It’s the design of the Hemi engine that made it special and highly desirable in racing and hot rodding. A spark plug is located in the top center of each symmetrically perfect, dome-shaped combustion chamber, shortening the flame travel and burn distance and therefore providing more effective lighting of the air/fuel mixture.
Domed pistons create enough compression, and the exhaust and intake valves are located on either side of the combustion chamber. The valves are positioned 58.5 degrees from each other, increasing the efficiency of crossflow, exhaust area cooling, and combustion. Two separate rocker-shaft assemblies per cylinder head control all of the valves. The hemispherical combustion chambers also allow the engine to run cooler, creating more surface area for heat transfer and large cooling passages. The engine also has a high volumetric efficiency and a high compression ratio.
The Hemi engine became known for its massive horsepower, low-end grunt, and physical size. Hot rodders appreciated its bore and stroke sizes. The Street Hemi produced 425 horsepower and 490 pound-feet of torque. It appeared under the hoods of no fewer than 13 models between 1966 and 1971, though it was never produced in large numbers. The last year of sales for the Street Hemi was 1971, and Chrysler reintroduced it in 2004 when Mopar Performance released it as a crate motor.
Early Instances of the Hemi Engine
The first recorded car that featured an engine with hemispherical combustion chambers was the 1903 Welch Tourist. Its two cylinders produced 20 horsepower. Two years later, Belgian automaker Pipe made a car with a four-cylinder hemi engine. In 1907, the Fiat 130 HP Grand Prix car had a hemi. Other automakers later produced cars that featured hemi engines, including Duesenberg, Stutz, and Offenhauser.
England’s Riley Motors manufactured an engine that used hemispherical combustion chambers, pushrods, overhead valves, and twin camshafts. The 149-cubic-inch engine was used by Healey and produced 104 horsepower (100 horses in non-Healey applications) and 134 pound-feet of torque. It had a small 3.17-inch bore and a large 4.72-inch stroke.
World War II
During World War II, Chrysler began testing and developing engines with hemispherical combustion chambers for aeronautical and military purposes. Chrysler worked with Continental to create the 1,792-cubic-inch AV-1790-5B V-12 in the M47 Patton tank. The monster of an engine put out 810 horsepower and 1,560 pound-feet of torque.
The 1940s P-47 Republic Thunderbolt fighter plane used another example of the hemi engine. The XIV-2220 V-16 had pushrod-activated valves and a displacement of 2,200 cubic inches, and at 3,400 rpm it produced 2,500 horsepower. The engine also used 58.5-degree separation between the valves.
The P-47 was the Air Force’s heaviest and largest single-seater aircraft at the time, and at 15,000 feet, it exceeded 500 mph—70 mph faster than the plane’s original engine. The nose of the plane had to be slimmed down to reduce drag and extended to accommodate the massive engine. The XIV-2220 never made it to production due to the end of the war, but engineers were able to learn some valuable information about cylinder head gas flow.
Chrysler Hemi Development
In 1948, Chrysler continued working on a compact V-8 version of the hemi engine. John Platner, a graduate of the Chrysler Institute of Engineering, and William Drinkard, manager of the engine development department, developed a 90-degree V-8 engine with hemispherical heads in the laboratory. After dynamometer testing proved its performance, Chrysler management gave the 330-cubic-inch A-182 engine the green light.
Some changes needed to be made, though, before the hemi could reach production status. Its components had to last 100,000 miles before requiring replacement. One of the major challenges was camshaft wear, as the area between the camshaft lobe and valve tappets would wear prematurely.
Chrysler engineers in Mel Carpenter’s department changed the material of the hydraulic tappets for this A-239 prototype, along with how the faces of the tappets were formed. The hydraulic tappets also allowed for quieter and smoother operation and extended valve life. They reduced the valve unit load and added a graphite coating. An additive was required in the engine oil.
Being able to replace the spark plugs also proved to be a challenge. To remedy the issue, engineers placed steel tubing through the valve covers and large ceramic boots over the spark plugs. The tube had an O-ring seal against the valve covers to help prevent oil leaks. The plug tubes and wires ran under the covers, exiting at the back of the engine.
