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The Chrysler new Hemi and its forerunners

by François Dovat

Hemispheric combustion chambers have been very popular among racing and aircraft engine since 1912. That year the original design of Ernest Henry according to the ideas of Boillot, Goux and Zuccarelli appeared and was applied to the Grand Prix winning 7.6 liter Peugeot.

After WWII some automotive engines with hemispheric combustion chambers combined to a single camshaft in the block instead of double overhead camshafts or side valves were mass produced both sides of the Atlantic Ocean. While most Detroit giants OEMs marketed V8s with overhead valves and wedge type combustion chambers, Chrysler chose the hemispheric way. Such engines cost a little more to manufacture and their overall width is increased, but they allow a higher specific output due to outstanding volumetric and indicated thermal efficiencies.

Figure 1

Figure 2

Introduced in 1949, the little 1.3 liter "hemi" of the Peugeot 203 developed 42 gross hp at 4500 rpm (fig. 1 & 2). Some more power was required for the US market: the Chrysler "Fire Power" of 1951 produced 180 gross hp at 4000 rpm out of 331 cubic inches (5.4 liters) and a weight of 322 kg (fig. 3). It was first installed on the New Yorker and Saratoga lines. Between 1951 and 1958, Hemi engines of up to 392 CI and 390 gross hp were offered on the Chrysler, Imperial, DeSoto and Dodge. They carried the various names of "Firepower," "FireDome", "FireFlite and "Red Ram" and were produced in separate plants, but they all shared a similar design. A comprehensive database about these engines is available at http://www.thehemi.com/engines.php

Figure 3

From 1966 to 1971 the famous 426 CI Hemi engine, nicknamed "Elephant, was a regular production option on several high performance cars. It first appeared on Feb. 23, 1964 at the Daytona 500, taking first, second and third places. An Hemi powered car also set a new average speed record for the track of 154.33 mph. The mighty engine would afterwards reliably serve Chrysler's NASCAR efforts and dominate as well other forms of racing, thus establishing its legendary reputation. Designed with a healthy stroke / bore ratio of 0.88 by 4.25 x 3.75 inches, long center to center rods of 6.77 inches, cross bolted main bearings caps, forged crankshaft and two 4-barrels Carter AFBs mounted on an aluminum dual-plane intake manifold, it was a totally new engine which had only its basic combustion chamber shape and valvetrain layout in common with the earlier Hemis of the 1950's. Except for emissions control, the 426 Hemi is still by no means obsolete and its production has recently been restarted for racing use. Its 425 advertised gross hp seem to have been a quite conservative rating. (fig. 4 & 5)

Figure 4

Figure 5

The same concept is also revived on the new 345 CI / 5.7 liters " Hemi Magnum " (fig. 6,7,8) whose production began in June 2002 at the Salitillo facility in Mexico, with a projected number of 440,000 engines a year. After two decades of multi-valves and multi-camshafts craze, it looks like a return to design simplicity and sanity. Spark ignition engines with 4 (or more) valves per cylinder are powerful at high revs, hence for racing when rules limit the cylinder capacity, but getting some substantial low end torque was a challenge that could only be overcome with still more complexity, such as port deactivation and variable valve timing. Double overhead camshaft are good at reducing valvetrain deflection, yet their drive leaves the designer with the choice of intricate timing gears trains, long chains or toothed belt with tensionners. In addition more rotating shafts mean more friction.

Figure 6

At 270 kg, the new Hemi is considerably lighter than is predecessors of the 50's and 60's which weighted between 330 and 385 kg; it is also 25 kg lighter than the 5.9 LA with same bore center distance it replaces. New thin walls foundry technology and aluminum heads make up for the weight savings. The 5645 cc are obtained by a bore of 99.5 and a stroke of 90.9 mm (3.92 x 3.58, 345 CI), dimensions very close to that of the 354 of 1956.

Figure 7

The cast iron cylinder block has cross-bolted main bearing caps as well but the crankshaft is made of nodular, sand-cast iron. The 85cc combustion chambers have small lateral flat surface areas to produce squish and dual spark plugs with a coil over plug ignition. Other features are hydraulic lifters with roller followers, double rocker shafts in each head with 1.65:1 ratio cast steel rockers, sequential multi-port returnless fuel injection, tuned-runner nylon plastic intake manifold, electronically controlled 80mm bore single-blade throttle body using a DC motor with reduction gears to open and close the throttle. The con-rods center to center length is 6.25 inches for a largely sufficient 1.75 rod length/ stroke ratio.

A compression ratio of 9.6:1 makes for unleaded 89 octane but the engine can also accept 87 octane gasoline, probably with slight power and efficiency loss. Its advertised net power is 345 hp (257 kW) at 5400 rpm with a maximum torque of 375 lb-ft (508 Nm) at 4200 rpm.

Figure 8

The advantage of overhead camshafts emerges at high rpm, due to reduced valvetrain inertia and deflection. But soaring revs are nowadays of no use on a street engine, so that a single camshaft-in-block is absolutely adequate, as are two valves per cylinder only. The hemispheric combustion chamber favorable surface to volume ratio and breathing ability can be exploited without overhead camshafts cost and complexity. Moreover, servicing and repairs are easier and less costly.

Apart of aircraft engines where dual ignition is mandatory for safety reasons, twin spark ignition was largely used on racing engines of the fifties and sixties, especially those of Maserati. Porsche and Alfa-Romeo fitted it also to some production engines such as the GTA and 2 liters "Twin Spark". It allows a faster burn rate, particularly on over-square engines and leaves more space for large valves than a centrally located single spark plug. Furthermore, with a domed piston and a single plug, flame front shrouding may lower the burn rates; this problem is avoided with dual spark ignition.

(© François Dovat)

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