Porsche's PDK shifts easily from track to street

Carmaker's double clutch transmission has faster gear changes and more fuel efficiency

Jul 25, 2009

Special to the Star

Birmingham, AlA.–It's not uncommon for Porsche to invite a bunch of writers to the Deep South to test its vehicles on country backroads and on the track.

But to test a new transmission partly for its fuel-sipping nature? I've done fuel economy trials in a number of obvious vehicles – hybrids, subcompacts, family sedans. But keeping a feather foot on the throttle of a Porsche 911 Carrera S was a new one on me.

I also drove it hard on a track. But Porsche's intention was to demonstrate that you can have your strudel and eat it too, thanks in part to its PDK seven-speed transmission.

I dubbed it "Pretty Damn Kwik," which is much easier than its actual name, Porsche-Doppelkupplungsgetriebe, or "double clutch drive." It replaces the previous six-speed Tiptronic automatic option on Boxster, Cayman and 911 Carrera models, at $4,660 extra on the Boxster and Cayman, and $5,560 on the 911.

The company was the first with the technology, using it for racing in 1983, but it took a while to tweak such a hard-core transmission for more comfortable street use. Several automakers now offer double-clutch transmissions under a variety of proprietary names, including Volkswagen, Audi, Nissan, Mitsubishi and BMW.

Given their advantages and how well they work, I won't be surprised to see them make their way into more mainstream vehicles when volume manufacturing brings the price down; auto-shift transmission choices of the future may well be limited to dual-clutch or continuously variable (CVT) models, for their improved fuel economy over traditional automatic units.

The PDK isn't an automatic transmission, but a manual gearbox that shifts itself. It contains two clutch packs, each consisting of a set of gears on a shaft, with clutch discs that operate in a sealed chamber of special oil that's cooled via a heat exchanger. One set contains the even-number gears, 2 through 6, while the other carries the 1-through-7 odd-number gears, plus reverse. There's no clutch pedal, just a shift lever with Park, Reverse, Neutral and Drive, along with a manual mode that lets the driver select gears either with the shifter or wheel-mounted paddles.

On a regular manual, pressing the clutch pedal disconnects the engine from the transmission, and the driver uses the stick shift to slide a collar over the chosen gear to select it. Releasing the clutch pedal reconnects engine and transmission, and sends power to the wheels. That brief disconnection period disrupts the continuous flow of power, and can cause the car to lurch if the driver doesn't do it smoothly.

With the PDK, one clutch pack engages the appropriate gear, while the other pack simultaneously disengages the previous one, and then gets the next one ready. There's no power disruption, and even the most skilled driver can't shift a manual as quickly as this. It's also lighter and more efficient than a traditional automatic. Porsche says that gear changes are up to 60 per cent faster than on the six-speed automatic that the PDK replaces.

Adding the seventh gear brings down the engine speed while cruising – 100 km/h in a Carrera with PDK is 1750 rpm, versus 2450 rpm with the six-speed auto, for better fuel economy and quieter ride. But PDK's main advantage is its immensely satisfying, performance-oriented feel, which you just can't get with an automatic, and especially not with a CVT. And while you get that experience with a clutch, you'll never outperform the PDK.

Using my best fuel-efficient driving habits on a combination of highways and rural roads, I coaxed 7.8 L/100 km (36 m.p.g.) out of my 911 Carrera S. By Transport Canada figures, the stick-shift model gets a combined 9.8 L/100 km (29 m.p.g.), the PDK a combined 9.4 L/100 km (30 m.p.g.).

Of course, few buy a Porsche for any real or perceived fuel economy, especially when you're paying the $131,330 my considerably loaded tester would cost. I'm guessing its real value to many owners will be as a comeback when they meet the same environmental disapproval that owners of large SUVs face.

As nice as it is on the street, the PDK really shines on the track. Pressing the "Sport" button alters the shift points, keeping the revs higher. If the car's equipped with the optional Sport Chrono Package Plus feature, which includes a dash-mounted stopwatch and lap counter, there's also a "Sport Plus" button that's even sharper on the throttle response, suspension and shift points. It's extremely aggressive and intended primarily for track use.

It also allows for Launch Control, offering maximum acceleration from a standing stop: hold your foot on the brake, put the throttle to the floor and then let the brake go. There are other cars with launch mode, but a Porsche rep says that, unlike them, the PDK can perform the stunt all day, with no wear or damage to the transmission's mechanicals or its factory warranty, thanks to its sealed and cooled clutch oil chamber.

You can row up or down the gears with the shifter or via paddles mounted on the wheel. They're not my favourite, though: the two buttons are identical, requiring that you push for upshift, and pull for downshift. I far prefer pushing or pulling both, one hand for gear up, the other for gear down.

Even so, when I'm rich, I'll order the PDK on my Boxster. I'll call it my go-fast, go-green option.

Toronto Star