Bringing Up the Rear: How the Demon Driveline Took Shape

The 840-horsepower 2018 Dodge Challenger SRT® Demon has the highest horsepower of any production car*. In fact, as magazine and online headlines the world over proclaim every day, Demon is even more potent than the previous muscle car champ, the 707-horsepower Dodge Challenger SRT Hellcat.  


And as any hot rodder will tell you, there’s more to a successful ride than just the engine. It takes a well-matched package of related mechanical components to ensure the power gets to the tarmac over and over again. To see what was involved in bolstering the Hellcat’s driveline to keep pace with the Demon’s added 133 horsepower, we chatted with Dodge performance engineer Jim Wilder.  

Before we continue, it’s often said that history repeats itself. Well, Wilder happens to be an avid drag racer who campaigns a hard-charging 1965 Plymouth in Midwest nostalgia super stock and muscle car shootout events. Going back to the first muscle car era, a similar scenario played out in 1958 when Dodge employees formed a club called The Ramchargers. By 1962, the team of weekend drag strip warriors (which included names like Tom Hoover, Jim Thornton, Mike Buckel, Tom Coddington, Dick Burke, Dan Mancini, Dick Jones, Dick Maxwell, Herman Mozer and others) were designing Dodge high-performance street and strip packages like the 413 Max Wedge, 426 HEMI®, 440 Six Pack and more. Today, Wilder’s weekend race activities continue the legacy of Dodge engineers who literally take their work home with them every night and weekend.  


Getting back to Demon, Wilder says: “The eight-speed TorqueFlite® 8 we developed for automatic Hellcat applications is based on the ZF 8HP 90. An incredibly robust unit, we didn’t have to make any major mechanical changes to handle the Demon’s extra power. But we did rethink the electronic software to include the TransBrake.” If you don’t know, drag racers have been using TransBrakes in all-out automatic race applications for about the last 30 years. But it took the forward thinkers at Dodge to bring it to life on a street car. The Demon is the world’s first mass-produced muscle car with a factory-installed TransBrake.*  

In a nutshell, the TransBrake engages the 3 1st gear clutches and 1 second gear clutch simultaneously with the car standing still on the starting line. This locks the transmission against vehicle movement. Then, the driver applies throttle via the accelerator pedal to bring the revs up to 2350. This allows the engine to reach torque levels that it couldn’t if held on the line with just the brake pedal and tires. So when the green light flashes and the driver releases the TransBrake (via the steering wheel shift paddle), the car virtually explodes off the starting line. And with sufficient traction there’s enough forward thrust generated to achieve 1.8 g and pull the front tires off the strip surface for a few feet as the Demon hits 0-60 mph in 2.3 seconds!  

Beyond the TransBrake, Wilder says, “We also took a page out of the classic Max Wedge recipe book and switched to a higher stall 151k (1.99:1) torque converter than the standard Hellcat’s 137k (1.70:1) item. Out back, the Hellcat’s 2.62:1 rear axle ratio (with automatic transmission) was bumped to a more aggressive 3.09:1.” Combined, these changes magnify the torque multiplication and launch force. Wilder adds: “We fought pretty hard for the looser converter and steeper gears; together they’re probably worth a 2-.25/10s drop in the quarter-mile e.t.” Indeed they are. In general, a 1/10-second reduction in the e.t. is roughly equal to a full car-length at the finish line. So the converter and gears made the Demon two full car lengths quicker than a comparable Hellcat – with no other changes.  


Oh, but there are other changes! To cope with the 770 pound-feet of torque (120 more than Hellcat’s 650 pound-feet), the driveshaft tube’s wall thickness was increased by 20 percent (from 2.5 to 3.0 mm) and made from stronger material. The ring and pinion gears are made from a different alloy and shot peened for improved durability. One key detail that separates the modern Challenger (LC body) rear suspension from the originals from 1970 – ’74 (E body) is the independent rear suspension system.  


Where the vintage E body used a simple live axle and rudimentary parallel leaf springs, today we’re treated to superior handling and ride courtesy of an independent rear suspension (IRS) with a rigid-mount differential and coil-sprung individual half-shafts leading out to each rear wheel. To add strength, the Hellcat’s 6-ball cross-glide joint (the union between the differential’s output stubs and the half-axle shafts leading outboard to each rear tire) has been enlarged to contain 8 balls which boosted torque capacity from 8,000 Nm to 9,300 Nm while reducing operating temperature by 30 to 40 degrees to prevent thermal breakdown of the lubricant.  

Another detail that might only be noticed by future Demon restorers and swap meet hunters is the specific differential case. Though visually similar to the finned cast aluminum unit seen in 6.4 SRTs, Scat Packs and Hellcats and packing the same 230-millimeter ring gear (9.06 inches), Jim tells us: “It uses specific metal alloys and heat treatment for improved strength and is quickly identified by an integrally cast “Demon” verbiage. It is visible from underneath the car on the bottom side of the diff”.  


Stopping for a moment here, let’s recall that the top-tier E body Dodge Challengers of 1970 and ’71 got a special Dana 60 rear axle when ordered with a 4-speed manual transmission and the 440 or HEMI V8. Those Dana 60 axles are practically worth their weight in gold with Mopar® collectors today. Wilder continues: “We’re fully aware Dodge performance enthusiasts appreciate details like this, so when the supplier for the differential casting asked us, ‘Can we add a Demon logo to the case to help our production people tell them apart from other castings during manufacture?’ — we said, ‘Absolutely, we have to do this!'”  

And so the next time you open the hood on a 2018 Dodge Challenger SRT Demon, don’t forget there’s more to the picture than the burly supercharged 6.2-liter HEMI V8 staring back at you. Thanks to guys like Jim Wilder, Demon buyers can rest assured that the entire package is ready to deliver reliable fun, run after run.  

*Excludes non-mass produced vehicles and hybrids.  


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