Transmission Talk

By Eric Peters, Automotive Columnist

We don’t have “three on the tree” (column-shifted three-speed manual transmissions) anymore, but what we do have is a profusion of different transmission types — both manual and automatic — each doing things no old-stlye transmission could have dreamed of.

It could use a little explaining.

It used to be that there were just two basic types of transmissions to choose from — manuals and automatics. The manual transmissions had a clutch you needed to engage and disengage with your left foot — while the automatics you simply left in “drive” and that was it.

Enthusiast drivers generally preferred manual transmissions because of the greater control and precision they offered — and also because there was less parasitic power loss through the torque converter, a round, fluid-filled device found near the front of an automatic transmission that spins with the engine and uses the force of hydraulic fluid under pressure to transmit the engine’s power through the transmission’s main shaft and internal planetary gears to the drive wheels. But because there is no actual mechanical connection between the engine and transmission in a conventional automatic — just the force of the fluid spinning inside the converter — a significant portion of the engine’s available power was lost.

This is why, all else being equal, the stick-shift version of a given car would usually be quicker (or at least, more responsive) than the same car with an automatic. The stickshift car also tended to get better gas mileage — again, because less of the available power was lost through the “slippage” inherent to an old-style automatic and its fluid-driven torque converter. Finally, absent modifications, few factory-produced automatics delivered the snappy gear changes enthusiast drivers prefer — and which made the most of the engine’s ouput. (Hence the derisive term, “slushbox.”)

The only real advantage the automatics of the past had was convenience and ease of use. The fluid coupling enabled the driver to idle “in gear” without stalling the engine — because of the torque converter and the use of hydraulic fluid/pressure to transmit power. Also, the driver didn’t have to worry about what gear to select or when. You put it in “D” and that was that.

But the performance gap has narrowed considerably in the last decade — and thanks to efficient “lock-up” torque converters (now universal) that physically connect the engine to the transmission at least part of the time (mostly when the car is at road speed) by means of a special clutching mechanism, the parasitic power losses associated with automatics have been reduced to the point where a stick car often has only a slight performance/fuel economy advantage over the identical car with an automatic.

Sometimes, the automatic-equipped version of a given car is actually more fuel-efficient.

Modern automatics are also electronically controlled — and can be programmed to deliver the crisp, high-performance shifts enthusiast drivers prefer. Some modern automatics are even capable of adjusting shift feel and quality to suit. Many even let you control gears changes and firmness yourself, via paddle shifters and “sport” vs. “normal” mode — and so on.

There are also “adaptive” automatics that alter their operating characteristics based on the habits/driving style of the person driving. Shifts become more aggressive — or less so — depending on how the car is driven.

It’s like having a custom-made race transmission — only better, because you don’t have to sacrifice comfort to get performance. The modern adaptive automatic delivers both.

Even those without this more advanced capability typically can at least switch from one pre-programmed mode to another via driver-selectable modes.

Another really neat development is the “shiftable” function found on many modern automatics that provides a degree of manual control over gear changes. There is typically a secondary gate for the shift lever — or buttons marked with “+” and “-” symbols to denote upshifts or downshifts. The driver either taps the gearshift lever (or buttons) to move the transmission through the gears manually. Of course, there is no clutch — and the transmission’s electronics usually do not allow full control over either upshifts or downshifts, as on a true manual transmission. The electronics will usually not allow the driver to shift down to first (or second) gear until the car’s speed has dropped below a certain pre-set level; this is a fail-safe to keep the driver from accidentally grenading the transmission — or over-speeding the engine. Also unlike a true manual transmission, you cannot raise the engine RPMs in neutral and then suddenly engage the clutch for a dragstrip-type launch from a standing start. That’s because, being a true automatic, there is no clutch.

You just floor the gas pedal — and go.

Modern manuals transmissions, meanwhile, have sprouted as many as seven and even eight forward gears, with six being the least you’ll typically find in a new car.

The top gear — or top gears — are often overdrive ratios, designed to reduce engine RPM (and thus, gas mileage) at highway speeds. Even a very powerful car like the current Chevy Corvette — which has a large V-8 engine — can deliver 30 MPG (or more) on the highway, courtesy of the fuel-sippy gearing.

The latest twist on the manual gearbox is the use of electronics and actuators to operate the clutch — so all the driver has to do is control the changing of gears. Current-year supercars such as Ferraris and many Porsches offer this technology.

The last type of transmission available today is neither an automatic nor a manual — nor a combination of these two. It is an entirely different animal known as the Continuously Variable Transmission (CVT). Instead of a torque converter, valve body and internal gears, the CVT uses pulleys and a metal belt to transfer engine power to the drive wheels. The pulleys have the ability to expand or contract continuously, effectively increasing or decreasing the drive ratio at any given moment. CVTs offer an efficiency advantage over a conventional manual transmission — with the ease of use of a conventional automatic.

Under acceleration, the driver of a CVT-equipped car will notice the engine speed move upward in a smooth, linear progression, without the drop-off between gear changes — and the attendant “shift shock” one experiences with a regular automatic. Even under full throttle, the CVT driver can sip his coffee without worrying about spillage during upshifts.

Because the engine can accelerate to its optimum operating range and stay there, straight line performance with the CVT is often as good as it would be with a manual transmission. The engine is always in the “right” gear for conditions and there’s no lag time between upshifts and downshifts, no hunting for the right gear. The engine computer relays information to the CVT, which uses hydraulic pressure to expand or contract either or both pulleys as necessary.

The selector ranges (“Park,” “Reverse,” “Neutral,” and “Drive”) are typically the same as in other cars with automatic transmissions. The CVT even “creeps” from a stop if you let off the brake — just like a car with a conventional automatic.

Another nice thing about the CVT is that it’s a simpler design than a conventional automatic. There are only a couple of moving parts and no gears to strip or grind. That should translate into fewer and less expensive repairs down the road — as well as better fuel economy potential.


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