There are now essentially four different types of transmissions available in various cars and SUVs and the media isn’t helping with the mixing of terms. This is a summary of their method of transmitting power and the advantages and disadvantages. Whether you want a sports car or the most fuel efficient economy car, this applies to you for a satisfying driving experience.
Part one will discuss the Automatic and the Automated Manual.
First, a brief discussion of gears or “speeds” a transmission may have. There are an increasing number of speeds offered in a transmission. The highest in production is now nine in an automatic for the 2014 Jeep Cherokee and Chrysler 200 (Update: A new 10 speed is shared with Ford and GM). There are a lot of models with eight speed automatics as well as several manual transmission or “stick shift” cars with 7 speed transmissions.
The advantage of more gears is twofold. For the enthusiast it enables optimal acceleration keeping the engine rpm closer to its peak power. This provides quicker acceleration provided the interval between shifts is quick. The other advantage is similar in that the engine can stay in its most efficient rpm range for better fuel economy.
Automatic Transmission
The most common and popular transmission is the Automatic. An automatic uses fluid to transmit power from the engine by way of the torque converter to a series of couplers and gear sets.
The advantages of the automatic are numerous. Of course, the name implies no input needed for shifting. They are also very smooth in operation although they can be manipulated by the vehicle controls for a firmer or softer shift. Shift speeds are can be very quick with little, if any power lost during the shift. It is an excellent transmission for drag racing provided the shifts are firm and the software is optimized for it.
Because of the torque converter, depressing the brake and the accelerator allows for the driver to decide what engine rpm is utilized for an optimized launch within design and software limits. This is called a “brake torque” and it “loads up” the torque converter. This is ideal for consistent and quick launches at the dragstrip.
The disadvantages are added weight and a heavier rotating assembly which used to be, even in modern cars with a lock-up torque converter, a very slight mileage penalty. For the enthusiast, a slight penalty on the dynamometer in terms of power transmitted to the wheels and on racetrack could occur as well.
Advancements in technology have now surpassed the mileage versus the manual transmission in some cars. Service intervals are long and while rare, repairs can be costly due to the complexity of the design with hydraulic lines, mechanical assemblies and electronic controls. While hot
laps can be done with an automatic, the weight, complexity, efficiency, and heat management make it unsuitable for a race car except on the dragstrip. Will the transmission “hunt” frequently trying to find the optimal gear and are the ratios optimized for acceleration or fuel economy?
Naysayers call it an “Auto tragic”. Another slang term is “slushbox” due to the hydraulics and I suspect, some notorious power losses due to inefficient design back in the day. When Chevrolet introduced the new Camaro with less horsepower than the manual, it was ridiculous.
MPG comparison:
BMW 428i 8-speed automatic: 22 city /32 highway; 6-speed manual 20 city /30 highway: Advantage automatic.
Automated Manual
Also known as a dual-clutch transmission or DCT (single clutch units have nearly disappeared with road cars), even professional journalists still refer to the automated manual as an automatic. It’s not a “true” automatic because it doesn’t have a torque converter and the associated limitations of the hydraulics. It is more accurately a manual transmission controlled by electro-mechanical means. The
proper term is an Automated Manual.
The DCT is found in many manufacturers’ cars except domestics (a familiar theme of behind the times). Because the efficiency of a direct mechanical connection like a manual or “stick shift” and shift speeds consistently quicker, more precise and more consisten than a human, it is the transmission of choice in many performance cars. The shift speed and engagement of one clutch while the other disengages means a nearly seamless power delivery. The efficiency and lighter rotating assembly means potentially better mileage than an automatic as well.
Two clutches and computer controlled shifting also imply a long clutch life. The long-term reliability should be excellent, but repairs will be an interesting proposition many years down the road. My guess, like an automatic, by the time a major repair is needed at 200k or 300k miles it’s a moot point anyway.
With shift speeds reported to be .1 to .25 of a second and always a perfect shift, it is no wonder the worlds quickest cars are almost all DCT equipped. Notable exceptions are the Dodge Viper and Chevrolet Corvette Z06/ZR1. Recently Porsche caused an uproar by only offering their excellent
track-focused 911 GT3 with a DCT. Time will tell if it will be offered with a manual transmission (Update: Porsche announced a manual option again).
The beauty of a perfect rev-matched downshift or a consistently .3-.5 second quicker quarter mile time is also compelling. The disadvantages could be considered weight and complexity. Also the “dumbing down” of the driving experience moving further towards autonomy and removing the skill necessary to operate a motor vehicle. Launch-control is a common feature because there is little, if any variance in engine rpm allowed for a dragstrip style launch. This and the aggressive ratios allowed by manual shifting means all-wheel drive cars can have ridiculous acceleration times but with some limitation in driver input. A single-clutch unit tends to be less smooth in low-speed type driving. It also doesn’t spread the wear between two clutches.
MPG Comparison below. Be sure to check the government site for the latest ratings: https://www.fueleconomy.gov/feg/Find.do?action=sbsSelect
BMW 335is coupe 6-speed manual 18/26; 7-speed DCT 17/24 Advantage: Manual
Porsche 911 Turbo 6 speed manual 16/24; 7-speed DCT 17/25 Advantage: DCT
Volkswagen Passat Diesel 6 speed manual 31/43; 6-speed DCT 30/40 Advantage: Manual
Automated transmissions obviously differ in intent and results. Continuing with the discussion of transmission, Part Two will cover the Manual and Continuously Variable Transmissions. Be sure to check out more mileage comparisons as well.
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