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Old 07-17-2012, 01:55 PM   #1 (permalink)
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Porsche Automatic coasting

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.
Powertrains - SAE Vehicle Engineering Online



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