Like so many of our story ideas, this one's been in the making for over two years. That was when Audi/VW first came out with their DSG (Direct Shift Gearbox) transmission and we launched a search for the ultimate street car transmission technology. The story started as just an acceleration and laptime comparison between two of VW's then-new Mk V GTIs, one with a manual 6-speed transmission and the other with the heavier but faster shifting DSG. Then we got our hands on some alternative technologies, like BMW's third-generation SMG (Sequential Manual Gearbox), found in the E60 M5, and the battle for the fastest shift began.
In the span of two years we've seen so many semi-automatic transmission variants pop up that this article just kept on getting postponed. We finally drew the line, even though BMW just released its first DCG (Dual Clutch Gearbox) in the new M3 and Porsche, the originator of the dual clutch concept, just hit production with their PDK (Porsche Doppelkupplung) transmissions. And, of course, there are the cars that we couldn't get our hands on, like the F1 transmission Ferraris, e-gear Lamborghinis and the Bugatti Veyron, which is equipped with a twin clutch box, built by Ricardo, to handle 1000bhp. What we ended up with are driving impressions, as well as shifting duration measurements, in acceleration (up-shift) and deceleration (down-shift) from the VW GTI (DSG and manual), BMW M5, Nissan GT-R, Lexus IS-F and the new Lancer Evolution X MR. We'll take a look at each transmission, how they work and drive, and how each measures up against the rest.
The Semi-Automatic Gearbox
We all know that an automatic transmission can shift faster than a manual transmission. It also offers clutch-less operation; a big plus if you spend a lot of time in gridlock. The reason that few enthusiasts, outside of the drag world, like automatics is because they slip. The same mechanism that allows you to just stomp on the gas and go, the torque converter, has to be able to slip when the car comes to a stop so that the engine doesn't stall. Slip, unfortunately, offers little car control because the wheels aren't directly connected to the engine. With slip, the driver's inputs from the go-pedal don't necessarily translate through the transmission to the wheels.
The Holy Grail of performance transmission technology would be a semi-automatic gearbox that has the best of both worlds and more: lightning fast shifts, intelligent clutch-less operation, and the direct connection to the wheels that the typical torque converter can't offer. Faster gear changes also translate to faster laptimes as anytime a gear change occurs; a vehicle is essentially not accelerating or decelerating.
Several concepts have popped up over the years, ranging from smarter automatic transmissions, that can be shifted to some degree by the driver, to dog boxes in race cars, that can be shifted through brute force without releasing the clutch but still require the use of a clutch for starting and stopping. But none offered the perfect balance. Dog boxes are too clunky and loud for the street while automatics with shifters still slip and take eons to shift.
The two technologies that finally came to production and stood out as the solution were BMW's SMG and the VW/Audi DSG, licensed through Borg-Warner. The SMG is basically a standard manual gearbox modified so that the clutch actuation and gear selection are computer controlled through hydraulic valves, offering clutch-less operation and faster shifting. The DSG transmission concept is a more radical departure, using essentially two independent gearboxes and wet clutches mashed into one case, so that each clutch and gearbox can take turns as the gears are shifted up and down. While both concepts worked well, the dual clutch technology seems to have won out the transmission war and is now the technology that most OEMs employ, including BMW and Porsche.
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