Automatic Vs. Manual Transmission: But What About DCT?

It's the same age old question: manual or automatic transmission for better gas mileage and performance? Well, up until recently it was commonly known that better fuel mileage could be had with a manual transmission over an automatic transmission. And in the arena of straight-line performance, it was always a tossup between the automatic and the manual, but most factory cars seemed to be consistently quicker on the strip with a competent driver in the case of a manual transmission. So, why is that? Well, manual transmissions are lighter for one; not by much, but enough. The other reason is because manual transmissions don't absorb horsepower and torque from the engine in order to perform their devious tasks like an automatic transmission. An excellent example of this horsepower robbing feature can be plainly seen on the 2010 Chevrolet Camaro SS which is rated at 26 horsepower less with an automatic transmission than its older brother, the manual transmission. The automatic transmission in the Camaro SS is electro-hydraulic; this means that hydraulic pressures using gear oil (transmission fluid) are needed to make the gears in the transmission shift and the torque converter to transfer power to them.

Anything that is hydraulic in nature needs pressure or energy from an external element to actuate it in a transmission. In the case of an automatic transmission it uses fluid pressure created by the torque converter to increase hydraulic pressure enough to engage or disengage automatic transmission gears (or clutch packs) if pressure is needed in an automatic transmission to change gears then there must be resistance. Hydraulics sometimes require thousands of pounds of pressure to operate in an automatic transmission; that being said the engines flywheel (which is connected to the crankshaft) is needed to transfer power to the automatic transmission's torque converter to make it spin to in turn increase internal fluid pressures. So, when an engine meets the kind of resistance that a hydraulically operated automatic transmission requires the engine bogs down and engine speed is reduced. If rpm's are reduced (or encumbered by resistance/ pressure), then the overall peak horsepower figures of an automatic transmission equipped vehicle will be lower. If horsepower and torque figures are lower in an automatic transmission when compared to a manual transmission, then you can best bet that the manual transmission will have the advantage in a straight-line performance tests over the automatic since it has more available power.

So, manual transmissions are generally better for performance and fuel economy than automatic transmissions, but what if there was a transmission that was the best of both worlds? Enter the new era of the DCT (dual clutch transmission). The DCT is the best of both worlds, it combines automatic shifting characteristics with manual transmission guts; but wait there's more. Even though the DCT uses manual transmission gears it requires no clutch pedal, making it seem like the ultimate winner. There's something else very unique about the DCT other than just being a manual transmission that behaves like an automatic; actually there are two things about the DCT. The first thing that is so unique about the DCT is the fact that it even gets better fuel mileage than both automatic and manual transmissions. For instance, the Volvo C30, S40, and V50 all got 8% better fuel economy over contemporary automatic transmissions. Even if these Volvo's were fitted with a manual transmission they would most likely only get one to two mpg difference on the highway; that being said, 8% is a huge jump in fuel economy even over a manual transmission.

So, you may be wondering how a DCT can achieve this fuel mileage? Simple, a DCT is always in direct power transfer from the flywheel and doesn't have a need for pesky power robbing hydraulics since all of the DCT's internal transmission parts are manual. The other thing that makes this happen is that the DCT's transmission shifts are much faster than a manual, about 150 milliseconds; this means that the engine never drops in rpm too far or falls out of its power range like when you push in the clutch on a manual transmission. So there's one more thing that makes a DCT better and that is when you're at the strip; your car will be even faster than with the best driver and a manual transmission. For example, the 2008 BMW M3 equipped with DCT was .20 seconds faster from zero to 60 mph than a professional driver could muster in a manual transmission M3. Furthermore, even a similar test was done in the Audi A3 with a DCT and the results were the same; DCT equipped Audi A3 went zero to 100 km/h in only 6.7 seconds as compared to a manual six speed version that could only do it in 6.9 seconds. This may not seem like much, but in the quarter mile test a car could be about a half a second faster if equipped with a DCT. To wrap it up, when buying a new car and you have the option of getting a DCT (or DSG if European) I highly suggest doing so. Your car DCT equipped vehicle will be faster, have better fuel economy, and you can choose whether to or not to change gears without the use of a clutch pedal.

Sources:

http://green.autoblog.com/2008/05/16/dual-clutch-powershift-gearbox-cuts-volvo-fuel-consumption-8/
http://www.leftlanenews.com/bmw-unveils-seven-speed-dual-clutch-transmission.html