All About the SRT® Hellcat, Part 4: Superchargers and HEMI® V8 engines; 1 + 1 = 3

As we marvel at the Dodge SRT® Hellcat HEMI® engine’s status as the world’s most potent regular production V8 offering*, we’re reminded that the history of the automobile has seen the appearance of a nearly countless multitude of engine configurations. Since the dawn of the worldwide auto industry, the liquid cooled, reciprocating, internal combustion engine has been rendered in more distinct configurations than there are breeds of cats, of which there are 39.  
 
Looking back over the past 115 years, car buyers have been offered piston engines with 1, 2, 3, 4, 5, 6, 8, 10, 12 and even 16 cylinders. The blocks containing those moving pistons have been arranged with the bores arranged inline, in a V, in a W, in a radial circle and even horizontally opposed. In other areas of this feast of horsepower, buyers have had access to overhead cams, cams inside the block, single cams, dual cams and even quad cams. 
 
Combustion chambers have taken the forms of bath tubs, wedges and hemispheres, while the quantity of valves has been juggled in a dizzying array with as many as five circular “poppet heads” crowded into each combustion chamber. It’s enough to boggle the mind. But, in the end, one engine configuration has emerged from the sea of whiskers and wagging tails as the dominant configuration when the utmost power is needed: the supercharged V8 with hemispherical combustion chambers. More specifically, the Chrysler brand HEMI V8 with a belt-driven Roots-style supercharger. As used in drag racing, no engine alive can rival the 5 to 8 thousand horsepower tendered by a typical blown fuel HEMI V8. 
 
If you didn’t know, a supercharger, or “blower”, is a sort of external air pump that’s driven by a belt connected to the crankshaft. When the crankshaft turns, rotors within the supercharger case rotate against each other – never touching – so as to compress and force feed the intake charge into the combustion chambers. The end result is that a much larger volume of fuel/air mixture is crammed into each combustion chamber than could be drawn in naturally by the piston during the intake stroke. When the spark plugs ignite this extra-dense fuel load, the increased cylinder pressure manifests itself as elevated torque and horsepower. 
 
Among the first people to discover the Chrysler brand HEMI engine’s inherent suitability for supercharging were drag racers in the 1950s. Many drag racers would experiment by replacing the carburetors on their 392 HEMI® V8 engine with belt-driven superchargers. 
 
With the 2015 arrival of the mighty SRT Hellcat HEMI V8, a new chapter is being written. Though previous supercharged HEMI engine creations were the work of independent shops, racers and weekend warriors, the SRT Hellcat HEMI V8 is the first factory-produced HEMI engine with a supercharger bolted down between its broad, muscular shoulders. The resulting 707 horsepower and 650 lb-ft of torque mark the SRT Hellcat engine as the most powerful mass-produced V8 of all time*. 
 
So, what is it about the HEMI engine that makes it so well suited to the effects of a positive-displacement, belt-driven supercharger? Let’s review some pictures and learn. 
 
1951 Gen I Chrysler Firepower HEMI® cutaway
 
This cutaway of a 1951 Gen I Chrysler Firepower HEMI® engine highlights the large, straight intake and exhaust ports. Also of significance are the generous coolant passages cast into the heads and block (seen above and below each port and surrounding the cylinder bore). The architecture serves to pull localized heat into the coolant flow instead of allowing it to linger, a crucial bonus in high-stress supercharged applications. Though FCA US LLC (formerly Chrysler Group LLC) never sold vehicles with supercharged HEMI V8 engines in the pre-SRT® Hellcat era, thousands of amateur and professional hot-rodders and racers have validated the concept. And now, in the SRT Hellcat era, an exciting new chapter of HEMI engine performance is being written. 
 
Gen II 5.7-liter HEMI® engine
 
Moving up to 2003, notice how the Gen II 5.7-liter HEMI® engine embraces similar architecture with generous port volumes and double rocker shafts. The valve stems are arranged at a 34.5-degree included angle so they open inward to the centerline of the cylinder bore. Thus when the 2.00/1.55-inch intake and exhaust valve heads lift off their seats, the resulting flow path is enshrouded by the cylinder wall. The 2015 SRT® Hellcat HEMI V8 cylinder heads and block are evolutions of this 5.7 cutaway engine but with even larger port volumes and 2.13/1.65-inch intake and exhaust valve head diameters and added beef. While the cylinder heads used on 6.4-liter HEMI engines are cast from A319F grade aluminum alloy, the supercharged SRT Hellcat HEMI V8 requires faster heat evacuation, which is provided by a switch to heat-treated 356-T6 aluminum-alloy construction. Flow tests reveal the SRT Hellcat HEMI V8 intake port flows nearly 350 cubic feet per minute, a number formerly seen in custom-built race engines. 
 
