At the burger joints, gas stations, and hangouts of Main Street America, Chrysler’s 1966-’71 426 Street Hemi captivated gearheads each time a raised hood revealed a pair of Carter AFBs and the giant black-crackle-finish rocker covers of the orange “Elephant.” On dragways, abandoned airstrips, highways, and backroads, its exhibitions of speed secured its performance legacy.
The hemispherical combustion chamber had existed for a long time prior, however. It had been employed by others, including Chrysler (the 426 was its second-generation Hemi). After developing the design for the military during WWII, the Corporation refined it for use on its new V-8 for 1951. The 331-cu.in. FirePower (hemi) debuted with an impressive-for-its-day 180 hp, and for 1955, the Chrysler 300 was named for its 331’s 300 hp. It grew to 354-cu.in. for 1956 and 392-cu.in. for 1957 and was rated at 380 hp for 1958.
Hemis offered high volumetric efficiency. A low surface-to-volume ratio in the chamber promoted thermal efficiency. The lateral valve arrangement allowed for larger valves installed at specific angles, and straighter intake and exhaust ports for a more direct path to and from the chamber. Shrouding of the valves was minimized. The spark plug was located near the center of the chamber for shorter flame travel and more even burn. Hot spots and carbon deposits were reduced.
De Soto and Dodge fielded their own Hemis as well in the 1950s, but practically no parts interchanged. Size, weight, complexity of the valvetrain, and cost, finally prompted the switch to the wedge chamber design for 1958 Dodge and De Soto cars, and 1959 Chryslers.
Hemis were built at the Marine/ Industrial Division engine plant using special procedures like select-fitting parts, and testing for power, durability, and smoothness. This classic cutaway illustration provides a peek inside the engine.
In the early 1960s, Engineering was already home to a group of drag racers called the Ramchargers, who were respected at the strip and applied lessons learned there to advance engine designs. As discussed in the December 2006 issue of HMM (#39), a founding member, engineer Tom Hoover, was the “race program coordinator for the engineering division.”
The 413- and 426-cu.in. Max Wedges had been highly successful on dragstrips, but less so in NASCAR, and Chrysler President Lynn Townsend was seeking a solution. Hoover, Willem Weertman (assistant chief engineer, engine design), and a skilled team developed the new 426 Hemi.
Hoover recalled, “For a high output and high air-flow configuration, we knew the most about and had the most confidence in the Hemi. We recommended in very short order that we adapt the Hemi to the raised B engine.” Following approval, “Beginning in April of ’63, we set out straightaway to win the Daytona Beach NASCAR race in February of 1964.”
They made the deadline, and Richard Petty drove his #43 single four-barrel 426 Race Hemi-equipped Plymouth to victory at the February 23rd running of the Daytona 500 at the Daytona International Speedway. Second and Third place finishers were also Plymouths, and a Dodge was Fifth. Hemis continued to do well that season, and Petty would go on to win more races and the championship.
The A864 Race Hemis were very strong on dragstrips as well in 1964, running two Carter AFBs early on and Holleys later, on an aluminum short-ram manifold and with a 12.5:1 compression ratio. For 1965, the A990 426 Hemi drag racing engine used a lighter magnesium short-ram and aluminum heads and water and oil pump housings.
Fans, competitors, and sanctioning bodies witnessed the Hemi’s on-track exploits. For 1965, NASCAR outlawed the non-production-car engine. Chrysler boycotted, but continued in other motorsports. In NHRA drag racing, Hemi cars were also being scrutinized.
Note the shape of the 426 Hemi’s combustion chamber and the valve and sparkplug placement (left) compared to the canted-valve big-block Chevy (right). This Hemi features Manley swirl-polished valves in the stock sizes.
A compromise was reached with NASCAR, and Chrysler returned midway through the 1965 season. To compete in the muscle car market and satisfy racing sanctioning bodies, the A102 426 Street Hemi was developed as an option for 1966 midsized Dodges and Plymouths and was rated at a prodigious 425 hp at 5,000 rpm and 490 lb-ft of torque at 4,000 rpm.
Street Hemis sported an inline, aluminum, dual four-barrel intake mounting a pair of Carter AFBs with staged throttle linkage. Iron hemi heads retained their overall design—hemispherical chambers, 2.25- inch intake valves angled 35-degrees from the bore axis, 1.94-inch exhaust valves at 23 degrees, and the spark plug nearly between them. Dual rocker shafts, forged-steel rocker arms, lighter dual-valve springs, and 3/8-inch pushrods were also employed.
