Powertrain engineers love understatement—as well as a bit of deception. That was made clear recently when Chrysler announced that its new "Hellcat" Hemi V8 will be SAE J1349-rated at 707 hp (527 kW) and 650 lb·ft (881 N·m) when it debuts in the 2015 Dodge Challenger SRT. The latest iteration of the fabled Hemi is by far the most powerful production engine in Chrysler history, making the Challenger the most powerful muscle car ever.
Automotive Engineering in early June reported Chrysler's "teaser" output numbers for the supercharged 6.2-L V8, which SRT Powertrain Director (and Chrysler Advanced Powertrain development head) Chris Cowland told us would be "in excess of 600 bhp and torque above 575 pound-feet." Strategic understatement, indeed. Named after the famous WWII Grumman F6F fighter plane—Mustang was already taken, we note—the 6.2-L Hellcat shows the Hemi architecture it’s based on still contains plenty of room for development.
This is the first production Chrysler Hemi V8 to feature a factory-engineered supercharger system. Supplied by Japan’s IHI Corp., the blower is a Lysholm type (twin-screw) positive-displacement unit, belt driven from the crankshaft. As opposed to a more traditional Roots-type supercharger with rotating lobes, the Lysholm compressor pushes intake air that’s received from the car’s large, functional hood scoop through a pair of close-meshing screws that resemble worm gears. The design features relatively low parasitic losses and air-leakage levels compared with Roots-type machines.
According to Cowland, his team “evaluated all of the current supercharger suppliers during the early stages of the program, relative to efficiency, power and torque requirements. IHI was selected as the best match to our requirements,” he said. The unit’s design and specifications were developed jointly between Chrysler Group SRT and IHI, he noted.
Displacing 2380 cm³, the IHI blower has integral charge coolers and an integrated electronic bypass valve to regulate boost pressure up to 11.6 psi (80 kPa). Its twin-screw rotors feature a proprietary coating containing PTFE and polyimides which enable closer rotating tolerances and help reduce air leakage for greater operating efficiency. The new supercharger has a 2.36:1 drive ratio and 14,600-rpm maximum operating speed. It uses synthetic lube and is sealed for life. A one-way clutch provides operating refinement without detracting from the mechanical “blower noise” that SRT fans adore.
Better known as a turbocharger specialist, IHI’s Lysholm-type superchargers have been used by AMG Mercedes (the S55 V8) and Mazda (Miller-cycle V6). Compared with a twin-turbo setup, the development team felt a supercharger would offer “a better match to the vehicle characteristics we were targeting, specifically instant throttle response and low-speed torque,” Cowland explained.
The Hellcat uses a mass-airflow sensor for bypass, throttle and OBD controls, Cowland said. The 92-mm throttle body is the largest ever used on a Chrysler vehicle and is supplied by a twin-inlet, 8.0-L air box.
For a Chrysler Powertrain video featuring the Hellcat V8, see: http://www.youtube.com/watch?feature=player_embedded&v=VAQqDyy8Fa0
Beefed up lower end
Asked why Chrysler’s highest-output V8 was developed around the 6.2-L displacement (103.9 x 90.9 mm bore x stroke), when Chrysler already had the 6.4-L (103.9 x 94.6 mm) version, the so-called 392, which is also available in the 2015 SRT Challenger, Cowland replied that the development team chose to destroke the 6.4-L in order to increase the strength of the crankshaft. He said 90% of the engine content is new compared with the 392.
While Chrysler’s original business plan for the modern-era (2003 to present) Hemi included a cast-aluminum cylinder block, production engines have retained the iron block for cost reasons, according to a ranking Chrysler executive. Despite the ability of an aluminum block to lighten the front of the vehicle by 50 lb (23 kg) or more, Chrysler Powertrain engineers have stuck with the iron casting due to the Hellcat’s increased bearing loads, compared with those of the naturally-aspirated 392.
The supercharged high-output Hemi demands “very high-strength block bulkheads, and cast-iron offers the best solution within the geometry of the Hemi family,” Cowland explained.
Accompanying the robust iron block are heavy-duty main bearings supporting a forged-steel crankshaft with induction-hardened bearing surfaces. The crank is capable of withstanding firing pressures of nearly 1600 psi (110 bar). Cowland noted that the increased peak cylinder pressure of the Hellcat version required significantly strengthened connecting rods and pistons, the latter being forged and featuring wrist pins with a special diamond-like-carbon coating.
The Hellcat V8 is not equipped with Chrysler’s cylinder deactivation system that’s used on the standard Hemi. It does feature an active-exhaust system, however. See video: http://www.youtube.com/watch?feature=player_embedded&v=m00JN6P2X6M. The non-SRT Challenger’s active exhaust is supplied by Faurecia. For more on the 2015 car, see http://articles.sae.org/13100/.
The trick “Valet Key”
Chrysler’s powertrain and electronics engineers have created a clever solution to ensuring all 707 of the Hemi Hellcat’s horses won’t be accessible to just anyone (parking valets, teenage drivers) who slips behind the SRT Challenger’s wheel. When equipped with the Hellcat engine, the car comes standard with two key fobs, a red one and a black one.
The red fob is the one that unlock’s the full Hellcat fury, explained Russ Ruedisuelli, head of SRT and Motorsports engineering. The black fob you give to the lead-footed parking valet, because when you turn on the ignition with it, the engine controller signals a reduced-output engine map, limiting peak revs to 4000 (and max output to 500 hp/373 kW). It also locks the Chrysler-built, ZF-licensed 845RE eight-speed automatic transmission out of first gear, while upshifting earlier than normal. It also disables the steering-wheel paddle shifters and launch control, and sets the car’s traction, steering and suspension to their “Street” settings.
Supplementing the new V8, the SRT Challenger also gets a new, selectable Driver Mode system, which Ruedisuelli said “allows a choice of shock settings, steering assist levels, horsepower, traction setting, auto transmission calibration, and shift points. There are over 125 different settings.” The 2015 Hellcat will be available with a Tremec six-speed manual in addition to the 845RE.
Final drive ratio on Hellcat-powered cars is 3.70:1 for the six-speed Tremec manual gearbox, and 2.62 for the automatic. Driveline upgrades include prop shafts, half shafts, axles, and cooling.Continue reading »