Don Roy
February 1, 2009

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.

From under the car, loosen the band clamp at the front end of the muffler inlet. The clamp is riveted to the pipe, so don't bother trying to take it off.

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.

There are hangers at either end of the muffler. They will prevent you from pulling the muffler forward enough to get it off the exhaust pipe. Each hanger is secured to the framerail with two bolts. For the rear hanger, it's reasonably easy to see where the fasteners are.

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.

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