Original Hemi V8: dual-rocker powerhouses

Chrysler Corporation was not by any means the first company to use hemispherical heads in their engines, but they are probably the ones best known for them, because of their high production and their high performance… and because they trademarked the term “Hemi™.” Ironically, the original Hemi engines were eventually stripped of their distinctive hemispherical heads, and continued, heavily modified, into the 21st century. But that’s a story for another time.

Chrysler was on the lookout for the most efficient engine design in the 1930s, as they sought replacements for their sturdy, reliable in-line six and eight cylinder engines; they even went to sleeve-valve and radial designs. Then World War II drew the engineers’ time and attention away from the search for the perfect future engine (leaving the straight six untouched until the slant six, then decades in the future).

1951 Chrysler FirePower Hemi V8

During the war, a team was assigned to developing a new aviation engine; this resulted in a powerful inverted-V16 for aircraft, a 36-liter monster dubbed XIV 2220 (2,220 cubic inches). It was installed in a P-47 Thunderbolt and reportedly broke 500 mph, around 70 mph faster than the prior engine. Five engines were reportedly made, and three have known whereabouts, but the project ended because jets clearly spelled the end of propellors for fighters. The airplane engine had hemispherical heads, as did an experimental V12 tank engine.

Get updates

Free: Get an email when we post a new article!

What makes something a hemispherical (in Chrysler’s case, trademarked Hemi™) engine? It is using a hemispherical (half-a-sphere) shape in the head, where the piston comes closest to the valves and the spark plug in an overhead-valve engine. The advantage of using this spherical cut in the head is bringing the spark plug to the dead center, with a valve on either side. Not only is combustion more efficient, because the flame starts in the center of the area, but the valves can be made larger than in a normal head.  The large valves and combustion pattern bring about higher-than-usual efficiency. Welch was likely the first outfit to use them, but many others followed before Chrysler.

After the war projects, Chrysler engineers renewed their search for engines. They did a systematic, methodical search with single-cylinder test engines reducing the cost of experimenting with heads and valves. The team included engineering chief Fred Zeder’s younger brother James Zeder, Mel Carpentier (head of engine design, soon succeeded by William Drinkard), Ray White, and Ev Moeller, who in 1939 was an early graduate of the Chrysler Institute.

John Platner was in charge of testing every setup they could find (including designs from Rolls-Royce, Fiat, Riley, Alfa Romeo, and Healey) on the single-cylinder engine; he found that the hemispherical heads used by Riley and Healey produced more power than other setups, while avoiding spark knock, partly because it could produce high power at relatively low compression. In an actual V8 engine, the valve life was also longer than with other designs. This was outlined in a report he and R. Dean Engle wrote in October 1947.

James Zeder took the next step, trying out hemispherical heads on a Chrysler Six. The modified engine was smoother than the stock one, but also produced much more power, with no real downside other than cost and complexity (in this case, it needed dual overhead cams).

The main problem with hemi-head engines thus far was the need to have dual overhead camshafts, greatly increasing cost, complexity, and tuning time, requiring difficult and painstaking adjustments to the valve system. The Powerplant Research and Engine Design teams worked together, according to Willem Weertman, to create a new single-cam design in both V6 and V8 form. The V8 was particularly important, since increases in bore size would allow larger valves, and the compact shape would improve car layouts.

The major downside of a V8 was cost: the company already had a smooth, reliable in-line eight-cylinder whose tooling had been paid for. Bill Drinkard and John Platner argued successfully that the engine would be a major breakthrough, and it was approved. (One story has James Zeder, in charge of powerplant, getting permission from his legendary older brother, Fred Zeder, who was the head of Engineering; Fred slammed James and V8s in general, but the president of Chrysler overruled him because he knew that GM was working on V8s.) In any case, creating Mel Carpentier assigned design of the actual production engines to Fred Shrimpton.

The first V8 was created in 1948, a small model to test the idea; and management gave the OK on a production-intent prototype. Dubbed A239, the new 331 cubic inch engine was created. Drinkard set a standard for 100,000 miles before any major parts would need to be replaced, then a high standard; years later, many still thought engines should only last 50,000 miles.

Camshaft wear was an issue, due to valve loading; it could take no more than half an hour to destroy a camshaft. This was fixed by using different materials, coating the tappet face, changing cam profiles to help rotate the tappets, and adding ZDDP to the break-in oil. Without these steps, taken by future racing chief Bob Rodger and future product planner Burton Bouwkamp among others, the “Firedome V8” would not have been seen. They also had to figure out how to change spark plugs (then an annual chore) without removing the valve covers, eventually deciding to add a steel tube that went through the cover; the spark plug was protected by a ceramic boot, and an O-ring sealed the tube from the oil under the valve cover. Another cover protected the spark plug tubes and wires, which were routed to the back under the cover, giving the engine a cleaner look.

Along the way, the engineers shot-peened the crankshaft for durability, and developed hydraulic tappets which increased valve life; Carter and Chrysler engineers worked together on a water-jacketed carburetor with an automatic choke built in, to prevent icing and stalling.

