The Legendary HEMI® Engine

"There's nothing humble about that HEMI^® rumble"

Hemi: (HEM -e) adj. Mopar in type, V8, hot tempered, native to the United States, carnivorous, eats primarily Mustangs, Camaros, and Corvettes. Also enjoys smoking a good import now and then to relax.

No other engine in U.S. history has the heritage and recognition of the Chrysler Group HEMI®. While completely re-engineered from the legendary 426 HEMI V-8 that powered Chrysler muscle cars of the 1960s, the two features that earn the new 5.7-liter HEMI power plant its HEMI name are:

1. The hemispherically shaped combustion chamber designed to accommodate large valves and put the spark plugs close to the center of the combustion chamber
2. The cross-flow valve arrangement that is perpendicular to the crank-shaft centerline

The modern Chrysler Group 5.7-liter HEMI engine is elegantly simple. It achieves power, fuel economy and emissions goals with a design that is uncomplicated and cost effective.

The modern HEMI is engineered to deliver outstanding performance and reduced noise, vibration and harshness (NVH), resulting in a highly refined powerplant. Fuel economy has also been improved, but not at the expense of HEMI performance. Chrysler Group has developed and was the first to offer Multi-Displacement System (MDS) on a modern, large-volume vehicle in North America with its debut on the Chrysler 300 and Dodge Magnum. The MDS seamlessly alternates between four cylinders when V-8 power is not needed, and V-8 mode when more power from the 5.7-liter HEMI is in demand.

The modern HEMI engine offers more horsepower and torque than any Chrysler Group vehicle passenger car engine since the legendary 426 HEMI of the '60s and '70s. The 5.7-liter HEMI engine debuted on the Dodge Ram 2500 and 3500 as the standard engine offering. Producing 330 hp at 4800 rpm and 375 lb.-ft. at 4200 rpm, the HEMI provides more power than competitive standard V-8 engines. Versus competitive vehicles with similar size gasoline engines, it provides best-in-class acceleration and towing capability. The HEMI is available in the Dodge Ram 1500 to create the most powerful mass-production, light-duty pickup on the market.

Every year since its introduction, the Chrysler 5.7-liter HEMI® has earned a place on Ward's 10 Best Engines list. "The HEMI's 10 Best Engines win for a third consecutive year is no accident. The HEMI is a powerhouse, both literally and as a marketplace phenomenon," said Bill Visnic, Senior Technical Editor, Ward's AutoWorld. "A look at the HEMI's installation rates proves it's a home run in every vehicle that gets it."

“The modern HEMI engine is one of our best examples of Disciplined Pizzazz,” said Eric Ridenour, Executive Vice President – Product Development, Chrysler Group. “We combined the best of our engineering heritage with modern design to develop an engine that puts smiles on the customers’ faces – one million customers’ faces.”

The HEMI engine first debuted in a 1951 Chrysler. The most renowned of all HEMI engines was the 426, built from 1964 through 1971. These engines powered the Dodge and Plymouth muscle cars that made Woodward Avenue and Van Nuys Boulevard famous. There were only 6,671 of these engines produced.

Why dual spark plugs?

Michael E. Gemmel wrote: “Each cylinder has an ignition coil pack over one spark plug, and a regular plug wire connected to the other spark plug. Further, the coil pack also has a plug wire attached to it that extends to the opposite cylinder bank. It appears that each cylinder shares a coil pack with another cylinder. Each of the two plugs on a given cylinder is fired by a separate coil. One plug has a coil directly attached, and the other is fired via an ignition wire connected to a coil located on another cylinder on the opposite bank. The benefits would be one-half the number of coils (8 vs. 16) compared to each plug having its own coil, and of course less weight.”

“Cryptojoe” wrote: “Being the good Motech Graduate I am, I can say that the extra plug fires during the power stroke in order to more full burn the hydrocarbons. While this does add to NOx (oxides of nitrogen) and ozone emissions, it relieves the catalytic converter from becoming overwhelmed with unburned hydrocarbons. Unlike the Japanese systems of the late 1970s and early 1980s, which avoided the use of catalytic converters, the second ignition allows additional power in the down stroke while lowering the need of restrictive catalyst plates in the converter. This increases breathing, and in turn adds to horsepower output as well.

“As you may recall, in the 1980s Japanese manufacturers skirted trade restrictions concerning EPA standards for unburned hydrocarbons by placing spark plugs either in the exhaust pipe (which fired with every piston ignition) or in the exhaust manifold (which fired each time their corresponding cylinder fired). Chrysler morphed this idea to include dual fired plugs on each cylinder, which allows the firing to take place closer to top dead center, and then again when the piston is on the back side of the power stroke.”

Specific advantages

The source for most of this information is Popular Hot Rodding.

The cam was placed high up in the block to keep the pushroads as short as possible. The hollow cam has oversized journals and lobes to minimize side loading on the roller-style lifters. The valve springs are beehive types, more effective than standard springs so they can be lighter, with less lifter collapse. Rockers have much less intertial mass than usual, with the form and size carefully designed for a conservative .500 inches of lift; but the valves flow well enough to make this more than enough.

