There's been so much buzz about the Chrysler Group's Hemi 5.7L V-8 — big power, big torque, big statement — it's almost been forgotten that Chrysler recently launched its fuel-saving Multi Displacement System cylinder-deactivation architecture for Hemis fitted in the Chrysler 300C and Dodge Magnum.
Further obscured by the Hemi hype are some intriguing engineering solutions necessitated by MDS, which shuts down half of the Hemi's eight cylinders when they're not needed. Chrysler says running the Hemi on just four cylinders can cut fuel consumption by as much as 20% in some situations.
But running a V-8 on half its cylinders provokes certain noise, vibration and harshness problems — not the least of which is exhaust sound. There currently is an industry-wide effort to develop pleasing, even “brand-enhancing” exhaust-sound signatures, and the Hemi's basso, sometimes guttural exhaust certainly is a key contributor to its macho image. Yet for MDS-equipped Hemis, simply fitting the exhaust system that makes such an impression with the standard Hemi wouldn't work. Running in 4-cyl. mode, an MDS Hemi is an entirely different aural animal.
A cylinder deactivation-equipped engine essentially is two unique engines to the exhaust engineer. But no vehicle can wear two complete exhaust systems. Chrysler looked to exhaust specialist Eberspaecher North America Inc. in Novi, MI, to develop an exhaust system to compliment the Hemi's divergent 8- and 4-cyl. personalities.
This was not a shot in the dark: Eberspaecher's European operation engineered the system for Chrysler parent company DaimlerChrysler AG's Mercedes-Benz SOHC V-12, which uses Mercedes' own cylinder-deactivation system that has been available in the European market since 1999.
Mathias Keck, Eberspaecher's vice president-development, says Eberspaecher North America won the exhaust-system contract for all Chrysler 300/Dodge Magnum variants (three engines are available: the base 2.7L V-6, a 3.5L V-6 and the Hemi), but “when we started the program, the (Hemi) V-8 wasn't in there. Then they introduced the Hemi (to the 300/Magnum development) — and MDS was an added feature.”
Keck says Eberspaecher had just 12 months to develop an exhaust system for the MDS-equipped Hemi.
“It was a challenge to tune (the Hemi's exhaust) for both 4-cyl. mode and V-8 mode,” says Keck, who stresses the fundamental knowledge Eberspaecher wielded to develop the system: “There are no black boxes — it's all about physics.”
Eberspaecher's exhaust system for MDS-equipped Hemis in the Chrysler 300C and Dodge Magnum comprises four mufflers: dual centrally located, large-capacity mufflers deliver the mellifluous sound when the Hemi is working on all cylinders, while a pair of smaller mufflers, each located near the split rear tailpipes, is responsible for generating pleasing tones when the Hemi is kicked back in 4-cyl. mode. The primary goal for the MDS Hemi, says Keck, was canceling low-frequency noise — mostly “booms” and “groans” — with muffler internals.
Eberspaecher engineers say it is the world's first “passive” exhaust system for a cylinder-deactivation engine. The system the company produces for Mercedes employs an actively switched valve to seal off the exhaust from one cylinder bank of the V-12. Cadillac's cylinder-deactivating V-8-6-4 of the early 1980s had passive exhaust, however.
The setup largely was dictated by the total muffler volume available in packaging the system for the two cars, because the muffler volume (9.86:1 in ratio to engine displacement) was predetermined before MDS was added, says Keck.
He says engineers strive to achieve any desired result with as little muffler volume as possible, and the 300C's 9.86:1 compares favorably to, say, the Lexus LS430 fullsize sedan, which has a muffler volume/engine displacement ratio of about 15:1. Making the 300C/Magnum system work with such efficient muffler capacity is a particularly worthy feat that Keck attributes to Eberspaecher engineers' mastery of computer-simulated design for exhaust systems.
Simulations have effectively replaced hard parts for exhaust-system development engineers. For the MDS Hemi system, there were 376 simulations and just 34 prototype parts. Bob Zola, engineering specialist-testing, says not long ago engineers would have filled a room with the prototype parts required to derive the eventual production-vehicle exhaust system.
“Ninety percent of tuning now is done via simulation,” says Keck. “It's very important to have an accurate engine simulation.” Eberspaecher, in fact, has helped to develop computer-simulation models that are used throughout the industry. The shift to highly accurate simulations also presents obvious cost advantages, and Keck says the company is proud the special needs of the MDS Hemi were met by an exhaust system that is no more expensive than that of a comparable-displacement engine without cylinder deactivation.
“There was literally zero cost premium,” for the MDS Hemi exhaust, says Keck, adding the system required no special materials, brackets or other add-ons.
Ah, but who had the final say on what sounded “right” for the Hemi's exhaust in the 300-Series cars? Ya-Juan Bewman, engineering specialist-acoustics, says Eberspaecher's experience often is a guide, but engineers working on the Chrysler 300C exhaust system presented a few choices to Chrysler upper managers, who made the ultimate decision. Bewman won't acknowledge if any single Chrysler exec made the call, but the Eberspaecher team clearly believes it was the right one.
Exhaust sound is “the NVH DNA of the car,” says Keck.