CNET On Cars: Smarter Driver: The wonderful world of transmissions
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CNET On Cars: Smarter Driver: The wonderful world of transmissions

6:06 /

Transmissions can be tricky, but CNET's Brian Cooley breaks them down and builds them up so you can understand the differences.

-A transmission is a sort of crutch. Your car's engine wants to rotate in a fairly narrow range of RPMs. But your car's wheels wanna rotate over a much wider range. So the transmission sits between them and makes these-- matching up their different rotational needs. The most common transmission today is a traditional automatic. It's a hydraulic fluid-filled thing that shifts like it sounds-- it's very hydraulic. It takes into account engine RPM, vehicle speed, accelerator position, and engine load, and whatever the various computers around the car demanding of it, and selects a gear. Two parts in particular-- the hydraulic valve body and the hydraulic torque converter give the automatic its traditional smooth, but often flabby driving feel. Hence, it rather derives a name that enthusiasts and purists have given the automatic, that of course would be slash box. -Yes, getaway with Merc-O-Matic, almost defies description. It's so silky smooth, you're scarcely aware of what's happening. -But their convenience made them the hero of the industry. In the US, well over 90 percent of new cars shipped with one. Now automatics used to have just 3-speeds. Then after the 1979 energy crisis, a fourth gear became common as a fuel saving overdrive. -Merc-O-Matic goes into overdrive cruising effect. You soar along smoothly, quietly, and without effort. -Overdrive is any gear with an input-output ratio lower than 1 to 1. That allows the engine to run at low RPMs while the car is cruising at a high speed. Today 5 and 6-speeds are common, with 8 and 9-speeds at the cutting edge. So, if more gears are better, why not an infinite number? Enter the CVT-- Continuously Variable Transmission. Instead of a specific number of gears or ratios, it uses variable pulleys to create almost any ratio. As the pulleys change diameter on the fly, they're turning a steel drive belt between them, and that brokers the RPM between the engine and the wheels on a continuum, instead of via big jumps between gears. -Conventional automatics hunt for the right gear going up a hill. Jumping between third and fourth, we really need to be in, say, 3-1/2. Well, because CVT chooses from a much wider range of ratios, it can nail that in between spot, and hold it as you make a steady, smooth climb to the top. -The knock on CVT is that they have tended to feel even more slippery and big than a traditional automatic, and can't handle the output of the most powerful engines. But both traditional automatics and CVTs have made big strides in responsiveness. And today, automatics typically deliver the best zero to 60 times and the best MPG at once. But purists don't care. For them there's nothing like manual. A manual transmission has typically 5, 6 or maybe 7-speeds-- a clutch and a shift lever. Now manual transmissions have not changed much since the 50s to be honest. And all that makes it work is the clutch pedal down there, the gear shift lever here, and all those gears I mentioned are arrayed on a couple of shafts inside the gear box. You make all the decisions, using the clutch to momentarily break the connection between engine and drive line long enough to grab the next gear without grinding everything to pieces. The benefit of them is simple. They're direct. They're really robust. And they let you pick exactly the gear that you want at exactly the moment you want it, and hold it for as long as you want it. They're completely personal in terms of driving. But they're really an anachronism today. Manuals are pretty rare these days, found typically on very fast cars or very cheap ones. But the manual transmission is having a resurgence of sorts, thanks to this guy. This is a dual-clutch automated manual transmission or a DCT here in this AudiRS5. It looks like basically an automatic and that shift gate would fool you. But beyond the lever it's a very different animal. A computer and some servos are in charge of operating not just one but two clutches, and moving the gears around. It's called a dual-clutch because it actually has two clutches, allowing one to keep hammering power through the current gear, while the other clutch decouples and selects the next gear on a different internal shaft. It's like having two manual transmissions in one box that hand off to each other. -The gear changes are so quick that the driver does not notice the transfer of torque into the other sub-transmission during an up shift. -The results are lightning fast shifts in 200 proof power delivery. From high-end Porsches, to hot Mitzus, and now everything Ferrari makes, the dual-clutch is the intersection of purism and tech. The bottom line is you get the precision and the direct power connects of a manual transmission with really fast shifts, but also the convenience of drop it and drive to technology, and just two pedals to deal with. Choose a transmission with these factors in mind. Your driving style. Automatics and CVTs win on convenience. Hands down, dual-clutch gearboxes can give you their convenience with a manual's responsiveness but at a cost. And a manual transmission's great if you really like driving, know how to drive one, and miss things like the Cold War and Ed Sullivan. MPG, transmission choice can make a big difference in the car's fuel economy. Compare the MPG of each transmission offered in the new car you're looking at. Cost, automatics tend to cost a little over $1,000 when optional. Dual-clutch gearboxes can be 2 to 3 times that. And serviceability, automatics and CVTs are often maintenance-free sealed units these days. Manuals will need occasional clutch replacement, but they're highly reparable.

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