Generally, the engine has been designed for lighter weight. The new Hemi is precision cast, which allows it to be lighter than a typical 5.7 liter engine, even with a taller deck height than Chevy's; and, partly to counter the intertia of its relatively long stroke, the pistons were made light as well, using cast eutectic alloy. The slipper-style piston has much in common with racing pistons, with a weight of 413 grams. For longevity, the Hemi pistons use a hard anodize on the top ring land, to act as a heat barrier and anti-micro weld mix, and to allow the top ring to be only 3 mm from the top of the piston, cutting emissions while bringing more power. As with the old 426 Hemi, the rings are also relatively thin. Also in common with racing engines is a reservoir groove undeneath the top ring, to reduce the pressure between the top and second ring.

The skirt is coated to allow for variance in production piston sizes, increase the fit for ring seal, and reduce piston noise. The lightweight wrist pin is also high-set.

The crank has larger inner counterweights than equivalent Chevy engines; but their weight is offset by the lighter pistons and rods. A windage tray sits underneath the crank, while the serpentine belt pulley also acts as a torsional vibration damper. The connecting rods are also designed for strength and low weight, using a powder metallurgy process first used by Porsche, and negating the need for a balance pad. A cap bolt is used instead of a through bolt.

One problem with the Hemi is that a speed density system is used for measuring air into the engine rather than air mass, so that cold air packages and such can throw the system off.

The 2 inch ports flow 270 cfm at .600 inches of lift, with peak flow at .375 inches. At .250, the stock head also had excellent flow. The exhaust port hits 161 cfm at .600 lift, with a 1.55 inch valve. Both have unusually good velocity and distribution throughout their range - even compared with a Chevy LS6 engine. The engine apparently takes well to simple porting.

General hemi engine notes

In January 2003, the public saw the first supercharged Hemi engine ever officially produced by Chrysler. It produces 430 horsepower and 480 lb-ft of torque, using the 5.7 Hemi engine and a Whipple supercharger, and was first shown in the Dodge Magnum SRT-8 concept. The 300C version had "over 400 horsepower" and 350 lb-ft of torque.

Hemi production began in June 2002 at the Salitillo plant in Mexico, with a projected production of 440,000 engines a year.

The original Hemi was smaller, but the most famous, the unbeatable 426 Hemi, was considerably larger - and produced quite a bit more power, albeit before emissions standards. That engine took first, second and third place at the 1964 Daytona 500 and became the de facto drag racing standard, not to mention the killer street engine - one which added about a third again to the price of a new car. It re-entered production for racing in the late 1990s. Click here for more "old" Hemi details.

The original Hemis and the current one both have dual rocker shafts, large (similarly sized) valves, and two valves per cylinder arranged in a similar pattern for great efficiency. By having only two valves per cylinder, Chrysler is bucking a trend; by having two plugs per cylinder, they are also bucking an emerging trend of single spark plugs with dual electrodes.

The engine's unique two-valve hemispherical combustion chamber within an aluminum head provides impressive air flow, torque and power (hallmarks of the original), and gives the engine its throaty rumble. Fifty-six pounds lighter than the 5.9-liter V-8, the 5.7-liter Hemi produces 41% more power and 12% more peak torque, and appears to have substantial reserve for more power gains.

An electronic throttle control system, Chrysler's second drive-by-wire gasoline engine (after the 4.7), moves the throttle in response to accelerator pedal position. The throttle system is based on engine torque control, and compensates for changes in the engine load needed by the air conditioning system, compressor, alternator, power steering pump and automatic speed control.

Dual spark plugs per cylinder ensure consistent, complete and rapid combustion, without knock, that increases peak power and torque, reduces exhaust emissions and increases fuel efficiency. It even smoothes the engine's idle. The 5.7 delivers an 8-10 percent better fuel efficiency than the less-powerful 5.9.

Cylinder deactivation, which shuts off fuel to some cylinders under idle and low demand situations, is expected in 2004. That will increase fuel economy.

Every person we've contacted who has driven a Hemi has been impressed by its power - and the economy beats the much less powerful 360 it replaces. However, the proportion of regulated pollutants is a problem for Chrysler. Bob Sheaves noted:

The Hemi design combustion chamber is one of the poorest designs for emissions - why do you think it took so long to get it into production? It almost did not make emissions test requirements even with the modifications. ... Today's Hemi is that (a "Hemi") in name only. [Editor's note: Bob is referring to the head design. It is not a true hemispherical head, but looks vaguely like a hemispherical head with parts filled in.]

John Veatch wrote:

"Seeing all the hoopla surrounding the new Hemi and the previous speculation on whether its displacement was going to be either the nostalgic 354 or 383, I am amazed no one has made mention of what is probably the coolest coincidence of all. The actual production engine came in at 345 c.i.d. @ 345 hp, the exact same specs as the 1957 DeSoto Adventurer 345/345 Hemi, the first standard engine to make the 1hp/ci mark (the 1956 300 optional HP engine had earlier accomplished the feat). Both preceded the much ballyhooed optional 283/283 hp 1957 Corvette engine." [The 1950s engines were gross horsepower while the current one is net.]