The Dodge Challenger SRT Demon’s 5 best horsepower tricks

Come this fall, Dodge will start deliveries of the new $86,090, 840-horsepower Challenger SRT Demon. Let that sink in: eight hundred and forty horsepower. That’s four times the engine power of the average sedan.

Five years ago, few would have predicted that a machine like the Demon was technologically feasible, let alone something that a major automaker would produce. Remember the muscle-car glory days of the sixties? They’ve officially been trumped.

One of the more interesting aspects of the Demon is not simply the horsepower produced by its supercharged 6.2-liter Hemi V-8, but the fact that Dodge managed to deliver that juice in a car that’s emissions compliant and comes with a warranty. Horsepower is relatively easy to obtain, but making exorbitant power within the constraints of a factory-produced street car is a heady challenge.

How did Dodge do it? We sat down with Chad Seymour, Dodge’s SRT Brand Manager, who recently brought a Demon by Hagerty headquarters in Traverse City, Mich. Here are the main engineering moves behind the Demon:

More Air: The Demon gulps oxygen like a whale inhales krill. The Hemi engine breathes through a massive cone air filter that’s tucked behind the driver’s-side headlight. That box is fed by three channels: the giant scoop across the front of the hood, a hole in place in one of the headlights, and the grill below the bumper. At speed, the rushing air very slightly pressurizes the box. Greater air volume is always the first step to increase power. Seymour said the combination of airways, which in total flow a maximum of 1150 cubic feet of air per minute, added 17 horsepower.

The upgrades to the supercharger, however, are more significant. The belt-driven pump retains the same design as on the Demon’s 707-hp sibling, the Hellcat, but it produces a greater volume of air. The maximum boost pressure was also increased from 11.6 psi to 14.5 psi.

More Fuel: Greater air volume is useless unless there’s more fuel to burn it. The Demon uses two fuel pumps instead of the Hellcat’s single unit. The Demon has also been tuned to run on 100-octane fuel. High-octane pump gas is typically 93; the higher the octane number, the less likely the fuel will spontaneously combust before the spark plugs fire. That resistance to pre-ignition means that the Demon’s engine computer can set the ignition timing—the relation of the spark plug firing in relation to the piston position—for higher horsepower.

The Demon’s 840-hp rating is only achieved with 100-octane fuel. On regular gas, the horsepower is a still-staggering 808. There’s a button on the dash that the driver uses to tell the computer that there’s 100-octane in the tank.

More Squish: Most of the Demon’s engine internals, like the connecting rods and pistons, were upgraded over the Hellcat. Those changes were done, in part, to increase the ratio, or how hard the piston squeezes the air-fuel mixture. The harder the squeeze, the more energy the engine extracts from the fuel.

More Revs: The Demon’s engine rpm limit is 6500, 300 higher than the Hellcat.

More Chill: One of the downsides of stuffing extra air into an engine is that as the pressure rises, so does the heat of incoming air. The higher temperature lowers the density of the air, reducing the oxygen content. To solve this problem, the Demon has not only a small radiator nestled between the supercharger and the engine, but it also uses the car’s air-conditioning system to further cool the incoming air. What’s interesting is that the A/C compressor typically saps engine power to the tune of 10-12 ponies. But Seymour said that the horsepower gain offsets that parasitic loss.

All told, there were some 25 major upgrades to the Hellcat Hemi to get to the Demon’s 840 horsepower. Sometime in the next few weeks, we’ll be back to tell you what it’s like to drive a car so drunk on horsepower.