Gen II 426 HEMI® V8 with top-mounted supercharger
 
The Gen II 426 HEMI® V8 aboard this 1968-vintage Top Fuel dragster shows off the traditional top-mounted supercharger configuration in use since the early ‘50s and made possible via aftermarket blower drive kits and intake manifolds. Though the stack-height of the “bug catcher” air scoop, fuel injection plate and supercharger case restricts forward visibility, this layout is the most efficient for its straight-shot flow path down into the engine. On the leading end of the supercharger, a crankshaft-driven belt turns the twin helical rotors inside the case to create boost levels as high as 20 psi. This championship-winning dragster ran the quarter mile in 6.43 seconds at 230.76 mph. The top speed, by the way, is only about 31 ticks faster than today’s Dodge Challenger SRT® Hellcat. How far we’ve come! 
 
6.2-liter SRT® Hellcat HEMI V8
 
Though based on the mighty 485-horsepower 6.4-liter HEMI® V8 used in SRT® and Scat Pack Chargers and Challengers, the 6.2-liter SRT Hellcat HEMI V8 is comprised of 91-percent new, redesigned, upgraded or revised parts to keep pace with the extra output. Fear not, the many features that make the naturally aspirated 6.4-liter HEMI V8 great, make the supercharged SRT Hellcat HEMI engine even greater. The top-mounted IHI supercharger is driven by the same 10-rib serpentine belt that runs the A/C compressor, water pump, alternator and other accessories. The supercharger’s drive pulley is sized to yield a 2.36:1 ratio, so when the SRT Hellcat HEMI V8 is running at its (electronically limited) 6, 200-rpm redline, the supercharger is turning at 14, 600 rpm. With every rotation, just under 2.4 liters of compressed fuel/air mixture is pumped into the combustion chambers. Maximum boost pressure is 11.7 psi. The 92-mm throttle body is the largest ever used on a Dodge production power plant. 
 
Twin die-cast aluminum rotors
 
Since supercharged HEMI® V8 engines like those used in Top Fuel drag racing only run for a few minutes at a time, heat soak is of minimal concern. But for a road-going high-performance passenger cars like the Dodge Challenger and Dodge Charger SRT® Hellcat, keeping the intake charge cool and dense is a primary goal. A peek inside the supercharger case shows off the twin die-cast aluminum rotors which direct air between each other during compression — and not the housing. This helps keep the inlet charge temperature down so it maintains as much density as possible before entering the combustion chamber. Visible to the left of the rotor is one of the twin liquid-to-air heat exchangers built into the case. The SRT Hellcat HEMI V8 engine’s dedicated intercoolers share nary a drop of coolant with the rest of the engine, so inlet air temperature is kept below 90 degrees C (194 F). 
 
SRT® Hellcat HEMI V8 combustion chamber cutaway
 
Like previous Gen I and II HEMI® engines, the SRT® Hellcat HEMI engine’s ability to endure the rigors of supercharging stems from robust architecture. Still, since the supercharger elevates piston loads to the equivalent of 22, 000 pounds at full power, the SRT Hellcat HEMI V8 enjoys a number of upgraded features including block coolant passages (in green) that extend all the way down to the main bearing bulkheads, thicker main bearing webs, strengthening gussets cast into the cylinder heads and a unique 3.58-inch-stroke forged steel crankshaft with increased main and rod journal overlap for extra strength. Also visible in this cutaway are the SRT Hellcat HEMI engine’s unique reverse-dome pistons which yield a boost-friendly 9.5:1 compression ratio. The twin spark plugs are inherent to all 2003-up Gen III HEMI engines. By initiating the flame kernel at two distinct locations, flame formation time is reduced for a faster burn rate and lower cycle-to-cycle combustion variation for cleaner tailpipe emissions. Early testing with the 5.7-liter HEMI V8 in 2003 showed that the twin spark plugs improved efficiency by 1 percent over a single-plug version (that never reached production). 
 
2015 Dodge Charger SRT® Hellcat
 
The end result of combining HEMI engines and superchargers is demonstrated here. And don’t forget, when the Charger SRT® Hellcat is driven with a gentle right foot, it is possible to achieve an EPA estimated 22 mpg on the highway. To paraphrase a 1968 Chrysler Corporation magazine ad; “Ask our engineers what makes a HEMI® V8 powered anything the one to beat and they’ll probably give you a lot of talk about volumetric efficiency, heat dissipation, flame travel, gas flow and that sort of technical stuff. Don’t believe it. You can’t make an engine like the HEMI V8 with figures and formulas alone. It’s gotta be voodoo, baby!” 
 
*Excludes non-mass production vehicles (i.e., low volume and “coach builders”) and hybrids. 
 
©2015 FCA US LLC. All Rights Reserved. Chrysler, Dodge, SRT, 392 HEMI Challenger, Challenger SRT Hellcat, Charger, HEMI, SRT Hellcat, the Hellcat logo and the SRT Hellcat logo are trademarks of FCA US LLC.


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