• 2016

Power Monster


The 5.7L Gen III HEMI® engine was introduced in 2003, but the fi rst engine in the production HEMI engine family was the 331-354-392 HEMI engine built in the 1950s. The second generation was produced from 1964 through 1971. Known as the 426 HEMI engine, it was based on the big block foundation. The latest monster in the family, the third-generation HEMI engine, has already been built by FCA US LLC in six different displacements: 5.7L, 6.1L, 6.4L or 392, 426 (crate engine), 6.2L (supercharged), and 354 (supercharged NHRA). There have been more than 3 million of these engines produced in 13 years of production and they are still going strong. While some of the displacements are similar to the older HEMI engines, this new HEMI engine is unique in many ways. Specifically because it has a lot of technology designed into its basic hardware.


The original cast iron 5.7L blocks use a 3.917″ bore, which is close to the 318’s bore of 3.91″. The aluminum block P5155507 has a 4.125″ bore and weighs about half of the cast iron block. All the Gen III HEMI engines use the A-engine (and Magnum) bellhousing bolt pattern. For added strength and stiffness, the blocks are also skirted designs (for instance, the 426 Gen II HEMI engine in which the main caps are cross-bolted).

The block’s production deck height is 9.24″, which is quite short compared to the 9.6″ for the A-engine and the 10.72″ for the Gen II HEMI engine. The camshaft height (centerline of crank to centerline of cam) is 7.44″ which is very high compared to the 6.125″ of the A-engine and even less for the Gen II blocks. This high cam height gives plenty of clearance to the crank and rods.


One of the interesting design features of the Gen III HEMI engine heads is that they are all dual-plug heads. In production, both plugs fire at the same time. There appears to be no horsepower advantage to either plug position. Also, each engine uses a right and left head like P5153345 and 346, which are CNC-ported heads for the 5.7L. The right and left feature is caused by the head’s oil drains. All of the Gen III HEMI engines have the intake valve opposite the exhaust valve, which is the same layout as the other two generations.

Mopar® also offers a performance Stage 3 ported cylinder head set P5160027 for the 2009-2017 5.7L.

The big difference with the Gen III HEMI engine layout is that the valves are at shallower angles than in the other HEMI engines. These shallow valve angles allow the combustion chamber to be shallow also. There are open and closed versions of the chamber. The basic chamber shape is a somewhat ovular sphere. This shape is made by pulling the right and left sides in toward the center, ultimately creating an oval. The shallow chamber allows the pistons to be lighter and the chamber has less surface area, which helps resist detonation.


The crankshaft has a timing wheel bolted to the rear counterweight. The wheel is designed to fit over the rear crank flange. There are two wheels — a 32-tooth wheel and a 58-tooth wheel. The ESM (ignition/fuel computer) and the specific timing wheel MUST be matched or you’ll have ignition trouble. The shallow chamber allows the pistons to be light, a small dome rather than a large dome common on earlier HEMI engines. The short deck contributes to the overall piston being short (about 1.21″ compression height), which helps the production 5.7L piston to be quite light (about 412 grams). Bigger bore pistons are heavier.


Some of the intake manifolds are made of plastic (lighter weight) but the 6.1L version is cast in aluminum. There is a single plane intake manifold P4510581AB (4-bbl carb) and a 4-bbl throttle body (fuel injection) version P4510582AB — both are designed for the 5.7L.


On all the production Gen III HEMI engines, the fuel and ignition functions are controlled by the EMS or engine management system (also called the ECM — engine control module or computer). The production ones are NOT programmable. MP offers two programmable EMS units, which are designed to be used  with the 5.7L (77072455AB) or 6.4L (77072454AB) HEMI engines installed into older classic cars or trucks. They are not designed for use in vehicles that already have an EMS.

The production Gen III HEMI engines feature a distributor-less ignition, which means there is no distributor and no boss for the distributor. There are no spark plug wires because the coils mount directly to the dual spark plugs. The spark advance, vacuum and total advance functions are manipulated by the EMS.


All of the Gen III production engines use a hydraulic roller cam. An HP hydraulic roller cam for the 392 is P5153691. Each one of the production HEMI engines tends to have a unique cam. These engines use high-performance beehive valve springs line P5160074. Similar to the Gen II HEMI engines, the Gen III uses different length pushrods for the intake and exhaust valves. When rebuilding the HEMI engine, remember that the tappets should be installed BEFORE the cylinder heads.

One unique aspect of the Gen III cams is that the cam journals are very large, especially the #1 journal. While the 5.7L engine uses valve lifts around .470″,the 6.1L, 6.2L and 6.4L HEMI engine cams use valve lifts of around .570″, which is very large for production engines and much larger than the production Gen II HEMI engine, which was around .480″. About 20 years ago, racers were pushing the valve lifts past .700″, which required much higher valve spring loads. These higher valve spring loads tended to deflect the camshaft between journals. To solve this problem, the racers wanted bigger cams, which required bigger cam bearings. Another advantage of this HEMI engine design is that with the cam raised in the block, the tappet angles are much less — about half of the small block engine. With the tappets at a much lower angle, they tend to push against each other so that cam deflections are less.


The oil pump is driven off the nose of the crank. The production engine oils the valve gear up through the block and heads to the rocker shaft similar to the Gen II engines and the B-engine and A-engines of the ‘60s and ‘70s. Conversions to pushrod oiling are available. These engines have windage trays but the tray is part of the oil pan gasket. There are several trays based on the stroke.

Though we’ve only discussed some of the common ones used in production, there are many options for blocks, heads, cranks, intakes and EMS hardware. For more information and added details, please contact the Mopar® Performance Tech Line – 1-888-528-HEMI (4364) or go to the website – www.mopar.com.

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