Engineers also shot-peened the crankshaft to reduce fatigue and added a dual breaker ignition to the engine to create constant hot sparking and to keep a reserve of ignition voltage. The engine underwent 8,000 hours of dynamometer testing, as well as 500,000-plus miles of reliability testing on the road. The result was the FirePower.
For the 1951 model year, Chrysler debuted the FirePower engine, the automaker’s first overhead valve V-8. It was standard on the Imperial and New Yorker and optional on the Saratoga. Most versions of the FirePower used a single two-barrel carburetor and produced 180 horsepower and 312 pound-feet of torque. The FirePower had a 7.0:1 compression ratio, along with a 3.81-inch bore and 3.63-inch stroke. Just one of the hemi heads weighed 119 pounds, and the complete engine weighed more than 700 pounds.
Chrysler continued developing and testing the hemi engine. The stock exhaust headers were replaced with streamlined units to increase volumetric efficiency. As a result, horsepower jumped up to 193 and torque bumped up to 330 pound-feet. Engineers smoothed and enlarged the exhaust and intake valves; the exhaust valves were opened up by 0.125 inch and the intake valves by 0.25 inch. A set of four inline single barrel carburetors were also added on top.
The test engine became known as the K-310. Its output ended up at 225 horsepower and 332 pound-feet of torque. Engineers discovered that a midrange camshaft added 50 horsepower and 20 pound-feet of torque. With a high performance grind cam and a four carb high speed manual, output jumped up to 308 horsepower and 361 pound-feet of torque with stock pistons. When 12.5:1 pistons were added to the mix, the engine produced 353 horses and 385 pound-feet of torque.
In 1955, a young member of Chrysler’s management team named Bob Rodger designed a hotter version of the engine using dual four barrel carburetors. Chrysler placed a 331-cubic-inch version of the engine in the C-300, making it the first production American passenger car with a claimed 300 horsepower. The following year, the displacement was increased to 354 cubic inches in the 300B, producing 340 or an optional 355 horsepower. It became the first American V-8 with one horsepower per cubic inch. In the Imperial and New Yorker, the engine made 280 horsepower. In 1957 and 1958, the hemi V-8 had 380 horses, but it left the lineup in 1959.
The Father of the Hemi
A hot rodder himself, Tom Hoover was one of the founding members of the Ramchargers. He graduated from the Chrysler Institute of Engineering and started at Chrysler in 1955. Hoover led the team that created the 413-cubic-inch Max Wedge cars in 1962, and in 1963, the wedge engine’s size grew to 426 cubic inches, when the ACCUS/FIA established a 7.0-liter displacement limit for racing engines.
While wedge engines performed really well in the racing world, there needed to be more. Lynn Townsend, who had been the president of Chrysler since 1961, called for a new engine for competition. On a tight schedule, Hoover and his team went to the drawing boards, altering the first generation hemi head and making modifications to the existing RB block structure. They changed the block to head fastener area and retained the 58.5 degrees of separation between the valves. They also changed the bulletproof bottom end to the reciprocating assembly.
Bred for Racing
The A-864, with its 12.5:1 compression ratio, was the first hemispherical head design that sported the trademarked Hemi name. Richard Petty won the 1964 Daytona 500 in a Plymouth Belvedere that featured the first A-864 426 Hemi, and the second and third place finishers used Hemi engines as well. The engine dominated both NASCAR and drag racing that year and grew in popularity among hot rodders, salt flat racers, and boat drag racers.
But in 1965, the NASCAR Grand National series banned Chrysler’s Hemi engine and Ford’s 427 from competition, simply because both engines weren’t available in production cars at the time (Chevrolet was already completely out of racing). Some of the competitors argued that the Hemi had an unfair advantage. Additionally, mid-size cars were no longer allowed on the superspeedways, which included cars such as the Belvedere and Dodge’s new Coronet.
As a result, Ronney Householder, a former IndyCar driver who was then working for Chrysler as the racing manager, ceased NASCAR efforts for the 1966 season. He urged drivers to compete in the USAC, ARCA, and IMCA series, and other racers, such as Petty and David Pearson, started drag racing.
In NHRA, the Hemi went on a diet for the 1965 season. The NHRA mandated that all Super Stock and Funny Car entries had to have steel bodies rather than aluminum. So, Chrysler developed the A-990 Hemi, with aluminum heads and a magnesium cross-ram intake.
