At a glance it might be easy to dismiss the SRT Demon’s supercharged 840 horsepower Hemi V8 as a Hellcat engine with a bigger blower; after all, they both displace 6.2-liters and share a common engine architecture. But SRT powertrain development chief Chris Cowland is having none of that. “This is not a tuned Hellcat,” he stated during the opening remarks of his trackside technical presentation at Lucas Oil Raceway. “There are no easy passes for durability requirements.”
But beyond strengthening the engine and adding more boost, the Demon’s powerplant sports a number of clever technologies that help the world’s most powerful production V8 deliver consistent performance despite the various abuses that are inherent to its drag racing aspirations. Here we’ll take a closer look at what motivates the quickest factory-produced vehicle in history and see how SRT has ensured that it will continue to perform like a world-beater long after it leaves the showroom.
Built alongside the other Hemi engines in Dodge’s Saltillo Engine Plant, in Ramos Arizpe, Mexico, 62-percent of the hardware that goes into a Demon V8 is unique when compared with the Hellcat V8, and that includes upgrades to 25 major components.
The foundation is a deep-skirt cast iron block that is honed with a deck plate to minimize bore distortion. The crankshaft, a forged alloy steel unit with a 90.9-millimeter (3.58-inch) stroke, is unique to the Demon and features revised balancing, while induction-hardened crank bearing surfaces and individual journal optimized main bearing clearances ensure that things continue to run smoothly.
New forged high-strength alloy pistons are on hand here with a 30-micron piston-to-bore clearance increase, while unique powder forged connecting rods with an upgraded shank and big end as well as revised ultra-high tensile fasteners offer even greater durability than the Hellcat units. And they’ll need it, considering the fact that the pistons and connecting rods are loaded with over 11 tons of force during each combustion event. The Demon’s piston oil squirters flow at twice the rate of the Hellcat’s in order to keep the slugs properly cooled.
A new gun-drilled hydraulic-roller camshaft delivers .561-inches of lift on the intake and .551-inches on the exhaust, while duration is spec’d at 224 degrees on the intake and 240 degrees on the exhaust side. The grind is unique to the Demon and helps to bump the car’s redline to 6,300 rpm, while the fuel cut-out kicks in at 6,500 rpm.
A 33 percent increase in oiling for the rocker tips and the revised valvesprings help improve cooling and lubrication, which is yet another strategy put in place to help extend the service life of these components.
Though they’re machined alongside the Hellcat’s, the Demon’s cylinder heads are optimized to help get more air into the engine and further bolster durability. Cast from A356-T6 aluminum due to its strength and heat-shedding properties, the Demon cylinder heads feature a Hemi-style valve layout with dual spark plugs and quench pads.
Rather than simply swapping out the pulley on the Hellcat’s 2.4-liter twin screw supercharger, SRT replaced it with a larger 2.7-liter unit which bumps the boost up to 14.5 psi, an increase of 2.9 psi compared to the Hellcat’s blower. Outfitted with rotors that are 1.1 inches longer than those used in the Hellcat’s supercharger, the blower features dual water/air heat exchangers integrated into the supercharger housing; the electric coolant pump flows up to 11.9 gallons per minute.
Of course with more air going in, the engine needs more fuel to mix with it, so SRT grabbed a second dual-stage fuel pump from the Hellcat parts bin, which are paired up with larger fuel injectors with higher injection pressures.
While the Demon’s beefed up powertrain certainly plays a pivotal role in getting Dodge’s big Challenger into the 9s, how it delivers the grunt is equally as important. “You can all run the drag strip calculators that will say that 840 horsepower is not enough to push a 4,200-pound car down a quarter mile in 9.65 seconds,” Cowland pointed out. “But when we’ve got an eight speed transmission and the technology required to make a low-speed torque curve like [the Demon’s], that is what makes the car so fast.”
Excess heat is the enemy of performance, and the big boosted Hemi generates quite a bit of it. In order to help combat that, Dodge offers another production car first with the SRT Power Chiller system equipped in the Demon. The system reduces charge air temperature by 18 degrees Fahrenheit by diverting air conditioning refrigerant from the HVAC system to a chiller unit that’s mounted by the low-temperature circuit coolant pump.
After being cooled by outside air passing through a low-temperature radiator at the front of the car, charge air coolant flows through the chiller unit where it’s further cooled before it is sent to the heat exchangers in the supercharger.
“But we also wanted to make sure you’d have the same power on your second run that you did on the first,” Cowland explained. “And that’s where the After-Run Cooler comes in.”
This system helps prevent heat soak issues and quickly preps the engine for another blast down the strip after shutdown by continuing to run both the engine cooling fan as well as the low-temperature circuit coolant pump in order to reduce charge air temperatures.
Drivers can keep track of the supercharger coolant temperature through the infotainment system to determine when the supercharger is at the optimum temperature for another pass. SRT says that when working in conjunction, the Power Chiller and After-Run Cooler can lower intake air temperatures by up to 45 degrees Fahrenheit.
While the Demon’s engine is undoubtedly an impressive engineering feat in its own right, it’s also combined with the grip offered by the specially-formulated Nitto NT05R drag radials as well as a factory-equipped trans brake, unique suspension tuning, steeper rearend gears and other performance tweaks.
Ultimately, SRT’s approach to turning the Challenger into a consistent nine-second car with a warranty was really a symphony of engineering strategies rather than a simple exercise of throwing more power at the car and hoping for the best.