Speed and power seem to have a mesmerizing effect on both automotive journalists and the car-buying public. Horsepower and foot-pounds of glorious torque grab headlines, but if a vehicle can't effectively control and transmit that power to the ground in usable doses, raw horsepower is useless.
The untouted link in this process is the transmission. Its technology used to be pretty simple, but it has continually evolved: First we had stick-shift cars, usually with three or four speeds, until sporty cars began to feature five and finally six speeds. Then came automatic transmissions featuring few as two speeds (remember the Power Glide?) before three became the standard for a couple of decades. Finally, automatics began featuring four speeds, then a few went to five, and for 2007 they took a leap when a couple of luxury automakers introduced seven- and eight-speed models.
If you spend a lot of time hauling materials home from the lumberyard (or carting your family around), you can appreciate the challenge of consistently channeling engine power to the ground. Although you may be conditioned to think that the most efficient system is the venerable stick shift, new technology and developments in transmissions and drive systems may make you rethink your next purchase.
Infinite gearing
There is an alternative to trying to keep score of available gears these days: a continuously variable transmission (CVT). Instead of gears that mesh and change, a CVT uses two cone-shape pulleys circled by a long, snakelike belt, usually made of stainless steel teeth. As speed increases, hydraulic pressure forces a narrowing of the opening in the primary pulley, which squeezes the chain-link belt and forces it to grow larger on the primary pulley while compressing its teeth for its trip around the secondary pulley, effectively up-shifting. Conversely, that primary pulley widens during deceleration to let the belt return to its previous size, effectively downshifting. You never hear a CVT shift because it is constantly making minute ratio adjustments.
Although they're more widely available in compact cars, CVT transmissions are slowly entering the crossover SUV market. Jeep puts CVTs in both its Compass and Patriot models, and Nissan is using its newest Xtronic CVT in its crossover model, the Murano. Nissan's system applies 90 percent of maximum torque from 2,400 rpm on up, and the revs rise only slightly during hard acceleration. In addition, Nissan has developed a wider range of gear ratios with the CVT system than is found in a normal automatic transmission. As a result, the CVT can be programmed for better acceleration and increased fuel efficiency – a combination that has always remained in conflict with conventional vehicles.
I recently put a Jeep Compass outfitted with a CVT to the test on some sand dunes at the model's introduction event just west of Portland, Oregon. In hills of deep sand, a normal stick shift or automatic vehicle might spin its wheels, and if you let off the accelerator to avoid over-revving, you'd get stuck up to the axles. With the CVT, I simply stayed on the power and let the CVT continue to adjust for the proper ratio to control wheel spin. I sawed the steering wheel back and forth, and I made it to the top of the dunes every time. Even in heavy off-road testing (and without a low-range four-wheel-drive lock switch), The Compass' CVT system performed well. To navigate the rocky terrain, I simply shifted manually to the CVT's lowest setting, switched on the hill-descent control and let the transmission do all of the work.
Although it may take some time for CVTs to become prominent features of larger trucks, that day is coming. Quicker performance, better fuel economy and lower emissions make a strong argument for further CVT development. But getting some drivers to overcome their need to hear the engine rev and the transmission engage may be a bigger obstacle than the technology. To determine whether a CVT is right for you, take a test drive. You may be surprised at how the vehicle responds and how it sounds during the process. To paraphrase the old Oldsmobile slogan, it's not your father's transmission.
Beyond the Transmission
In conjunction with developments in transmission technology, manufacturers are offering new methods of assuring the effective transfer of power from the transmission to the wheels. Traction-control devices use computers to read the drive wheels, and if they detect that one wheel is spinning more than its partner on the same axle, they reduce power to the spinning wheel and send proportionately more power to the wheel that's not spinning.
An even more sophisticated technology is stability control, which combines traction control and antilock brake systems. This gives the vehicle an almost magical ability to maintain a straight course. Computers read the gas-pedal force, traction, yaw angles and any steering or braking input to safely govern any tendency to stray. If the car hits a patch of ice and the rear end starts to skid to the right, the stability-control sensor decreases power to the outside rear wheel and applies some degree of braking to slow the vehicle while simultaneously maintaining or slightly increasing the power to the inside left front wheel. The resulting slight increase in speed to the left front and the corresponding decrease to the right rear coaxes the vehicle to straighten.