Inside 911’s PDK and DFI
BY MARTON PETTENDY | 17th Jun 2008
PDK will replace the long-running five-speed Tiptronic S automatic transmission across the entire six-cylinder boxer-powered Porsche sportscar range, starting with the 911 Carrera and Carrera S coupe and cabriolet models that arrive in Australia in September.
Though the PDK name first appeared in 1983 on Porsche’s 956 Group C race car, its pair of dry clutches was housed inside a much larger, heavier housing and the system was built for speed, not comfort.
But the system worked and a year later a PDK-equipped Porsche 962 racer won a Supercup race at the Nurburgring.
Porsche claims it has worked to refine the system for its road cars ever since and that electronic advances mean the original PDK system’s control unit would have been as big as a car’s rear bench seat to match the new system’s electronic ability.
Similar in concept to Volkswagen’s highly regarded DSG and the BMW M3’s M-DCT dual-clutch transmissions, the new PDK system is supplied and manufactured by German transmission specialist ZF after a six-year co-development program with Porsche.
Like them, it features seven forward gears and, matching the M3, comprises a launch control function, which in the 911’s case comes as an optional extra as part of the Sports Chrono Plus package.
Operating just like a conventional automatic transmission but comprising a full manual control via the gear shifter or steering wheel paddles, PDK does not employ a torque converter like a traditional automatic but instead comprises a pair of wet clutches, one mounted inside the other, to activate two separate sets of gear ratios simply by engaging each clutch.
Each clutch operates a sub-gearbox, with the larger (and stronger) outer clutch coupled to a gearset that includes first, reverse, third, fifth and seventh ratios, and the smaller inner clutch acting on sub-gearbox two, which looks after second, fourth and sixth gears.
The gear ratios either side the one in use are preselected, then activated simply by disengaging their clutch. The electro-hydraulic system comprises eight sensors - including four position sensors, two pressure sensors and two rev sensors – which receive digital signals from the ECU and activate gears hydraulically.
The lubrication system features two separate oil chambers, each with specific oil, and can circulate 42 litres of oil per minute in extreme situations. A dual-mass flywheel is also fitted.
According to Porsche, the PDK system’s efficient mechanical design and reduced oil level minimises churning losses and makes it 10kg lighter than the Tiptronic S auto. At 115kg fully wet, however, it is still about 30kg heavier than the manual.
Its optimised gear spacings include first and second ratios that are nearly identical to the auto’s, and much shorter third, fourth and fifth-gear ratios, with the PDK’s fifth gear roughly equal to the auto’s fourth and the PDK’s sixth gear (with an 0.881 ratio) similar to the auto’s fifth.
The PDK’s seventh gear is also overdriven (with a tall 0.617 ratio), and is exclusively for fuel consumption. Porsche says it reduces revs by about 30 per cent compared with the outgoing auto’s top gear, with the 911 Carrera PDK registering 1750rpm at 100km/h in seventh, compared to the previous Carrera Tiptronic’s 2450rpm. As such, top speed is achieved in sixth gear, not seventh, which is locked out when Sport mode is selected.
The significantly overdriven top gear ratio brings obvious fuel consumption gains on the highway and, according to Porsche, CO2 emissions reductions of up to 15.2 per cent compared with the auto.
Then there is the reduction in gearshift times, by up to 60 per cent. Porsche says the auto changed gears in 0.75 seconds, while PDK does it in 0.5, making 911 models 15 per cent quicker to 100km/h than their automatic forebears.
In launch control mode, that reduces to just 0.42 seconds, making it 19 per cent quicker than the auto and reducing the Carrera S coupe’s 0-100km/h sprint time to just 4.3 seconds.
With no clutch pedal, launch control is activated by fully depressing both the brake and accelerator pedals, at which point the engine will hold a constant 6500rpm, then simply releasing the brake and letting PDK do its work.
Pushing the throttle pedal through the floor-mounted detent will see the transmission change up by itself at redline for optimum acceleration (such as when overtaking for safety reasons), but otherwise the system will not override the driver and allows the rev-limiter to be reached.
Like the manual-shift functions in many automatics, full manual control is available by pushing the gearlever across into the manual shift gate before changing gears via the shifter or the steering wheel paddles, which can be operated temporarily at any time (even in Drive mode).
The PDK transmission also adapts to driving styles in auto mode like a traditional auto does, and can change down multiple gears at once by holding the paddle or lever. In the 911, it has a torque capacity of 440Nm, but will be modified for use with higher-performance models including the 911 Turbo.
Fitted with PDK, the 911 Carrera coupe returns combined fuel consumption of just 9.8L/100km – 0.5L/100km better than the manual version and 1.4L/100km or 13 per cent better than the auto version it replaces.
But at 10.3L/100km, the manual Carrera 3.6 coupe is already 0.7L/100km or six per cent more fuel efficient than its MY08 forebear – and that is a direct result of Porsche’s completely overhauled 911 boxer engines, which now come complete with direct injection for the first time.
Direct injection has been commonplace in modern diesel engines for some time and is now widespread in many new petrol models, including from Porsche’s sister brands Volkswagen and Audi – in which it is known as FSI, as featured on the 3.6-litre VW V6 that powers the entry-level Cayenne, whose V8 engines now also comprise direct injection.
While the displacement of both 911 engines remains the same, Porsche says not a single component is carried over to the new engines, which now meet strict new Euro V emissions standards due to be implemented in Europe in September 2009.
Both engines feature 40 per cent fewer components, with for example the camshafts and bearings now integrated for easier assembly and repair, and both are now 30mm lower than before.
Rotary masses are said to be reduced by 7kg and overall engine weight is down by 5kg, thanks in part to the deletion of a lay shaft and a plastic intake manifold – and despite the addition of direct-injection components, which are made exclusively in stainless steel to accommodate differing fuel qualities around the world.
Reduced friction comes courtesy of new piston ring and cup tappet coatings, while mechanical losses are reduced by a new fully variable, on-demand oil pump.
Combined with a new, lighter integrated dry sump system that electronically regulates oil pressure, the latter is alleged to reduce fuel consumption by two per cent and to improve performance by about 2kW because it runs only when required.
DFI injects fuel directly into the combustion chamber, instead of the intake manifold, which lowers the fuel/air mix temperature and so allowed Porsche to increase the compression ratio to 12.5:1, which inturn lifts performance, lowers consumption and improves throttle response.
Offering up to 120 bar of fuel pressure, the DFI incorporates a second injection cycle to use unburnt gases to more quickly heat up the exhaust catalyst, avoiding the need for a secondary air injection system. The result is that both engines now produce the naturally aspirated specific engine output benchmark of 100hp and 110Nm per litre for the first time.
Maximum power output for the base 911 engine is up by 15kW, from 239kW to 254kW, while the Carrera S flat-six now produces 283kW – up a substantial 22kW from 261kW. Peak torque, meanwhile, rises by 20Nm for both engines - from 370 to 390Nm in the 3.6 and from 400 to 420Nm in the 3.8.
Both engines produce their power 100rpm lower than before, at 6500rpm rather than 6600rpm – despite more oversquare cylinder dimensions. The 3.6 now has a 97mm bore and an 82.5mm stroke, while the 3.8 measures 102mm x 77.5mm. At the same time, both engines’ electronic rev-limiters have been raised, from 7200 to 7500rpm.