2006 Volkswagen GTI
The Direct Shift Gearbox (DSG), which was first introduced in the Audi lineup, quickly trickled down to the Volkswagen line. The fifth generation GTI was the first to offer a manual six-speed or a DSG, both mated to the same 2.0-liter turbocharged, direct injection engine. This would be our baseline to compare between manual shifting and semi-automatic shifting because, with an identical platform, the only difference is the additional 50-pounds of weight from the heavier DSG transmission.
The DSG, or dual clutch concept, is actually fairly simple. Instead of using just one clutch or torque converter to hold power transferred through each gear, shifts could be achieved far faster by using two different concentrically mounted clutches that overlapped in operation. As one clutch released in the original gear, the second clutch (which is already pre-staged in the next gear) would already be engaging, providing, in theory, almost seamless torque handover. Obviously, this means that each clutch is connected to every other gear, so that there is a tag team action. For smoother engagement, wet clutches immersed in lubricating oil, like the ones found in clutch type LSDs, are used. In order to save space, multiple plates are used in each clutch to increase the surface area and holding torque. Wet clutches also provide more efficient transmission of power and reduce heat generation. One drawback of this concept is that shifting has to be absolutely sequential, which in some instances, like a two gear downshift from cruising to full acceleration, may consume more time.
As the first affordable semi-automatic gearbox, VW's DSG hits the spot in terms of everyday driveability. But its software refinement and intelligence falls behind the other examples here. The DSG operates in two modes. The first is Standard Drive mode, which is geared completely toward smoothness and fuel savings, and is a bore to drive outside of crawling in traffic. The low shift points don't take advantage of the engine's power or engine braking from higher rpm's. Sport mode sets the shift point at 5000 rpm from first and roughly 4000 rpm in later gears. While this works well for winding roads, it is too aggressive for the streets as RPMs always seem to hang a little too high, even at part throttle. Manual shifts are typically met with a slight lag in response, which often causes unintended double gear changes by the driver. The wheel mounted paddle placement makes manual track shifting troublesome as the paddles are often unintentionally touched while shuffling the wheel. However, the smoothness of the shifts is apparent between the manual and DSG acceleration speed profiles.
Newer variants of the DSG, or S-Tronic transmission, feature 7-speeds, further refined tuning, and vehicle dynamics integration.
2006 BMW M5 (E60)
As the final variant of BMW's SMG technology, this third-generation 7-speed transmission, built especially for the M5 and M6, is by far the best of its kind. The SMG was born as a standard manual transmission with a conventional dry clutch. But, by using electronically controlled hydraulic systems, the clutch actuation and gear shifting are now controlled by a computer that evaluates throttle input, speed, load, and steering sensors to optimize the shifting intensity, timing, and quickness. This is similar to Ferrari's F1 and Lamborghini's e-gear transmissions.
While drivers have typically complained about early SMG transmissions, in the Z4 and E46 M3s, the SMG III is a huge leap ahead of its predecessors. Unlike SMG II transmissions, which are retrofitted manual transmissions, the SMG III was built only for SMG applications. Higher torque bearing gears are placed closer to output shaft bearing carriers so that the gear placement arrangement becomes very awkward to shift through a standard H-pattern shifter. The order would be more like 2-4-6-7-5-3-R-1, instead of the 1-2-3-4-5-6-7-R.
SMG transmissions operate and shift more like conventional manual transmissions in terms of feedback and stall time. Engagements are more positive, letting the driver know a shift has happened. The transmission can be driven in a very intelligent automatic mode or a manual shift mode. Both modes can also be adjusted in terms of shift intensity, feedback, and response by the driver. One advantage of the SMG design is that, since all the gears are mounted on the same shaft, it is possible to perform an instant double down, jumping two gears at a time. The SMG has the ability to do this in drive mode; when the gas is floored from cruising, the transmission will kick down two gears under the right conditions. The drawback is the driveline shock, when shifting manually in the most aggressive transmission mode, can on occasion spin the wheels and upset the chassis during cornering.