The tones emanating from a car’s exhaust pipes — a V8’s deep bellow, say, or the piercing wail of a high-revving V12 — have long been inspirations to an enthusiast’s ear. What others may deem mere noise can, appropriately tuned, become a symphony of internal combustion.
With the help of computer modeling and some mechanical gadgetry, automakers are tailoring exhaust notes to more precisely define the character of new models, most vitally in those cars with high-performance credentials. No longer is the goal just to muffle the explosions taking place in the combustion chamber, but to give the engine a voice as readily identified as a Carnegie Hall patron might distinguish a clarinet from an oboe. ODM Universal-Fit Catalytic Converter
“Engines and their exhaust systems are very much like musical instruments in that they have different sound signatures based on the physics of the exhaust gas pulses and how they travel through the pipes,” Garry Dunne, director of sound quality engineering at Jaguar Land Rover, said in a telephone interview.
Each “bang” exiting a cylinder and hurtling down the tailpipe includes multiple harmonics, a result of factors including the size and number of engine cylinders and the sequence and timing of their firing. Engineers “tune” the frequencies to create a pleasurable soundtrack — and to meet performance targets and noise regulations — by varying the exhaust pipe diameter and length and installing resonator chambers and mufflers of various shapes and sizes along the route. Materials matter, too: Exhaust manifolds made of thick cast iron attenuate sound differently than those fabricated from thin steel tubing.
Full-time loudness is not the goal. Drivers are more likely to want a hushed growl around town, saving the full-throated roar for the open road.
This duality is how Dr. Dunne describes Jaguar’s S versions of the new F-Type sports car, which, along with the Chevrolet Corvette Stingray offer active exhaust systems. In both cars, electronically controlled flapper valves can open or close when the driver selects specific driving modes. Responding to the throttle, the valves operate at very high speed.
“They go from fully open to fully closed in about five milliseconds,” said Traci Melville, an acoustics engineer at Tenneco, the supplier company that developed the Corvette’s valves. As a concept, active exhaust systems have been entertaining drivers back to the Model T era, when accessory catalogs bulged with ads for “cut out” valves. By the 1990s, with the advent of miniature solenoids, drivers could control the valves in the exhaust system of the Mitsubishi 3000GT.
Since then, automakers have introduced both electronic and pneumatically actuated (Chevrolet Camaro ZL1) valves, while others have opted to pipe real (Cadillac CTS) or electronically reproduced exhaust sound (BMW M5) into the cabin to achieve the same effect.
Creating more sonorous exhaust concerts has become a sophisticated science. It begins with a mathematical model of the engine and its combustion system, said Tim Jackson, Tenneco’s executive vice president for technology.
“The model lets us test up to 1,000 variations of an exhaust system right on the computer before we build the first part,” he said. “We don’t need to start with a real engine. We run the test all night, then the next day we pick up to five systems that look the most promising.”
The computer models create audio files, which are played to an automaker’s jury through headphones in a dedicated quiet room. The jury chooses the sound packages that most closely match the intended character of the new vehicle.
At Jaguar Land Rover, a team responsible for all new-car performance attributes signs off on the exhaust system acoustics, Dr. Dunne said. He noted additional tricks of the trade that automakers were using to add spice to the tailpipe chorus. On the F-Type S, custom fuel injection and ignition calibration make the exhaust pop and bang like a Tommy gun when the driver lifts off the gas.
Diesel Oxidation Catalyst “It’s the bit that makes the whole driving experience exciting,” Dr. Dunne said. “It’s a very fine balance in delivering the sound character we want for this car.”