Chrysler 1957 392 Hemi FirePower V8 engine

Chrysler was not to have the engine as an exclusive; DeSoto and Dodge were told to propose their own versions. Plymouth, as the mass-production brand, had no V8 plans at the time. Oddly, though all the engines were made from the same family, they all had different internal parts, so DeSoto, Chrysler, and Dodge Hemi internals are often not interchangeable.

The first Chrysler Hemi—then called a “dual rocker” by the engineers, and dubbed Fire Power by Chrysler—was standard in the 1951 Chrysler New Yorker and Imperial, optional on the Saratoga, and not available on the Windsor. Generating 180 gross horsepower and 312 pound-feet of torque with 7:1 compression, it would not be seen as high-performance today, but it beat the Cadillac V8 by a full 20 horsepower (Cadillac being substantially above Chrysler in cost) with lower compression. Both engines were 331 cubic inches in displacement.

The Hemi-powered Chrysler Saratoga ran a 12-second 0-60 run, easily beating Cadillac (13.5 seconds) and Oldsmobile (12.5 seconds), not to mention any Ford. The quarter mile came in 18 seconds at 82 mph, and the Chrysler New Yorker was the Indy 500 pace car.

YearGross Horsepower and Torque
331 Chrysler FirePower 
1951-53180 hp / 312 lb-ft
1954235 hp
1955-56250 hp
1956300 hp / 345 lb-ft (C300)
354 Chrysler FirePower 
1956-57280 hp / 380 lb-ft
1956340 hp / 385 lb-ft (300B)
1957340 hp / 380 lb-ft (Dodge D-501)
1957295 hp
1957-59Dodge pickups
1956355 hp (300B option)
392 Chrysler FirePower 
1957325 hp (375 hp, 420 lb-ft in 300C)
1958290 hp
1958310 hp
1958345 hp
1958380 hp (2 x 4 bbl), 435 lb-ft
1958390 hp (EFI), 435 lb-ft
1958360 hp (Facel Vega)
Dodge Red Ram and Super Red Ram 
1953-4 (241)140 hp / 220 lb-ft
1955 (270)183 hp (245 lb-ft) or 193 hp (Super Red Ram with Power Pack)
1956 D500 (315)260 hp / 330 lb-ft
1956 D500-1 (315)295 hp
1957 D500 (325)285 hp / 345 lb-ft
310 hp / 350 lb-ft
1957 Super D500 (325)310 hp / 350 lb-ft
DeSoto Fire Dome 
1952-54 (276)160 hp / 250 lb-ft
1955 (291)200 hp / 274 lb-ft (185 hp alt)
1957 (341)270 hp / 350 lb-ft
DeSoto FireFlite 
1956 (330)230 hp (255 w/4bbl) / 305 lb-ft
1957 (341)295 hp (4-barrel) / 375 lb-ft
DeSoto Adventurer 
1956 (341)320 hp / 356 lb-ft
1957 (345)345 hp / 355 lb-ft

It had a 3.81 inch bore and 3.63 inch stroke; the intake valve was 1.81 inches and the exhaust valve was 1.5 inches in diameter. Rockers rode on twin shafts, hence “dual rocker.” The idea behind the short stroke was to have slower piston speeds for longevity. The pistons themselves went between crank counterweights at the bottom of the stroke to smooth the idle. The 1952 DeSoto version had 160 horsepower, and was put into around 50,000 cars; the 1953 Dodge version was 241 cubic inches and 140 horsepower.

The 1952 Chrysler, despite a Caddy compression-and-power boost, still won the Daytona Beach speed runs. The first racing win, though, was done by Tommy Thompson on a half-mile dirt track (with a 250-mile race length). By then, James Zeder was in charge of Engineering; he brought in performance-minded engineers to get more power out of the engine.

The best showcase for the original Hemi was Bob Rodger’s idea, the Chrysler C-300—the first production car to have 300 horsepower, and the hottest car of the year for many people. The base price was nearly double that of a loaded Plymouth. The sheet metal was almost entirely shared, but there were clear indicators of what was under the hood.  Getting to 300 had required higher-flowing heads,  larger ports, a special cam with solid tappets, dual exhausts, dual four-barrel carburetors, and a mild 8.5:1 compression. With all that, it was reliable and easy to drive; 0-60 came in under ten seconds, excellent for the time if slow today. The car ran to 128 mph at Daytona, on the sand.

There were numerous changes to the various Hemis through the years, best shown in the table. For example, in 1956, the 331 was bored out by 0.130 inches, moving up to 354 (the same as the modern 5.7 Hemi), and a set of altered heads raised compression to 9.0:1; the 300B reached 355 horsepower with another set of dealer-installed heads (to bring compression to 10:1), the first time any production engine beat one horsepower per cubic inch. The 300 series “letter cars” (C300, 300B, 300C, etc.) were a full package, with fine-for-the-time handling and braking to match the acceleration.

