Modified Mustangs & FordsHow To Engine
Shelby Cobra Mustang Exhaust System Upgrade
Exhaust Systems Can Make Or Break Your Mustang Experience
MMF Tech Corsa Performance Exhaust
You've likely experienced or heard of someone with a "drone" in his or her Mustang exhaust system. This is a noise that is typically encountered during highway driving when the engine is under light load and running at about 2,000 rpm. If you think that maybe you've heard it, you haven't. When you do hear it, you'll know for sure. A good, proper drone fills the interior cabin, and your skull, with a penetrating resonance that rattles your eyeballs and makes your hair hurt. While your Mustang's exhaust can be the source of a most glorious symphony under acceleration, it can also become your worst nightmare when you just want to cruise.
The science and mechanics of correcting noise, vibration, and harshness (NVH) in an automobile are complicated, largely because an automobile is a rather complex piece of machinery. Anything that rotates faster than about 1,200 rpm is a potential source of noise because that rate is also 20 cycles per second-the practical lower frequency limit of our hearing. Of course, you can hear the engine running at less than 1,200 rpm, and there are many reasons for this.
The alternator and other front-end accessories are being driven at different speeds than the crankshaft. Moving down the road, driveline components-from the back of the transmission reduction all the way out to the tires-will change speed depending on what gear the transmission is in. There is a complex symphony in place, and that is just the beginning of it. In spite of the fact that a component is rotating at a given speed, it can put out sounds at a higher frequency. This is called a harmonic and is responsible for the uniquely glorious "rumble" that American V-8 engines have.
Secondary vibrations in the crankshaft are responsible for this and happen because of the dual-plane design of a typical North American V-8 crank. Listen to the noise made by a Lotus V-8, with its single-plane crankshaft, and you won't hear that rich rumble. What a loss-no kind of exhaust system can restore what isn't there in the first place, and that's something many four-cylinder owners need to recognize.
Please, No Physics
While we're not going to look at any formulas or get you working with a calculator, it's important to visualize sound in your mind as waves. Like waves at the beach, they arrive at a particular rate that determines what they sound like. Most of the sounds in your car are in the lower end of the audio spectrum-from about 20 per second to about 6,000 per second. This is their frequency.
Now, waves are also a little like numbers, in that they can be added and subtracted. When one wave is rolling out on the beach, it can take force away from an incoming wave. When two waves going in the same direction meet, they can combine to make a stronger wave. When 20 or more different waves meet in an important place, like the interior of your car, well, you can never be quite sure of what is going to happen. It might be glorious, or it might be an almighty drone.
Little else could be more disappointing than just installing a new exhaust and finding that you also have a drone that can loosen the fillings in your teeth. It could be related to the new exhaust, but before calling customer service or ranting hopelessly on the Internet, there are a couple of things to check first to make sure that you didn't cause it yourself.
Now, if it is happening under wide-open throttle, it is not the drone we know and love. It is probably a "short"-a place where the exhaust is touching the body when the engine rocks under full acceleration. You'll want to go back over the entire length of the exhaust, looking for places where it sits too close to the body or suspension pieces. Worn engine and transmission mounts can contribute to this kind of problem.
Photo GalleryView Photo Gallery
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.
Pudding & Proof
Our victim, er .test car, was a local GT500 that had previously undergone exhaust surgery with typical results. With the OE system installed, drone is not really a big problem, but flow restriction can be. With a more open exhaust, the drone was evident-big time. So we put the Shelby up on a hoist and got to work. You can do the job in your driveway, but a hoist makes it go more smoothly. Once the mufflers were changed, it took no time at all to verify what the manufacturer claims. The sound output at idle and low speeds is quiet. You would have no complaints at all from the neighbors in your underground parking garage. Out in the open, it is another story. As soon as you get on it, the exhaust note changes to a glorious, deep and throaty rumble that has no raspy overtones whatsoever. Finally, the payoff is that when you are out on the highway, cruising along around the 2,000-rpm drone zone, there is nothing to worry about. Your fillings are going to stay in place, and "drone" is nothing more than a bad memory form the past.
The Corsa product is of full stainless steel construction and is a little heavier than the OE mufflers they replace. They are physically smaller, so getting them in is easier than getting the originals out. The polished exhaust tips are beautifully done and fit very nicely in the cutouts of the Shelby's rear fascia. There's no doubting the quality that goes into building this product, and the exceptional design delivers exactly what the company claims. Just how does it get any better?