New Hemi Engines 2003-Present. Larry Shepard
was introduced in 2014–2015 and became Chrysler’s first supercharged engine. The Hellcat engine was similar to the 6.1L and 6.4L Hemis. It used a cast-iron block with a 4.09-inch bore, which was the same as the 6.4L 392. It also used the 6.1L’s 3.58-inch stroke. This made the engine a 6.2L version at 378 ci. The engine was rated at 707 hp and 650 ft-lbs of torque, the highest horsepower rating for any production engine. The Hellcat blocks were painted orange.
The Hellcat used a similar cam as the 392 with a 0.571-inch lift, but it had 8 degrees fewer intake duration and 16 degrees more exhaust duration. The twin-screw supercharger was built by IHI Turbo America and displaced 2,380 cc. The boost pressure was electronically regulated to 11.6 psi. The supercharger was driven by the serpentine drive belt and the aluminum heads used smaller valves than the 392 engine. It maintained the long valves and tall installed height.
When the Hellcat was introduced, no one thought that this production output level could be matched because racing and custom builds have unique emissions and warranty situations that relate to production engines. Many felt that if it was going to be topped, it would only be by 10 or 20 hp. That held true for a couple years, and then the 2017 Demon burst on the scene with 840 hp! The Demon uses the same basic 6.2L engine as the Hellcat, but the supercharger is larger on the Demon at 2,900 cc and 14 psi boost pressure compared to the Hellcat’s 2,380 cc with 11.8 psi.
The dual-plug feature of all Gen III Hemi engines is easier to see with the valve cover removed. The tops of the twin spark plug towers for each cylinder look like a figure-8 on its side. You can also observe the two rocker shafts (intake at the top), which are a feature of the Hemi-head design.
Family Identification
The quick and easy way to distinguish the Gen I Hemi engines from the Gen II engines was to look at the distributor. The Gen I had the distributor at the rear, while the Gen II had the distributor at the front. This can be seen even in the bare blocks. The Gen III doesn’t use a distributor, but this isn’t obvious at first glance.
If you are used to looking at bare Gen II Hemi blocks, a bare Gen III Hemi block will most likely look small. Based on the block’s bore centers, it is actually the size of a small-block. In its normal position, the block’s pan rails sit square to the floor because of the skirted design. The most unique feature of the Gen III Hemi V-8 bare block is the high-point in the center of the front face. This high point is created by the intersection of the two deck surfaces of the bare block. The front face creates the shape of a capital A. It is lovingly called the bird house. This A shape is not carried to the rear face.
If the engine is assembled, the A shape of the front face is now located behind the front cover, water pump, and all, making it more difficult to see. When assembled, perhaps the most obvious features of the Gen III Hemi are the dual spark plugs for each cylinder and the valve cover is wider than the traditional wedge-head valve cover. All of the production Gen III engines use a beer-barrel shaped intake manifold, and all Gen III Hemi engines use a serpentine front accessory drive. These last two features are not unique to the Gen III Hemi; they were also shared with the earlier 1992–2003 Magnum engines, so they are not as good to use as identifiers.
All of the naturally aspirated production Gen III Hemi engines use a long-runner tuned intake manifold. The longer runners give better low-speed torque and are suited to street applications. All of these manifolds gain the long runners by rounding the ends into a round, beer-barrel shape. Note that the manifold flange is cut off parallel to the ground (horizontal). Shown is the 6.1L cast-aluminum intake.
The Gen III Hemi engines all use a serpentine belt for the front accessory drive. In this system, there are two idlers. A race crate motor system is shown here. The main difference is the large pulley at upper right for the power steering will be much smaller and a different length belt will be on the production versions.
Unique High-Tech Features
Many of the unique high-tech features of the Gen III Hemi engines are not visible externally. In this section, I cover internal leading-edge high-tech features of the Gen III Hemi engines including the six-bolt head pattern, multi-displacement system (MDS), multi-point injection (MPI), variable valve timing (VVT), and more.
The six-bolt head-attaching pattern is one of the unique features of the Gen III Hemi engines, but it is not visible once the engine is assembled. There are four large (M12) bolts around each cylinder and two smaller (M8) bolts at the top outside of the tappet bay. The five small bolts form a line across the top. The big bolts form two lines: one just above the cylinder bores and one just below. Also visible at the bottom are the five cross-bolts for the cross-bolted main caps.
Six-Bolt Head Bolt Pattern
All of the production Mopar small-blocks used a four-bolt head pattern around each cylinder bore. The Gen II Hemi or 426 (along with the 383s and 440s) used five bolts around each bore. In the late 1990s, Mopar Performance introduced the six-bolt head pattern on the race aluminum small-blocks and many of the R3 and R4 cast-iron race blocks.
The Gen III Hemi engines also used a six-bolt pattern around each cylinder, but the six-bolt Gen III Hemi pattern is not the same as the six-bolt race small-block pattern. The Hemi pattern used four large bolts around each chamber and two additional small bolts along the top. These two smaller bolts are in a line directly above large bolts because the block extends upward to cover the tappet chamber. This surface provides anchors for the bolts and increased stiffness for the block itself. The technology is similar to the race pattern, but the Hemi pattern provides stiffer parts and stiffer, stronger assemblies.
Not all Gen III Hemi engines use the multi-displacement system (MDS) feature, but they are all machined for the four MDS solenoids (two at the bottom and two at the top). Here the solenoids have been removed. The tappet chamber is sealed, which means there are no front or rear china walls.
This cutaway model of the Gen III Hemi allows you to see the cam and tappets, which you couldn’t see otherwise. The top of the block and three of the four head bolts are visible along with the two extra small head bolts. (Photo Courtesy FCA US LLC)
Multi-displacement System (MDS)
The multi-displacement system (MDS) system drops four cylinders when the engine is under light throttle (steady speed). MDS is operated by four solenoids located in the tappet chamber cover. These solenoids are controlled by the engine control module (ECM) or computer, which is the same computer that controls the fuel and spark.
This cylinder drop is accomplished when an oil stream to four intake and four exhaust roller tappets pushes a pin in the lifters, allowing them to compress rather than move the pushrod. The tappets can be reactivated within 40 milliseconds if the accelerator is pushed. Because of this ultra-quick response time, the switch in either direction (four to eight or eight to four) is undetectable from the driver’s seat.
Cross-Bolted Mains
Similar to the multi-point injection (MPI) feature, cross-bolted mains are not new. The center three mains on the Gen II blocks were cross-bolted. The Gen III engines’ unique feature is that all five mains are cross-bolted. Chrysler racing and Mopar Performance added this feature into the second and third redesign of the Gen II cast-iron blocks in the late 1990s. This added stiffness seemed to help make the blocks stronger.