Solid-lifter cam specs were tamed to 276/276-degrees advertised duration and less lift. Tuned steel headers were replaced with individual-runner iron manifolds, and the compression ratio dropped to a pump-gas viable 10.25:1.
The bottom-end remained a 4.25- inch bore cast-iron block with reinforced main-bearing web area and cross-bolted main caps for the 2.75-inch mains and the forged steel, shot-peened, 3.75-inch stroke crankshaft with hardened journals and an 8-bolt flywheel flange. Beefy forged I-beam 6.861-inch connecting rods were attached via full-floating pins to domed forged pistons with valve reliefs.
There was a high-capacity oil pan and large-diameter oil pickup tube and passages in the block. Provisions for a fifth head bolt for each cylinder remained. A stud threaded into the head passed through a boss in the block’s lifter valley and was retained by a nut.
A front view of the inner workings is revealed in this rendering.
A dual-breaker ignition, high-speed water pump, high-capacity fuel pump, 3/8-inch fuel line, HD cooling, and a 2.5-inch diameter exhaust with crossover, mufflers, and 2.25-inch tailpipes were used.
Some mods were required in the engine bay to fi t the Hemi, which was mounted on a specifi c K-member that also had a skid plate to protect the oil pan. Torque boxes (also in convertibles) were added at the rear just ahead of the forward leaf-spring mounts. Hemi cars had HD torsion bars, shocks, rear leaf springs, and front anti-roll bar, 11-inch drum brakes (front discs optional), and 14 x 5.5 wheels and 7.75 x 14 tires. A few years later, 15-inch wheels and tires would be included.
The Hemi four-speed had a stronger 18-spline input shaft and other mods, used an HD 11-inch clutch and cast-iron bellhousing, and delivered torque to a Dana 60 Sure-Grip rear with a 9.75-inch ring gear and 3.54 gears. An external cooler came with the TorqueFlite, which also benefitted from internal upgrades and higher shift points. The 8.75-inch rear-end with 3.23 gears complemented the automatic. Other ratios and Sure- Grip were optional, but A/C was not.
B-bodies from the frugal $2,371 Plymouth Belvedere I V-8 two-door sedan to the new top-line $3,122 Dodge Charger could be optioned with the Street Hemi for 1966, and even a handful of four-door Dodges were built with Hemis that year, but no wagons. Adding about $600 to just over $900 depending upon car model and year, not including required additional equipment, the Street Hemi option wasn’t cheap.
Its reputation grew in the automotive mags of the day, posting mostly high-13 to low-14 second ETs right out of the box with the potential to improve them with minor tweaks. For 1967, a run of B/Stock WO23 Dodges and RO23 Plymouths fi tted with the Street Hemi were also formidable ¼-mile competitors.
Two views of the rocker arm assemblies illustrate how long the exhaust rockers needed to be.
A stronger 284/284-degrees duration cam, single valve springs, and oiling system updates arrived for 1968 Street Hemis, yet power ratings weren’t adjusted. For 1970, a hydraulic cam of the same specs replaced the solid-lifter version and eliminated the need for periodic valve adjustments.
With insurance premiums and emissions regulations on the rise, the 426 Street Hemi engine was retired after the 1971 model year, yet strong aftermarket support brought further development and parts, ensuring its continued use in drag racing. By the early 1990s, the supply of Street Hemi engine cores had dwindled and Hemi car values were rising, so new Hemi blocks began appearing in the Mopar Performance catalog followed by heads, and more. Later came 426, 472, 528, and 572-cu.in. Hemi crate engines.
With the arrival of the Gen III 5.7-liter Hemi Magnum V-8 (345-cu.in) for 2003, the famed name was revived (though the chamber was a modern interpretation), and has continued on V-8s, including the high-powered supercharged 6.2-liter Hemi with 797-hp in the 2019 Challenger SRT Hellcat Redeye and 840-hp (with race fuel) in the 2018 Challenger SRT Demon.
Built in small numbers, the factory Street Hemi-powered Mopars from 1966 to 1971 remain highly collectible, with top prices generally paid for the 1970-’71 E-body ‘Cudas and Challenger R/Ts, the B-body 1969 Daytonas and Charger 500s, and 1970 Superbirds, and any convertibles. The 1968-’71 Road Runners, GTXs, Charger R/Ts, Coronet R/Ts (through ’70), and Super Bees also draw high prices, while some of the various 1966-’67 models can still be relative bargains.
Chrysler likely couldn’t have predicted that the 426 Street Hemi would become a legend of this magnitude with a thriving following 53 years after its debut.
Providing exhaust heat to the intake manifold resulted in quicker warmup for street driving.
