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Inside the Challenger SRT8's Hellcat Hemi

Dissecting Chrysler's 707hp, 6.2L, Supercharged Game-Changer

By Stephen Kim, Photography by Chrysler

Don't fret. If you're sick of all the Hellcat hype, it's nothing that a trip down to Yoga class in your 50 mpg hybrid can't cure. Stock up on overpriced organic produce at the local farmer's market and adopt a foreign baby on the way back, and order will be restored in your planet-saving universe. The rest of us can't get enough of the 6.2L Hellcat V-8 and its 707 supercharged horsepower. By now, you already know that the Challenger SRT8's Hellcat Hemi is the most powerful domestic production engine ever built, and one of the most potent engines in the world. What real car guys want to know is how the Hellcat is different from earlier Gen III Hemis, what parts will interchange, how easily it swaps into old-school Mopars, and how much hp it's got left in the tank.

Not surprisingly, Chrysler's PR department is working overtime to diffuse the Hellcat media storm. Presumably, engineers have better things to do—like engineering stuff—so the rest of the world is merely regurgitating the exact same tidbits of Hellcat info off the Chrysler press release. Yawn. Since we never settle for fluff, we exhausted our connections to arrange a chat with Director of Advanced and SRT Powertrain Engineering, Chris Cowland. Simply put, Chrysler says 91 percent of the Hellcat's components are all-new, so we want to know how different they are from existing Gen III Hemi hardware. Until engine builders get to see the Hellcat firsthand, this is probably the most detailed information you're going to find on it anywhere. You're welcome.

Displacement Reduction

While some internet chatter has bemoaned that the 6.2L Hellcat is a few cubes smaller than its less potent 6.4L Apache Hemi stable mate, from a durability standpoint the reduction in displacement makes a lot of sense. The new 6.2L combines the 4.09-inch bore of the 6.4L Hemi with the 3.58-inch stroke of the 5.7- and 6.1L Hemis. The shorter stroke (3.72- vs 3.58 inches) offers three distinct advantages. First off, it increases the crank overlap—the portion of the main and rod journals that overlap each other—thus increasing strength. Secondly, considering that the Hellcat utilizes the same block deck height and connecting rod length as the 6.4L Hemi, the shorter stroke enabled Chrysler engineers to increase the compression height of the pistons. In a forced induction application, having more piston material between the wristpin centerline and the piston crown not only provides more space for thicker ringlands, but it also allows pushing the top ring farther down the bore. This isolates it from the extreme cylinder pressure and heat generated through supercharging. Lastly, reducing the compression ratio is a must in any forced induction combo, and the Hellcat Hemi relies on a larger piston dish to reduce compression from 10.9:1 to 9.5:1. A taller compression height makes it possible to fit a larger dish into the piston without compromising durability.

Beefier Short-Block

In extreme boost applications, hot rodders have already pushed the factory Gen III Hemi block well past 1,000 hp, so it's a plenty durable piece of cast iron already. Not surprisingly, outside of cooling enhancements, the Hellcat's block is otherwise unchanged. "The 6.2L Hellcat Hemi block features a new and distinct water jacket geometry," Cowland explains. "The increased thermal loading on the cylinder block necessitated changes to the water flow around the top of the cylinder to keep the bore walls and head gaskets at acceptable temperatures. The main bearing caps are common for both the 6.4L Apache and 6.2L Hellcat."

The Hellcat's crankshaft is similar to the Apache forging, but with several key enhancements. To improve impact and wear resistance, the crank journals are induction hardened. Although this process is more localized than nitriding—which treats the entire crankshaft, not just the bearing surfaces—induction hardening offers much deeper penetration. Bolted to the crank is a damper capable of withstanding 13,000 rpm. Additionally, the pistons swing on floating pins that utilize a carbon coating for enhanced load resistance. "The reduced-stroke Hellcat Hemi crankshaft is machined from the Apache forging, but it sees a number of key production process differences to increase durability," says Cowland. "In order to cope with the higher firing loads, the crank fillets are rolled under a higher load to increase fatigue strength. In addition, the bearing surfaces are induction hardened and run in conjunction with higher load capacity bearings. The bearing clearances and the micro geometry of the bearing surfaces on the crank are also optimized on the Hellcat Hemi."

Since driving a supercharger with a belt places tremendous loads on the crank snout, Chrysler stepped up its game once again. "The Apache crankshaft already featured a rolled fillet on the nose of the crank to elevate fatigue strength, and this process is carried over on the Hellcat Hemi. In order to provide the necessary drive torque for the supercharger, the size of the pulley/damper bolt has been increased by 2 mm, and a diamond-coated washer between the pulley and crank provide a very high friction coefficient," Cowland reveals.

The Hellcat's forged pistons and connecting rods are all-new as well. "The Hellcat's rod length is the same as in the Apache, but the small end structure and design is entirely different. They are manufactured from a different alloy as well," Cowland explains. Hot rodders will naturally be curious about what specific types of alloys are used in these forgings, but a direct comparison to popular aftermarket alloys such as 4340 steel and 2618 aluminum aren't possible. "The materials used for both the connecting rod and piston forgings are proprietary blends that do not conform to traditional alloy nomenclature. That said, the Diamond-Like Carbon (DLC) coating on the piston pins increases the load-carrying capacity and also reduces friction."

Heads and Cam

Fixing things that aren't broken makes no sense, so it's not the least bit surprising that the 6.2L Hellcat shares the same basic cylinder head architecture as the 6.4L Apache. In fact, Car Craft's independent testing has already revealed that the Apache heads are phenomenal performers capable of flowing over 340 cfm at .600-inch lift. One important difference is that the Hellcat is fitted with hollow-stem, sodium-filled exhaust valves capable of sustaining temperatures up to 1,472 degrees Fahrenheit. "The Apache ports already flowed extremely well while creating the desired charge motion for the Hellcat Hemi when combined with the lower-compression piston bowl," says Cowland. "The main change to the cylinder head is the move to a higher strength alloy with increased conductivity, which is required in order to withstand the additional thermal and mechanical loading. The head gasket is also specific to the Hellcat to improve sealing at high firing pressures."

By Stephen Kim
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