The automotive engineer's vocabulary is typically broad and precise, which makes it intriguing to hear the word "emotion" used increasingly when new technologies are introduced. In the case of Porsche's new 911, emotion was cited by engineers as a key factor in developing the car's acoustic signature.
"Sound was a major design and engineering focus," explained Wolfgang Hatz, Porsche's Main Board Member for Research and Development, who also heads VW Group's powertrain development.
At a recent technology workshop for the 2012 Carrera and Carrera S, Hatz and other Porsche engineers noted that creating a unique sound quality "between the engine and the tailpipe" has been a leading criterion for the 911 ever since the iconic sports car's 1963 debut. For the new Type 991 series (see http://www.sae.org/mags/aei/10167), the development team focused on “mechanical engine sounds characterized by high frequencies with tonal elements” together with low-frequency intake noises.
So significant is the 2012 car’s symphony as deemed by Porsche that the interplay between induction and exhaust was part of the car's design and engineering specification. Computer modeling mapping, as well as analysis of the configuration and dimensions of manifolds, pipework, catalytic converters, and mufflers, was carried out at the company’s Weissach Development Center.
The required sonic profile had to satisfy all possible operating states—from engine starting and idling (conveying power without disturbing the neighbors) to wide-open throttle on a racetrack. The goal was not only to deliver the aural “emotions” expected of a 911 but also to provide the driver with feedback about the car’s mechanical status while meeting Europe's 74 dB(A) drive-by noise requirements.
Electronically synthesized noise is not a Porsche solution, so the engineers developed a new Sound Symposer that is standard on both versions of the car. An acoustic channel picks up intake vibrations between the throttle valve and air filter and a membrane incorporated in the channel reinforces the vibrations and transmits them as an engine sound into the cabin. The system is driver activated or deactivated via a “Sport” button that controls a valve ahead of the membrane.
The result is a direct acoustic link between the cabin occupants and the engine, providing optimal transmission of the load-dependent sounds. The sound of gearshifts can also be enhanced.
The Sound Symposer incorporates a tunable Helmholtz resonator to achieve an harmonious sound pattern, damping out unpleasant noise occurring at around 5000 rpm.
If the Sound Symposer is deactivated, an aperture in the air filter housing still allows the basic engine and exhaust tones to be heard—"because we don’t want it to sound boring!” explained Dr. Bernhard Pfäfflin, General Manager of the Noise and Vibration Department.
New ZF 7-speed manual
Gear shifting is also part of the new 911's "emotion" equation. Upshifts of the twin-clutch (Doppelkupplungsgetriebe or PDK) transmission are aided by cylinder deactivation, the interruption of combustion providing the quickest drop in revs apposite for the speed of the next gear.
This is complemented by shorter signal propagation delays in manual shifts. The result is a more than 30% reduction in overall gear change time between operating the control to the completion of the engine speed adjustment phase, while still achieving a comfortable shift effect. The use of cylinder deactivation also provides the “typical race car" exhaust note, the engineers said.
Downshifting of the PDK is also now quicker, with greater double-declutch effect than with the outgoing 997 version of the 911. The Porsche engineers were totally up front about the point of all this, saying that notably in Sport mode with the exhaust flap open, the transmitted sound to the interior is like that of a race car.
The PDK incorporates a coasting (Porsche calls it “sailing”) capability, which returns the engine to idle if the accelerator is gently lifted in the cruise. This can provide a fuel saving of up to 1.0L/100 km, the company claims.
And just for good measure, completely lifting off the accelerator at high revs (both regular Carrera and S version produce maximum power at 7400 rpm) generates a transient backfire.
Complementing this “emotion” is the use of an all new, ZF-supplied seven-speed manual gearbox as standard with the seven-speed PDK an option. The instrument panel includes a “gear selected” indicator to aid the driver.
The manual transmission provides up to 19% lower engine speed for the same road speed and 10% lower fuel consumption at constant speed, compared to the 997’s six-speed manual. Fuel burn is now similar to that provided by the efficient PDK transmission, the engineers claim.
A shift gate lock prevents selection of 7th gear when shifting from 4th to 5th. Top speed is achieved in 6th but can be held in 7th, explained Dr. Pfäfflin.
All of the emotion-generating machinery would not convince if the 911 became obese. Therefore, it was essential not only to offset the extra kilos gained by the 991’s higher structural and equipment specification but also to trim it in net terms.
The added weight generated by a 100-mm (3.9-in) longer wheelbase, extra systems, and improved safety requirements added 58 kg (128 lb) compared to the outgoing 997. But Porsche managed not only to counter that increase but also to reduce overall curb weight by 45 kg (100 lb). The Type 991 Carrera with manual gearbox weighs 1380 kg (3042 lb). The new Carrera S weighs 1395 kg (3075 lb)—approximately 36 kg (80 lb) less overall than the previous generation vehicle.
This overall mass reduction has been achieved by a careful materials balance. Aluminum is used for the front end, middle, and rear of the vehicle, as well as in the front and rear lids, doors, center console crossmember, and in the rear-seat backrest. High-strength boron is among the variety of lightweight steel alloys employed in key safety sections including the inner roof frame and B-pillars. The cross-car beam is magnesium. The wall thickness of plastic components throughout the car is optimized for weight savings.
Joining techniques for the 991's structure include riveting, clinching, and adhesives.
The car's deployable rear spoiler sits on a diecast thinwall aluminum bracket. Steel, aluminum, and fiber-reinforced plastic comprise the spoiler, the blade of which has a thermoplastic polymer surface mounted on a rigid substructure.
In the car's drive unit, chassis and electrics saved about 10 kg (22 lb) compared with the outgoing model. In HVAC alone, using brushless motors in the cooling fans saved 1 kg (2.2 lb). A redesigned front axle with compact lightweight struts shaved another 5 kg (11 lb).
While the new 911 bears a strong aesthetic similarity to that of its forebears, it is a very different car. As Dr. Hatz stressed: “We had to reinvent what we regard as the epitome of the sports car; we practically started with a clean sheet of paper.”