street hemi was born because Chrysler engineering realized that while the 361/383/413 and 426 cubic inch Stage Wedges
were fine for drag racing, head cooling was too poor for sustained power. Chrysler was clearly thinking of NASCAR
's Grand National
racing series in mind. Only a complete redesign could make the max wedges viable. Since they were forced to start with a clean sheet of paper anyway they decided to do away with the wedge head
design and go with a hemi
The decision to go with a hemi head was radical. Hemispherical combustion chambers have a long history in racing, dating back to turn of the century Peugeot and Delage race cars. The famed Miller 91 and Duesenberg 122 engines had hemispherical heads during the twenties. Hemi heads allow for larger valves and better spark plug placement. Intake and exhaust ports are straighter, promoting higher flow. The head area is larger, permitting better cooling and a higher compression ratio. Which means more power. However, these racing engines generally employed an overhead cam valvetrain layout, and the 'opposed' alignment of valves in hemi heads is almost perfect. However, Detroit iron has traditionally preferred an overhead valve arrangement. The reasons were practical and involved cost considerations.
Overhead cam layouts have the advantages of a greatly simplified valvetrain layout and reduced mass, particularly reciprocating mass. There is no need for lifters, as the cam can operate the rocker arms, or even the valve stems directly. Because of this greatly reduced mass, overhead cam engines can be both more reliable and rev higher than their overhead valve cousins. It is generally agreed that overhead valve engines can rev no higher than 8-9,000 RPM before failure occurs, usually involving the lifters. Overhead cam engines in racing trim may reliably rev to 14-15,000 RPM. The limiting factor there is the valve spring which at those speeds suffers from harmonic induced material failure. Higher revolutions are possible using pneumatic valves. Such systems have existed for a long time in Formula One racing. However, such systems are very expensive, and required compressor consumes enough power that you must use the added revs in order to justify the parasitic power loss.
Overhead cam systems are very simple in engines employing an in-line cylinder layout. All you need is a slightly longer timing chain. Chain speed itself is not affected. But in-line engines of more than four cylinders become very long, leading to significant packaging problems. It is for this reason that V or opposed cylinder layouts are common for six or more cylinder engines. For a V-8 engine, the cam drives required become very complex. Multiple timing chains, or a complex gear drive system is required. That exacts penalties of increased cost, moving mass, and complexity. It is for this reason that only racing, or exotic OHC V-8 engines were built until the Ford 4.6L modular V-8.
Overhead valve V-8's employ a single camshaft located in the block between the cylinder heads, driven by a single timing chain. The valvetrain limitations are not seen as relevant for street considerations. Large displacement engines produce plenty of power and torque right off idle, and at highway speed RPM's are often less than 4,000 RPM.On the street, Detroit V-8's are very durable, because they are rarely pushed near, much less at, their limits. The preferred layout is a wedge head design. It is chosen because the valves are located in a line, making for very simple lifter and a single rocker arm shaft, while the hemis required two shafts. Wedge heads are simpler to design and manufacture, and again, the disadvantages nonexistent for street use.
Racing changes all that. Breathing is now paramount, and operating at max RPM a fact of life. But the valves in hemi heads are located at 90 degrees from each other, rather than a line. Rocker arm and lifter design is very complicated, particularly when low mass is sought. Chrysler engineers had a lot of trouble perfecting the valvetrain for the new Hemi motor. However, the choice of a hemi head was natural for the Chrysler design team. They had begun designing hemi head engines in the early fifties, most notably the 331 engine found in the first Chrysler 300 letter cars. That engine proved very successful in racing. NASCAR banned the Chrysler 300B after a single, dominant season. Road racers like Brooks Cunningham dropped the hemi into his Sebring winning Cunningham C4R race cars, and Allard J-series roadsters. The 277 Red Ram, 361 and 392 engines were also popular. In fact, the 392 Hemi remained popular as a drag racing engine into the seventies. The Donovan engines aimed at top fuel competition, were based on the 392.
In addition, Chrysler engineers designed the 426 to be an oversquare engine, with a bore to stroke ratio of about 1.5 to 1. For a given displacement, oversquare engines use a crankshaft with shorter journals, cutting mass, and greatly reduced piston speed. Which makes them much more reliable at higher RPM, and quicker revving. The large bore, though expensive, permits larger valves.
The engine proved a huge success in racing. Using the Hemi, Richard Petty enjoyed the most dominant season ever in the history of NASCAR. Pro stockers such as Ronnie Sox, Dick Landy and "Akron" Arlen Vanke simply dominated Pro stock until 1972 when the NHRA re wrote the rules to end the mopar victory parade. In Top Fuel and Funny Car competition with their nearly unrestricted nature, the Hemi simply became the engine of choice. Only the very rare-- and hideously expensive--- SOHC Ford 427 Cammer was even remotely competitive. Modern top fuel engines are all refinements of either the 392 or 426 basic designs.
The 426 Hemi was available on street cars from late 1964 through 1973, when pollution control and fuel economy considerations led to its cancellation. It was available in most body styles, and all that was required was to check the order sheet and come up with the cash. This stood in stark contrast to Ford and Chevy who produced several racing variants-- e.g. the Ford 427 side oiler and the Chevy ZL-1. All were unobtanium for those without a racing reputation. The hemi generally came with two Carter AFB four barrel carburetors and was rated at 425hp. The NHRA rated the hemi at 480hp, as 425 was probably available at almost any RPM, with stump pulling torque. Hills simply do not exist for Hemi-powered cars.
Note: Noders wishing more information on road racing in the early fifties are advised to read Burt "B.S." Levy's books The Last Open Road and Montezuma's Ferrari. The story is dead-nuts accurate and often hilarious. The book is self-published and available online. And you'll learn more about Allard and Cunningham racing cars, powered with early versions of the Hemi.