In the latest issue of evo, (issue 117, on sale 2 April) John Barker explains the mysteries of today's myriad gearbox designs. And now there's another: a new seven-speed, longitudinally-mounted DSG unit launched in the Audi S5.
The idea of a double-clutch transmission in a production car began with the VW group's DSG unit combined with a transverse engine, and it wasn't until the Bugatti Veyron arrived that we saw a longitudinal DSG. The Bugatti's Ricardo-designed unit was somewhat over-engineered for lesser applications, though, which brings us to the next stage in the DSG's evolution.
At the Geneva show Audi revealed the S5 S-tronic, a seven-speed DSG matched to the S5's longitudinal V8 and quattro transmission and which is entirely an Audi in-house design. The principle is the same as in the transverse DSG, with a hollow input shaft running concentrically around a solid one and each shaft connecting to alternate gears, but this time there's no need to squeeze it into a short package as there's a whole transmission tunnel to play with.
So the clutches can have their own compartment, separate from the gears, which means the clutches can bathe in ATF lubricant and the gears in normal hypoid gear oil to the benefit of both. The inner input shaft, engaged by the larger clutch, is mated to first, third, seventh and fifth gears (that's the order, working rearwards), while the outer shaft, whose clutch sits within that of the inner shaft, carries fourth, sixth and second gears. The centre Torsen differential sits on the back of the output shaft, which runs below the input shaft. The first three gears plus reverse, that is the gears with the biggest gap to the next ratios and thus the biggest loads on their synchromesh, have triple-cone synchronisers.
Second gear is pre-selected at 3mph to give a quick shift, and thereafter pre-selection changes to the nearest higher or lower gear according to sensors which help the system predict what will be needed next. The transmission has a 406lb ft torque capacity which makes it suitable for most likely Audi engine applications, and more will follow – particularly diesels whose narrow rev band makes a seven-speed transmission very useful – after this S5. The plan is for the S-tronic to replace the less-efficient torque-converter automatics in some cars, but it won't replace the CVTs.
The usual Tiptronic-type selector lever and steering-wheel paddles are carried over to this S-tronic, and you drive it in just the same way. It behaves very much like a sportingly-programmed torque-converter auto such as a Jaguar's ZF, with a solid, mechanical drive once moving just as in that ZF whose converter stays locked once under way. The shifts are smooth, down or up, helped by the usual well-judged throttle-blip when needed. Seventh gear gives very relaxed cruising; the ratio spread between first and seventh is eight-to-one. Unlike an epicyclic gear train as used in torque-converter autos, in which mathematics dictate what the ratios of the interconnected gears have to be, the S-tronic can have ratios tailored exactly to the car's needs.
Sport mode speeds the shifts (to 100 milliseconds from 200), livens up the kickdown and locks out seventh gear. Manual mode feels near-instant and of course there's the impression of a continuous torque delivery during the shift. Our prototype test car had an occasional slight snatch when shifting but Michael Schöffman, Head of Geared Automatic Transmission, says that will be fixed with the final production calibration before sales start in the summer.