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June 9, 2003

USA: Sun never set on Pontiac GTO exhaust note engineers

General Motors engineers on both sides of the globe have redesigned the dual exhaust system of the Australian Holden Monaro coupe to give the upcoming 2004 Pontiac GTO derivative an exhaust note said to be reminiscent of the classic original.

By bcusack

General Motors engineers on both sides of the globe have redesigned the dual exhaust system of the Australian Holden Monaro coupe to give the upcoming 2004 Pontiac GTO derivative an exhaust note said to be reminiscent of the classic original.

Mechanically, the exhaust system on the GTO consists of two completely independent exhaust paths that run from the manifolds to the outlet pipes, ensuring there is no mixture of exhaust gases.

However, creating the necessary “voice” of the vehicle required much more than simple mechanics.

“Anyone who has ever heard a classic GTO knows that distinctive ‘sound,'” said Pontiac-GMC general manager Lynn Myers. “It’s very much a part of the character of both the GTO and Pontiac, so we knew we had to recreate that same feeling in the modern version.”

The Monaro, on which the new GTO is based, didn’t have the “sound” Pontiac needed. “The Monaro is a great performance car, but it’s sold as more of a luxury vehicle in Australia so the exhaust note was fairly quiet,” said GTO marketing director Robert Kraut. “From the beginning, everyone knew that the exhaust would have to be ‘dialled-up’ to give us that special sensory cue that says, ‘This is a Pontiac.’ “

“We had three main priorities for this vehicle right from the outset: power, launch feel, and an exhaust note with lots of character,” said GTO programme engineering manager Dave Himmelberg. “Our sound objective wasn’t about making the exhaust loud. It was about bringing a smile to someone’s face as soon as they turned the key.”

Early in the vehicle development process, engineers test-drove an original 1964 GTO on loan from the Pontiac historical collection to acquaint themselves with its acoustical traits.

“We wanted to get a real good feel for the sound character of the vehicle,” Himmelberg said, “and that drive provided us with an acoustical baseline from which to start.

“We then listened to the 2002 Corvette, which also has a really good sound with traits we were looking for. With those two baselines, we pretty much had in mind what we wanted to accomplish. Our goal was to come in at a pass-by rate of just under 80 decibels, which is the legal sound limit in some states. But, we knew it couldn’t be objectionable. It needed to be a pleasing 80 decibels.”

At that point, GM engineers in Australia and North America worked in parallel to develop an exhaust system that delivered the desired sound and performance characteristics.

“It became a true global effort because the exhaust needed to be developed with parts available in Australia, but acoustically tuned to the spirit of an American classic,” Himmelberg said.

A true dual exhaust system was developed and acoustically tuned to meet the heavy-breathing, low back pressure needs of the 340-horsepower LS1 V8 powerplant.

It consists of dual catalytic converters, one on each exhaust bank; two resonators between each catalytic converter and muffler; two mufflers with different internal flow paths (the right flow path is longer than the left) to achieve a harmonic imbalance between the exhaust paths, enabling engineers to essentially mix tonal qualities to achieve desired sounds; specifically sized and routed stainless steel pipes to ensure reduced back pressure as well as desired tonal qualities; and a single tubular brace that holds the paths together between the converters and resonators to minimise vibration that could adversely affect the tonal qualities of the system.

Using advanced exhaust modelling capabilities, literally dozens of math-based models were analysed during system development to refine exhaust flow and detail paths in various components for desired tonal qualities.

“We analysed and re-analysed sound qualities at every point of the performance curve,” Himmelberg said. “We wanted the rumble at start up, more of a throaty roar during acceleration, and it needed to have a pleasing presence at cruise speeds.”

However, the true test came down to what the exhaust note sounded like to the human ear, and the emotions it evoked.

“We had about five prototype builds of the system,” Himmelberg said, “and for each of them, it really boiled down to listening to them on the car. We had 14 different scenarios where we’d listen to and analyse the sound – during acceleration, pass-by on streets, pass-by on highways, idle . . . you name it.

“In the end, we knew we had exactly the sound we wanted. It’s hard to hear it and not just grin.”

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