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TVD or not TVD, that is the question…

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For hard-core automotive enthusiasts seeking the utmost in cornering and handling prowess, the advent of torque-vectoring differentials that can distribute torque between left and right-side wheels are a godsend. Once primarily the province of performance-oriented all-wheel-drive vehicles such as longitudinally-engined Audi S and RS models (S4 thru S8, as well as A8 V8s); BMW X5 M and X6 M; and Mitsubishi Evo, the technology not only inspired the far cheaper and simpler brake-based torque vectoring found on many contemporary economy cars and crossovers, but led to the world’s first torque-vectoring differential on a rear-wheel-drive vehicle, as an option on the Lexus RC F coupe.

Soon enough, the debate started on whether or not TVD (Torque-Vectoring Differential, of course) was worth the extra cost, complexity and reported weight increase (in an already porky, overweight car) over the standard Torsen mechanical limited-slip differential. Online forums asked “Torsen or TVD?” The long-awaited response came via Car and Driver‘s Eric Tingwall, the magazine’s Technical Editor whose “twisted combination of degrees in mechanical engineering and journalism” certainly serve him well in his quest to answer that question in an article titled What’s the Diff? We Put the Torque-Vectoring Differential to the Test.

Mr. Tingwall indeed confirms that the TVD weighs 67 pounds more than the standard Torsen limited slip. Yet, eagle-eyed readers of the C&D article will notice that the specifications chart comparing the two RC F’s shows only a 41-pound increase for the TVD RC F? Is someone mathematically challenged? Or a sloppy copy editor? Highly unlikely. Instead, two possibilities arise to explain the 26 pound discrepancy. First, the two RC Fs compared (Torsen and TVD) were identical (down to the Ultrasonic Blue 2.0 exterior) except that the Torsen-equipped RC F was equipped with an optional moonroof missing from the TVD-equipped car. Not that you can get it anyway, as the Torque-Vectoring Differential is packaged, at a minimum, with a lighter carbon fiber roof and rear spoiler. RC Fs equipped with this bare-bones $1750 iteration of the Performance package, however, are something of a unicorn (or, perhaps, a special-order item). As Tingwall notes,

Lexus…requires that buyers take either the $4400 Premium package or the $5500 Performance package with the TVD.

Indeed, that was the case when yours truly entered his Southeast Florida zip code into the Lexus USA Build Your RC F page. Something that begs clarification, however, is that the $5500 version of the Performance package is nothing more than the bundled sum of the 3 components of the components of the “basic” Performance package (TVD, CF roof and spoiler) with the full contents of the Premium package (Heated & ventilated front seats, rain-sensing wipers, intuitive parking assist, triple-beam LED headlamps, auto-dimming outside mirrors, blind spot monitor with rear cross-traffic alert, Lexus memory system and black carbon fiber interior trim).

OK, enough of equipment and weight semantics. Does the Torque-Vectoring Differential make enough of a performance, driving feel and handling improvement to justify the extra cost and weight? In a word, yes. Numbers-wise, TVD sends 1% more of the weight distribution towards the rear and drops the center of gravity by 0.5″. On a 300-foot skidpad, roadholding improved from 0.91 g on the Torsen-equipped RC F to 0.94 g on its TVD-equipped counterpart. On a 610-foot slalom, the TVD car was 0.5 mph quicker and was 0.4 seconds faster on a 1.7-mile road course than its Torsen sibling.

Beyond the raw numbers, here’s how Mr. Tingwall compares the driving feel of the two cars:

On the skidpad, the standard RC F loses cornering grip at its front tires first, while the TVD-equipped car circles with restrained but consistent oversteer…

The RC F with the conventional limited-slip diff entered corners with pressing understeer, but in several turns made an abrupt, midcorner transition to oversteer. That kind of high-maintenance behavior makes the car unwieldy and more unpredictable than fun. With the TVD, the car’s attitude remains consistent throughout the corner. From turn-in to track-out, the TVD minimizes the effort required to hold the line.

There’s no question that torque vectoring improves objective performance, yet the strongest selling point for this differential is how the car feels from behind the wheel. Whether it’s at the limit on the track or winding down country roads, torque vectoring makes the car livelier and more controllable. Without it, limit cornering is a trying exercise in traction management, load transfer, and other subtle variables. The torque-vectoring differential feels like a subtle push from behind. Turn the wheel and the car dives in so eagerly and effortlessly that you’ll want to attribute its behavior to magnetic forces or supernatural powers. Or, you might just tell passengers that it’s all due to your peerless driving technique. And isn’t that worth the money?

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