All About the SRT® Hellcat, Part 3: A Look at the Driveline

In this third exploration into the SRT® Hellcat’s many fascinating engineering details, let’s go under the Challenger’s sleek exterior for a road’s-eye view of the all-important driveline. This vital link between the 6.2-liter supercharged Hellcat’s crankshaft and its beefy Pirelli supercar rear tires is packed with cutting-edge engineering that ordinarily goes unnoticed. We raised our donor 2015 Ivory White Pearl Challenger SRT Hellcat with a lift for a closer look. 

Just as it did in the sixties with the then-new 426 Street HEMI® V8, Dodge has prepared several Hellcat cutaway engines for show duty. Here the IHI supercharger’s case has been opened to allow viewing of the twin, high-helix aluminum rotors. Driven at 2.36 times crankshaft speed, they turn 14, 600 rpm at full power and can move as much as 30, 000 liters of air per minute. Seen another way, the Hellcat supercharger moves just under 2.4 liters of air with every revolution. The resulting 707 horsepower and 650 lb-ft of torque far surpass the 426 Street HEMI engine’s output of 425 hp and 490 lb-ft. The 92-mm throttle body is the largest ever used on a Dodge engine. It takes a rugged driveline to handle this kind of power, and Hellcat delivers. 

Manual-transmission-equipped SRT® Challengers with Hellcat power use the same rugged Tremec TR-6060 six-speed gearbox as naturally aspirated 392 HEMI V8 models with one difference: With 650 lb-ft of torque available, the Hellcat simply doesn’t need the 392-spec transmission’s 2.97:1 first-gear ratio. Rather, the Hellcat’s first-gear ratio is 2.26:1, yielding slightly less torque multiplication but also keeping engine rpm’s lower at comparable vehicle speeds. Likewise, the rest of the ratio spread is finessed to suit the Hellcat’s bounty of torque. Here’s a comparison: Hellcat ratios are 2.26, 1.58, 1.19, 1.00, 0.77 and 0.63:1. Meanwhile, the naturally aspirated (non-supercharged) 6.4-liter 392 HEMI V8 with its 485 horsepower and 475 lb-ft of torque has the following gear ratios: 2.97, 2.10, 1.46, 1.00, 0.74, 0.50:1 (this transmission is also installed behind the 5.7-liter HEMI V8 in Challenger R/T applications). In the days of the 1966-’71 426 Street HEMI engine, a large single-plate 11-inch diameter clutch disc mated the flywheel to the pressure plate. Now, two Sachs clutch discs do the job. Sized at 258 mm (about 10.2 inches) per disc, torque capacity is nearly doubled. The same dual-disc Sachs clutch technology is shared with naturally aspirated 5.7-liter and 6.4-liter HEMI V8s but with slightly smaller 240-mm (about 9.5-inch) diameter clutch discs. 

Since our donor Hellcat is a stick model, this cutaway display unit reminds us that the new 8-speed TorqueFlite 8HP90 is optional for Challenger buyers who prefer an automatic transmission (all Charger SRT Hellcats are automatic equipped). Weighing less than 10 pounds more than the 5-speed automatic it replaces, the benefit of 8 forward gear ratios is that they keep the engine in its most efficient rpm range longer than a transmission with fewer forward gear ratios. Speaking of which, the TorqueFlite Eight’s ratios are 4.71, 3.14, 2.10, 1.67, 1.29, 1.00, 0.84 and 0.67:1, with a 3.30:1 Reverse ratio. Looking back to the days of the 1966-’71 426 Street HEMI engine, its 3-speed automatic transmission had 2.45, 1.45 and 1.0:1 forward ratios and required extreme 4.10:1 (or lower) axle ratios for strong acceleration. Thanks to the modern TorqueFlite’s 4.71 first gear, the Hellcat uses a mild 2.62:1 rear axle ratio and delivers a 12.34:1 torque multiplication factor in first gear. To achieve the same value with a 1970 HEMI V8 Challenger’s 2.45 first gear would require drag race-only 4.88 gears in the rear axle. And while 6-speed drivers may enjoy the connected feeling they get from the stick shift and clutch pedal, there’s no beating the 160 milliseconds it takes the TorqueFlite 8HP90 to move from gear to gear when in Track mode. 

If you’re a performance fanatic, the Hellcat’s free-flowing, full-length, dual exhaust system and independent rear suspension are almost too pretty to hide under the car. In the first golden age of the American muscle car, Dodge exhaust systems were noted for being among the most efficient. In particular, the Street HEMI engines featured (then) huge 2½-inch head pipes. Today, the Hellcat uses even bigger 2¾-inch diameter tubing, and the system is even electronically tuned for optimum performance and an exciting auditory experience. We’ll focus on this in an upcoming blog. 

Dodge led the field with very strong rear axles in the 426 Street HEMI engine era, and the theme continues. For 2015 Dodge introduced this new aluminum differential case for added strength and lower weight. The prominent ribs act as heat sinks to dissipate heat into the slipstream. Inside, a 230-mm (9.3-inch) diameter ring gear is over a half-inch larger than the classic Hotchkis-type Dodge 8-3/4 differential used under legends like the original Coronet Super Bee, Dart GTS, Charger Daytona, Six Pack Challenger and automatic-equipped 426 Street HEMI engine models. Exclusive to Hellcat (and 392 HEMI V8-powered SRT and Scat Pack models) is a fourth mounting point to better stabilize the unit under violent, full-power drag strip launch conditions. Inside, a four-pinion, clutch-type limited-slip unit works with the electronic stability control (ESC)* system to direct power with minimal tire spin. 

The Challenger SRT Hellcat’s NHRA-certified 10.8 second @ 126 mph quarter-mile sprint* — on soft-compound drag radial tires — wouldn’t be possible without the high-strength unequal-length half shafts (finger points) and matched CV joints. This is the final link in the chain of command between the Hellcat’s flywheel and rear tires — where all of the Hellcat engine’s furious power is unleashed to the pavement. 
*Always drive carefully, consistent with conditions. Always wear your seat belt and obey traffic laws.


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