allpar, the Chrysler - Dodge - Plymouth - Jeep site

Much More

Origin of the Mopar Hemi engine, Ardun Heads, and Riley cars

The question of who invented the hemispherical combustion chamber is rather a silly way to waste time. Chrysler didn't, and never claimed otherwise. Zora Arkus-Duntov didn't, either, and I don't believe he ever claimed to.

1966 chrysler street hemi engines

Chrysler never claimed to have the first mass-produced Hemi, either. I don’t know who was first, but BMW had their 2.8 liter inline six cylinder engine in the mid-1930s; it had an in-block camshaft with 18 pushrods, 18 rockers, and two rocker shafts. The extra pushrods were horizontal, crossing the cylinder head to operate the exhaust valves. Despite this Rube Goldberg setup, the engine was good for its time. It was also made under license by Bristol. In the 1950s, an AC Ace sports car with Bristol engine was a winning racing combination in a production sports car; it became the Shelby Cobra.

hemi engineThe advantage of the Hemi layout, with valves angled away from each other, lies with increased valve size, and greatly enhanced porting.

The first mention of angled valves and overhead camshafts in The Grand Prix Car Volume 1 (Lawrence Pomeroy) is in his description of the 1912 three-liter Peugeot Grand Prix car. This was not a hemispherical chamber, but it had four valves in a pent-roof chamber, so it’s a level above the hemi. Hence, the inventor of the Hemi came up with a way to reduce the potential of the overhead cam, multiple valve design.

Numerous other makes copied Peugeot, and by the 1920s, angled valves in crossflow heads were the norm in racing engines, both in Europe and the United States. Delage, Ballot, Bugatti, Chevrolet, Deusenberg, Frontenac, and Miller, to name just a few, were all either hemis or had pent-roof chambers with four valves per cylinder. More recent marques include Jaguar, Aston-Martin, Coventry-Climax, Coswoth, Ferrari, and Maserati.

street Hemi

The major difference between hemispherical-head engines is the means of running the valve train: dual overhead camshafts, a single overhead camshaft, or pushrods. Chrysler chose pushrods and they worked amazingly well.

By the way, the original hemis were called (by Chrysler) “the Double Rocker.” It wasn't til the 426 that “Hemi” became standard, and Hemi is now a registered trademark of Chrysler.


Anyone who has visited the Walter P. Chrysler Museum has seen the first Chrysler Hemi, the XIV-2220 V-16 aircraft engine developed during World War II. The engineers gained valuable experience which they later applied to further experiments on corporate six cylinder engines (including a stillborn Hemi V6). I imagine more than one was aware of the existence of the Ardun heads for Ford engines. If there were features which applied to the new engine design, I hope they adapted them.

Chrysler sold Ford V-8s with modified Ardun head kits in Brazil

This work lead to the 1951 Firepower V-8 of 331 displacement, the 1952 Desoto Firedome V-8 of 276 cid, then to the 1954 Dodge Ram V-8 of 241 cid; all were hemis (culminating in the famed 392).

What Chrysler could claim as a record would be the mass production of more hemi-type engines than anyone. Period.

In 1951, James C. Zeder wrote an article for Motor Trend, reproduced with permission in the WPC News; he referred to the predecessors of the Chrysler Hemi, including a 1904 Belgian Pipe Company twin-cam powerplant (criticized for the poor airflow of the heads). He called the 1927 Delage engine a “thoroughbred masterpiece” with nearly two horsepower per cubic inch (the best muscle car engines achieved one horsepower per cubic inch). However, its dual overhead cams were expensive and “often temperamental.” 

1954 Hemi engineMr. Zeder mentioned the Bristol 328 engine as well, based on a BMW design, using a single cam with pushrods; and the Lea-Francis, with twin cams and pushrods (to avoid having cams in the head), noting that the layout was used by Riley. The Chrysler engine used pushrods, with self-adjusting hydraulic tappets to compensate for pushrod length changes as the temperature changed.

Mr. Zeder referred to the Ardun inclined overhead-valve heads, which raised output from 100 to 175 hp, as “building engines the right, but costly way.”

The earliest “hemi” I can find is the 1903 Welch, built by Chelsea Mfg. Co. (Michigan). It had a 20-hp, two-cylinder engine with overhead valves. The company moved to Pontiac, Michigan in 1904 and began producing larger cars with 36-hp four cylinder engines, still with overhead valves and hemi-heads. Their 1906 model had an adjustable steering column. Welch also built a 75 hp six-cylinder engine with overhead valves, hemi heads, and a single overhead camshaft. [This precedes the engines found by Mr. Hagenbuch.]

In 1909, the company was acquired by General Motors, and built the Welch-Detroit. In 1911, used the body tooling for the single-year Marquette (not connected to the later Buick-built Marquette), which used a four cylinder engine with a T-head sidevalve arrangement. No more hemis or overhead cams.

The Healey (Riley) engine also had dual overhead camshafts. Some old British repair books verified that the Riley did, indeed, have two camshafts in the block with short pushrods (because the camshafts are just below the heads). The high placement of the camshafts may give some the impression of being in the head.

There were two valve covers, one for each valve rocker shaft, so the spark plugs were mounted directly in the head without the need for spark plug tubes as used on Chrysler hemi engines.

The 1946 Riley RM used torsion bars up front, with the adjuster at the rear (beneath the driver's feet) with the forward end mounted in the lower A arm. Eleven years later Mopar would adapt a similar version and call it Torsion-Aire.

