Chris Cowland, Chrysler’s director of advanced and SRT powertrain, recently came clean with the engineering secrets behind the 707-hp 6.2-liter Hellcat V-8. In case you just stirred from suspended animation, this Dodge Challenger SRT engine is the most potent powerplant ever sold to customers for road use by any American manufacturer.
Revelation 1: While this Hemi is a cousin to 5.7-, 6.1-, and 6.4-liter Chrysler-built V-8 engines, the Hellcat is, for all intents, a new design sharing only basic architecture—such as a 103.9-mm bore—with its predecessors.
More than 90 percent of the components (by value, not parts count) are new. The cast-iron block has thicker webs and larger cooling passages to support the added power and stress. The induction-hardened forged-steel crankshaft provides the desired 90.9-mm stroke. The powder-forged steel connecting rods have cracked bearing caps for more secure clamping. New forged-aluminum pistons withstand more than 21,000 pounds of combustion pressure. The 24-mm wrist pins have a diamondlike coating for minimal friction.
READ MORE: Dodge Challenger Hellcat Tested!
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Revelation 2: While the uninformed may deem an iron cylinder block to be yester-tech, Cowland insists it’s the ideal material for this application given the intense loading. While a fresh aluminum design might save a few pounds, package dimensions would surely be larger due to aluminum’s significantly lower stiffness. The hard-working, fast-spinning Hellcat crankshaft is well supported by the iron block’s deep side skirts and twenty bolts securing the five main-bearing caps. A meaty closed upper deck resists cylinder-bore distortion.
Revelation 3: While the heat-treated A356 aluminum cylinder heads do not have hemispherical combustion chambers, they do carry huge 54.3-mm (2.14-inch) intake valves and 42.0-mm (1.65-inch) exhaust valves. Both have hollow stems to save weight; the exhaust stems are sodium-filled to dispense with excess heat.
Revelation 4: IHI Turbo America (formerly Warner-Ishi) manufactures the entire 80-pound supercharger—aluminum housing, aluminum rotors supported by steel shafts, and the twin air-liquid intercoolers—in a Shelbyville, Illinois, plant. A one-way clutch on the blower’s input shaft prevents the high-inertia rotating components from back-driving the engine during lift-throttle conditions. More potent than a street sweeper, this blower can move 30,000 liters of air per minute. IHI earned this business in a fly-off against two other supercharger suppliers.
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Revelation 5: Spinning the twin-screw IHI supercharger is a substantial engineering feat. Washers securing the 10-rib drive pulley to the crankshaft are infused with industrial diamonds to provide sufficient clamping friction. Driving the blower 2.36 times crankshaft rpm to deliver 2.4 liters of air per revolution consumes 80 horsepower. The asymmetrical supercharger rotors spin at different speeds thanks to their interlocking configuration and 3:5-ratio phase gears at the back of the blower. At the 6000-rpm engine horsepower peak, the rotor spun by the belt from the crankshaft turns at 14,160 rpm while its mate spins at 23,600 rpm.
The rotors are coated with a Teflon material that helps maintain the tight clearances needed to prevent airflow leaks. In the event of accidental contact, the soft coating abrades instead of the aluminum rotor material. Gears and rotor bearings are lubricated by a sealed-for-life splash system containing synthetic oil. Durability was proven by canting the blower at 47 degrees from horizontal and running it at different speeds and loads for 300 test hours.
READ MORE: First Challenger Hellcat Auctioned, Raises $1.65 Million for Charity
Revelation 6: Noise generated by the supercharger is engineered to please the obvious Hellcat users: individuals with a stout performance bent. During idle and part-throttle driving, the bearing, gear, rotor, and airflow ruckus falls below the audible threshold. Kick the gas and you hear a subtle whine mixed in with the louder intake and exhaust sounds. No artificial noise is added via the car’s sound system.
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Revelation 7: To cool the intake air and maximize its density, a 250-watt (1/3-horsepower) thermostatically controlled electric pump moves 45 liters of coolant per minute through the two heat exchangers (intercoolers) built into the flanks of the supercharger and the two radiators positioned at the front of the car. The goal is to reduce the temperature of the intake air to less than 140 degrees Fahrenheit so the water and antifreeze mixture is maintained at about 100 degrees F.
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Revelation 8: The fuel lines are 0.5 inch in diameter to quench Hellcat’s thirst for gasoline, while the air-fuel ratio is regulated by using both speed-density and mass airflow calculations (doubling up was necessary to satisfy onboard diagnostics requirements). Although direct injection was considered, suitable high-pressure delivery pumps with the required capacity do not exist.
The eight fuel injectors positioned in the intake runners deliver up to 630 cc of gasoline per minute (or one pint every six seconds). The injector orifices are drilled at a 20-degree angle to aim the 17-degree cone of atomized fuel at the back side of the intake valve. Delivery is nearly continuous to ensure that adequate fuel to support 707 horsepower is delivered. The heat of the intake valve vaporizes the microdroplets of gasoline before they enter the combustion chambers. Two air intakes are provided in the Hellcat Challenger: one at the center of a driver’s-side lamp located in the grille, the other at the bottom of an underhood filter housing.
READ MORE: Everything You Ever Wanted to Know About Modern Pistons
Revelation 9: A high-capacity engine oil cooler is standard Hellcat equipment. The factory-fill lubricant is Pennzoil Ultra Synthetic oil in a 0w40 viscosity grade. A total of six heat exchangers live at the front of each Challenger SRT powered by this engine.
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Revelation 10: The Hellcat V-8 is offered with a choice of a Tremec TR-6060 six-speed manual transmission or a ZF/TorqueFlite paddle-shifted 8HP90 eight-speed automatic. Both are upgraded for this application and equipped with lubricant coolers.
A twin-disc 10.2-inch clutch feeds torque to the manual ’box. Launch control is provided with both transmissions to help novices burn less rubber and waste minimal time off the line. The automatic starts in second gear in launch mode. Thanks to the wide ratio spreads and tall top gears, the Challenger SRT achieves up to 22 mpg in EPA highway tests. Unfortunately, its 13 mpg in city driving results in a $1700 gas-guzzler tax penalty.
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BONUS Revelation: Each Hellcat V-8 receives a 42-minute dyno run at the Saltillo, Mexico, manufacturing plant for break-in. Maximum rpm is 5200 at 90-percent load.
Chris Cowland was generous in sharing his expertise but seemed anxious to move on to the next project, which we believe will be the as-yet-unconfirmed 800-hp supercharged V-10 that would give the Dodge Viper enough venom to bite both the feisty Challenger and Charger SRT Hellcats.
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