle-back exhaust is intended for modified, high-horsepower cars that can produce up to 800 or more horsepower.