Someone sent me a link to your Mopar-Ardun-mystery page and I found myself quoted, not unpleasantly.

I am glad to see some of your readers care for this ancient legend about a possible link twixt the 1947 Ardun and the 1951 Chrysler Hemi. It is undeniably true that there are a number of similarities.

1951 Hemi Firepower

One wag has stated that the 1951 Hemi merely consisted of a set of iron Ardun heads atop a 1949 Cadillac shortblock, and that today's NHRA Top Fuel Hemi is nothing more than a fully-developed Ardun!

My friend Mr. George Kudasch, 89 [at the time of writing], retired mechanical engineer, was hired in 1947 by Zora Arkus-Duntov to work as co-designer on the project at Ardun Mechanical Corporation. After that project, Mr. Kudasch went on to a successful career at Sikorsky Helicopters. He recalls that in 1947, Zora had some visitors from Chrysler Corporation and showed them what he was doing on his Ardun heads project.

Jay Fitzhugh touched on this story in an Ardun article last year in The Rodder’s Journal. George (born 1916 in Russia) and I worked on an in-depth interview about his life which has just completed its four-parts publication in Bonneville Racing News.

Hemi and the Ardun heads

One source wrote:

I asked Zora if Chrysler ever contacted him for a license or any consultancy on their heads. He said "No, the first I knew of Chrysler's V8 head design was in 1953, when I had gone to work for GM. One day I happened to see a poster on the wall with a cross-section drawing of the new Firepower V8. It was apparent that the head design was very close to mine of 1947. Standing there, I felt a great sense of pride that this huge corporation had followed my design and was making tens of thousands of successful engines with it. I didn't care to make an issue of it; engineers have always learned from and built upon earlier designs."

1951 Hemi V8

Bill Watson countered:

I suspect the similarities between the Ardun and Firepower were more due to the cylinder bore size and block bore centres. With a given bore size, the valves can only be up to a certain diameter. Any larger and they will hit the top of the block when they open.

By 1953 Chrysler had three Hemi engines in production - Chrysler (Firepower), DeSoto (FireDome) and Dodge (Red Ram). And Zora knew nothing of these engines’ heads until he went to work for General Motors in 1953? What rock was he hiding under since 1951? Every auto magazine and car show had the Chrysler Firepower hemi displayed and written about, complete with illustrations, when it was introduced in 1951. And magazines such as Motor Trend, Road & Track, Auto Topics, Popular Science, Popular Mechanics, and Mechanix Illustrated all made the introduction on their front covers. So how could he not know?

A311 Hemi engine from Chrylser

Patrick Clement wrote that Chrysler diverged from common European hemi-head designs, which had dual overhead cams, by using a single five-bearing camshaft in the valley of the V-8, running pushrods up to work rocker arms which were on parallel rocker shafts.

James Zeder and Ev Moeller were on the team after the war that looked into the Hemi for Chrysler. They looked at a Healey engine with a Hemi head and found it to be the most efficient design that they had seen. They built a head for a Chrysler six cylinder engine, and called the experimental unit the A161. It was tested in the real world in a car driven by the head of testing Wallace Zierer. It passed the test and by 1948 they had a 330 cubic engine called the A182, a V8 with Hemi-heads. [In 1951, another Hemi V6 engine was designed.]

Pete Hagenbuch wrote:

The Chrysler engine design and development people already knew more about the characteristics of Hemi type cylinder heads than Arkus-Duntov could. They had just recently gone through a long and rigorous program of bringing their fantastic V-16 liquid cooled aircraft engine to the threshold of production. Its output was reported as 2500 hp at the time the project was cancelled. Not bad for a new design, still in development.

Whatever help Chrysler engine people received from a study of Arkus-Duntov's, their own experience stood them in good shape for designing the world’s first mass produced V-8 engine with hemi heads and pushrod-actuated valve gear. That's my claim, not Chrysler's.

And finally...from Bill Thomas:

cutaway view of the Chrysler Hemi engine - 331Former Chrysler engineer Peter Hagenbuch really tells it straight, and Bill Watson got it right too. The Welch, made in Chelsea, Michigan, was the first hemi.

The Cadillac and Olds were really not the same program, although both evolved from Kettering’s high-compression combustion research. These two engines both had wedge chambers.

Olds developed a V-6 at about the same time as the V-8, which looked just like the V-8, only shorter. It never made production. A later experimental Olds V-6, circa 1961, used a Lancia V-6 hemi as a design study. We also tested and looked at the Willys in-line six truck engine, which had a chain driven single overhead camshaft hemi. One of the later Olds V-6 engines is in the Reo Museum in Lansing, MI.

Note: the term “Hemi” was a registered trademark of Chrysler Corporation and is now a registered trademark of FCA US, LLC.

Modern Hemis 5.7 Hemi • SRT 6.1 and 6.46.4 Truck Hemi • Supercharged “Hellcat”
Classic Hemis 426 HemiOld Hemi History • 392 Crate HemiDevelopmentPlane Hemi

We make no guarantees regarding validity, accuracy, or applicability of information, predictions, or advice. Please read the terms of use and privacy policy. Copyright © 1994-2000, David Zatz; copyright © 2001-2015, Allpar LLC (except as noted, and press/publicity materials); all rights reserved. Dodge, Jeep, Chrysler, Ram, and Mopar are trademarks of Fiat Chrysler Automobiles.