The Street Hemi
While Chrysler did sell some Coronets, Darts, and Furys to the public with the racing version of the Hemi, there still wasn’t an official street variant available. However, in a letter from executives Bob Rodger and Bob Cahill dated January 6, 1965, Chrysler Product Planning requested that changes be made to the Hemi so that it could become a package for the NHRA Stock Eliminator class.
Though there were already intentions to add a second four barrel carburetor to the 426S Wedge engine late in 1964, the new Hemi version’s design had to incorporate streetability. The team had to develop it so that it could run efficiently in varying weather conditions and require less maintenance. The engine would run on high octane pump gas and retain all structural changes, such as the heavy-duty bottom end parts and four-bolt mains.
On January 12, 1965, W.J. Bradley of Chrysler Product Planning released a description of the Street Hemi package. Chrysler kept the Hemi as a higher rpm engine. The biggest change was the new aluminum, inline twin four barrel carburetors. Chrysler opted to use Carter AFBs (a 4139S in front and a 4140S in the rear). The Street Hemi also had a dual plane aluminum intake and a large, chrome semi-silenced air cleaner.
The cast-iron heads were largely unchanged from the A-864 variant, minus a design change to the front pad and valve cover rail for street accessories. The rocker gear and valve designs were identical to those of the race version. A milder street cam provided better low end response, and the ports remained the same. The valve spring rates also were altered. While the spring force still wore out the camshafts faster than those in other Chrysler engines, the rates weren’t as steep as the race version’s. Chrysler strongly advised that owners performed regular rocker arm inspections and adjustments.
A pair of cast short-runner exhaust manifolds ran down the sides of the V-8 and were designed primarily for easier installation at the production plant. Additionally, the right exhaust manifold utilized a pair of steel heat riser tubes to help with cold weather driveability. During warm-up, exhaust heat circulated through a port in the back of the intake and a small chamber under the rear carburetor. The operation was controlled via a coiled, bimetallic heat valve located on the exhaust valve, which closed the riser tubes once the engine reached its operating temperature.
The Street Hemi engine had a 10.25:1 compression ratio and used baffled oil pan and a lower compression piston that was forged with minor valve relief. A forged crankshaft was Tufftride treated to reduce wear and corrosion and increase its fatigue strength.
The option for the Street Hemi added about $900 to a car’s price, though the cost included some of the heavy-duty upgrades that, for other automakers, would have otherwise been added separately.
Back into Racing
By mid-1965, Bill France at NASCAR decided to revoke the no-Hemi mandate, and thus the Hemi engine came back into the oval racing world. In 1966, Richard Petty won his second Daytona 500 in his 1966 Belvedere, and David Pearson claimed his and the Dodge Charger’s first NASCAR championship that year.
In the NHRA Stock Eliminator class, Jere Stahl dominated the class with his 1966 Belvedere and won three of the four national event titles, along with the season championship.
Modern Day Hemi Engines
In 2003, Chrysler revived the Hemi name with the 2003 Dodge Ram. Codenamed “Eagle,” the third-generation 5.7-liter Hemi V-8 had near perfect hemispherical combustion chambers with two spark plugs per cylinder. The technology allowed for improved efficiency through spark advance and fuel injection. The 5.7-liter Hemi was updated for 2009 with variable valve timing, modified cylinder heads and intake manifolds for increased airflow and an available variable displacement system.
From Chrysler’s Street and Racing Technology (SRT) division came the 6.1-liter Hemi in 2005. Output increased to 425 horsepower and 420 pound-feet of torque thanks to an improved engine block, lightweight pistons, and a modified intake manifold.
In 2007, the “Apache” 6.4-liter Hemi crate engine debuted with 525 horses and 510 pound-feet of torque. Three years later, the 392 Hemi was available in the 2011 Challenger SRT8 with 470 horsepower and 470 pound-feet of torque. This Apache Hemi was largely based on the 5.7-liter V-8 and also appeared under the hoods of the 2014 Ram Heavy Duty and Power Wagon with increased fuel economy and towing capabilities.
Named after Grumman’s F6F Hellcat WWII fighter plane, the 6.2-liter supercharged Hemi V-8 debuted in the 2015 Challenger SRT Hellcat. It produced 707 horsepower and 650 pound-feet of torque and had variable valve timing but no variable displacement.