Racing versions

A racing test engine, K-310, was used to experiment with better exhaust headers; smoothed and expanded ports, and different carburetion and camshafts (with computers simulating different camshafts as well as physical trials). It ended up putting out 308 hp with a 361 pound feet, using stock pistons; adding 12.5:1 compression (requiring race gas) brought 353 horsepower and 385 pound-feet of torque. James Zeder noted in writing that the engine’s power was then “comparable with Indianapolis engines, which have been developed for power without regard to any other purpose.”

An Indy engine, dubbed A311, used fuel injection, bigger valves and ports, racing manifolds, and a racing cam; it ran the same lap speeds as purpose-built racing motors. In June 1954, the top four Indy drivers brought their cars to run the new 4.7 mile long oval test track, which had wide lanes and banked curves that could handle 200mph speeds. The fastest lap was 179 mph; then the Hemi A311 test car ran a 182 mph lap. Hearing of this, the Indy officials ordered that only 272 cubic inch stock-based engines could run, and Chrysler dropped its attempt.

Meanwhile, Buck Baker won the NASCAR championship with a 1956 Chrysler 300B.

The DeSoto engine grew to 330 cubes with a raised block and larger bore, hitting 255 hp with an optional power setup; the Adventurer series, a late-1956 launch, had its own special engines as well, boring the 330 to 341 cubic inches and 320 horsepower (clearing the way for DeSoto to be the Indy 500 pace car). Dodge also had a raised-block version, moving to 315 cubic inches, and up to 260 horsepower with a factory upgrade; dealers could install twin four-barrels, but no power rating was revealed. It was less than the Adventurer, though—while the hot Dodge D-500 trim dominated drag strips, the pecking order so far was clearly Chrysler first, then DeSoto, then Dodge, in performance, with Plymouth still absent (though not for long).

The biggest and boldest of the Hemis was the 392, created for 1957 with a 4-inch bore and 3.9 inch stroke—both greater than the prior biggest Hemi. This new Fire Power was good for 325 horsepower, in standard form; in the 300C it went to 375, and with optional higher-compression heads, it reached 390 horses. 1957 was also the first year for the three-speed TorqueFlite automatic (replacing the two-speed), and something that would be on every Chrysler rear-drive car until the 21st century—a torsion bar front suspension.

The 392 300C was able to do 0-60 in 7.8 seconds (yes, that’s the same time recorded by the 1995 Neon, which is an odd coincidence). The handling improved with the torsion bar front suspension, too. For 1957, DeSoto bored out its own dual rocker V8 to 345 cubic inches, hitting 345 horsepower with dual four-barrel carburetors. This was the first standard engine to reach one horsepower per cubic inch, and it was only slightly behind the 300C’s 392 in acceleration. Dodge grew the 315 to 325 horsepower and sold it in several forms, including one with a dealer-installed manifold and dual four-barrel setup.

Chrysler 1957 392 Hemi FirePower V8 engine

The 1958 Chrysler 392 was similar to the 1957 model; it couldn’t be bored or stroked more, at least not without losing reliability. There was an electronic fuel injection setup created by Bendix, and rejected by the other automakers, with good reason: it wasn’t reliable over time, due to the sad shape of materials technology. Bosch would eventually buy the rights to it and dominate the fuel injection industry in the 1970s and 1980s with a modernized version—the basics were, though, present in 1957. It was rated at 390 horsepower but nearly all the systems were replaced by the 380 horsepower carburetor system.

Changes for 1958 included replacing the old crank with a forged steel unit, and cutting back on rear axle ratio variations, which had grown out of control. DeSoto dropped the Hemi for 1958, using new wedge engines; and Dodge kept only the 325, with 252 or 265 horsepower. It too mainly moved to new wedge-head engines. The only Hemi car for 1959 would be the Crown Imperial.

The first-generation dual rocker/Hemi had a good run, not just in cars; they were popular for air raid sirens, pumps, farm power systems, boats, and other industrial uses. Reliable and efficient, their main problems were size (they were quite wide), weight, cost, and manufacturing time; Chrysler couldn’t pump out a million dual-rocker V8s very quickly.

The engines were not dropped; they were refitted. While a brand new series of big-block wedge-head V8 engines were developed (the B series), the Hemis were first stripped of their hemispherical heads and given polyspherical heads with simpler valve actuation—the “poly V8” series—and then redesigned for easier manufacturing with wedge-style heads, the “A engines.”  The latter were really needed for Plymouth, which at the time provided the lion’s share of sales; when the Chrysler engineers had developed their first V8, they had assumed it would only be for premium brands. Once Chevy and Ford started selling V8s in large quantities, Chrysler needed an engine that could be made quickly, cheaply, and in huge numbers for Plymouth.  They didn’t start from a clean slate; they started from the Hemi and worked it and worked it until they had something.  Not long afterwards, Engineering took one more step, making a lighter-weight block—the lightweight A—the LA series. This is the engine that took some of the original Hemi and pushed it all the way into the modern era.

Reply and be heard