Although Chrysler is most identified with the use of "Hemi" as a marketing term, many other auto manufacturers have incorporated similar cylinder head designs.

In return, hemi head engines tend to have complex valve trains, are expensive to build, and both larger and heavier than conventional designs.

A hemispherical combustion chamber is an efficient shape, with an excellent surface-to-volume ratio, minimal heat loss to the cylinder head,[clarify] and room for two large valves.

The intake and exhaust valves lie on opposite sides of the chamber and necessitate a "cross-flow" head design.

However, if the hemi-head hemisphere is of equal diameter to the piston, there is minimal squish for proper turbulence to mix fuel and air thoroughly.

The hemi head always has intake and exhaust valve stems that point in different directions, requiring a large, wide cylinder head and complex rocker arm geometry in both cam-in-block and single overhead cam engines (dual overhead cam engines may not have rocker arms).

Significant challenges in the commercialization of engine designs using hemispherical chambers revolved around the valve actuation, specifically how to make it effective, efficient, and reliable at an acceptable cost.

However, the exercise gave Chrysler engineers valuable research and development experience with two-valve hemi combustion chamber dynamics and parameters.

Eventually, three of the four Chrysler divisions had their own version of the FirePower engine, with different displacements and designations, and having almost no parts in common.

This lack of commonality was due in part to the three engine versions using different bore spacings (the center-to-center distance between adjacent cylinders).

Cunningham switched away from these designs in 1959 when Chrysler temporarily abandoned the hemispherical concept in favor of the wedge-head B engine until 1964.

Carl Kiekhaefer also used the Chrysler engines in his NASCAR cars from 1955 and 1956, winning the Grand National Series championship both years.

[5][6] The 331 engine was used in the following applications: The 354, released in 1956, had a bore of 3.9375 (100mm) in and stroke of 3.625 (92.1mm) in, and the same 10.385" (264mm) low deck height, for a displacement of 5,787 cc (353.1 cu in).

A dual four-barrel version of the 392 available in the 1957–58 Chrysler 300C and 300D cars was rated at 375 bhp (280 kW); the 300D, and some marine and industrial engines, used a (now rare) adjustable rocker.

[citation needed] The DeSoto Adventurer had a compression ratio of 9.5:1, using a special camshaft profile for hydraulic lifters, and produced 320 bhp (239 kW) using dual Carter WCFB four-barrel carburetors.

[4] Dodge did not have a V8 engine until one was developed specifically for the line in 1953 based on the 1951 Chrysler hemi design, but downsized for these smaller cars.

In higher trims like the Dodge Royal, the "Super Red Ram" featured a four-barrel carburetor and produced 193 bhp (144 kW) at the same compression ratio.

[14] For 1956, Dodge increased the displacement to 315 cu in (5.2 L) with a longer 3.80 in (96.5 mm) stroke and a taller raised-deck block, but used polyspheric rather than hemi heads on the base version.

Also, a "race only" package called the D-500-1 or DASH 1 was available with a special aluminum dual four-barrel intake that sported a pair of Carter WCFB carburetors similar to the ones on the Chrysler 300B and DeSoto Adventurer.

Even though the hemi headed offerings sported "dimples" in the valve covers for mechanical adjuster clearance, all engines had hydraulic lifters.

Complaints by Ford regarding its power led to the "Race Hemi" being banned from NASCAR's 1965 season due to its unavailability in production vehicles.

Although all manufacturers were familiar with multi-valve engines and hemispherical combustion chambers, adding more valves per cylinder and designing the complex valvetrain they require were expensive ways of improving the high–revolutions per minute (rpm) breathing of production vehicles.

It uses a coil-on-plug (distributor-less) ignition system and two spark plugs per cylinder to shorten flame travel leading to more consistent combustion and reduced emissions.

A new variable displacement technology called Multi-Displacement System (MDS) is used in some versions which can shut off two cylinders on each bank under light load to improve fuel economy.

The 5.7 L HEMI was released for model year 2003 on the Dodge Ram pickup trucks to supplant the Magnum 5.9 engine.

At lower engine speeds, the valve was closed, resulting in improved low-end torque from the longer runners.

[24] Engines mated to the six-speed manual transmission or used in the Ram Heavy Duty did not feature the Multi-Displacement System (MDS).

[26] The supercharger is a 2,380 cc (145 cu in) twin-screw IHI unit with integrated charge coolers, capable of producing 11.6 psi (80 kPa; 0.80 bar) of boost.

Applications: Chrysler displayed a larger and more powerful 392-cubic-inch (6.4 L) HEMI in 2005 with a factory-rated output of 525 hp (391 kW; 532 PS) and 510 lb⋅ft (691 N⋅m) torque.

[29] The production version of the 392 HEMI was developed in 2010, launched in the 2011 model year Dodge Challenger SRT8 with variable camshaft timing as well as MDS in cars with automatic transmissions.