Each time someone learned that my 2015 Dodge Challenger wasn't a 707-hp Hellcat, he let out a sigh loud enough to drown out the flathead V8s screaming down the drag strip. The Challenger was "just" a Scat Pack with 485 hp, but as I continually pointed out, I had also recently driven a stick-shift Hellcat—and this automatic Scat Pack felt just as fast.

Dodge

I was full of it. The Challenger Scat Pack ran the quarter-mile in 12.7 seconds at 110.2 mph. Impressive, especially at Sonoma Raceway's uphill, into-a-headwind drag strip, but nowhere near as quick as a Hellcat. The last one we tested did the quarter 0.7 second sooner and a staggering 14.6 mph faster. Either my derriere-dyno was totally out of adjustment, or something else was going on.

I poked at a calculator in search of an explanation, or at least an excuse. I found one: Despite having 222 fewer horsepower than the Hellcat, the automatic Scat Pack briefly pulls harder in every one of its first five gears than the manual Hellcat does.

Like many muscle cars, the Hellcat is long-legged, with a first gear good for 63 mph—a ploy to eliminate a shift on its way to 60 mph. By the time the Hellcat maxes out second gear (at 90 mph), the Scat Pack's eight-speed automatic is in fourth, its V8 having slapped redline in first, second, and third gears. Running a vocal engine through closely spaced, short gears is one of life's great pleasures, with each redline crescendo heightening the feeling of acceleration. That experience alone is enough to trick someone into thinking a car is quicker than it is.

Torque and gearing might change how a car feels, but horsepower always wins the race.

Another factor is that our brains learn to ignore constant acceleration. Ever notice how a jet plane only feels like it's accelerating hard for the first third of the runway, even though it's gathering speed the whole way? Shifting gears highlights how hard a car is pulling because the momentary pause acts like a reference mark for zero acceleration. The more often a car shifts, the faster it feels.

That's the subjective stuff, but physics provides an even clearer explanation. The shorter a gear—that is, the higher its numerical ratio—the more it multiplies an engine's torque. The more torque at the Challenger's rear wheels, the faster it will accelerate. The Hellcat's driveline has an overall first-gear ratio of 8.4:1, meaning its rear wheels turn only 8.4 times as quickly as its crankshaft. In exchange for the speed reduction, the wheels receive 8.4 times the torque. Behold the miracle of gearing. Multiply the V8's 650 lb-ft by that number, and the Hellcat's rear axles endure some 5400 lb-ft of peak torque at full throttle in first gear, enough to fling the car forward at a maximum of 1.02 g. In other words, the Hellcat's maximum acceleration is about the same as it would be if it fell off the Golden Gate Bridge.

The Scat Pack accelerates even harder. Its V8 produces only 475 lb-ft of torque, but its super-short gearing multiplies that output by 14.5 times, and its rear tires are asked to cope with up to 6900 lb-ft. If they could put this power to the ground (they can't), the Challenger would rocket forward with a maximum 1.37 g of acceleration. That's a ride you would not soon forget.

It's the same in every other gear. The Scat Pack briefly accelerates harder—and thus feels faster—than the Hellcat. But because the Hellcat pulls hard for longer, it's ultimately the faster car.

Remember the airplane, though—a long pull dulls the sense of acceleration. So, too, do the slowly ascending engine note and the relatively infrequent shifts. Short gears do the opposite, diddling your butt-dyno with higher peak acceleration and a repetitive screaming-engine redline rush.

Well, diddle me sideways. I need to remember when to trust my butt-dyno and when not to. Torque and gearing might change how a car feels, but horsepower always wins the race.

Jason Cammisa is a senior editor at R&T. He likes to take long walks on short gears.

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