Chrysler HEMI(r) Engine Facts

Everything old is new again. Hemispherical combustion chambers date to the early years of internal combustion, and provide excellent thermal efficiency and power production, especially in a relatively long-stroke, low-revving engine. They also work well with supercharging. The combustion chamber is that space between the piston and the cylinder head in which combustion takes place. Most early racing engines were, by today's standards, long-stroke and low-revving, and many were supercharged. Hemispherical combustion chambers were commonly used in racing engines built up to the mid-1960s, when the success of the Ford-Cosworth DFV Formula One engine led to a resurgence of four-valve-per-cylinder designs with narrow valve angles incompatible with hemispherical combustion chambers. By then, too, successful racing engines revved up to 10,000 rpm and the large chamber volume of a hemispherical chamber would have caused serious gas-flow and hence power production problems. (Parenthetically, four-valve heads were nothing new, either - the seminal 1913 Peugeot Grand Prix racers had engines with dual overhead cams and four valves per cylinder. This was arguably the most influential engine, ever.)

Chrysler first built an engine with hemispherical combustion chambers in the World War II era, for fighter aircraft. In 1951, Chrysler joined the postwar horsepower race with its first ``hemi'' V8 engine, and the design became associated with to the company, at least in the public eye. The Chrysler Hemis were expensive to build, and to buy, and low sales led to discontinuation after 1959. (Note here that ``hemi,'' lower-case is generic, ``HEMI'' all uppercase is a Chrysler trademark and applies to the current engine).

But performance made a comeback in the 1960s, and Chrysler needed an edge on the competition in stock car racing. And so the now-legendary 426 cubic-inch Hemi was born. It won its first race, the 1964 Daytona 500, convincingly - Richard Petty lapped the entire field. But it wasn't quite a real production engine, and so was banned - until 1966, when it became available in street form to anyone with the extra cash to buy one. And it took plenty - the Hemi was a $600 to $800 option in cars that cost $2,500 to $4,000. In the late 1960s, Hemis were nearly invincible in stock car and drag racing. But after 1971, the Hemi was finished, at least as a street engine, the victim of emissions requirements that were far beyond its design specification. It lived on in drag racing, where it and its derivatives dominated the top classes to the present day. As mentioned, hemispherical combustion chambers are amenable to supercharging - and Top Fuel drag-race engines are supercharged to enormous pressure, enough to make something like 5,000 horsepower from 500 cubic inches. (Note that they don't do this for very long!)

Chrysler brought the HEMI(r) back in 2003, with the debut of the newest generation of Ram pickups. Now it's in the Chrysler 300C and Dodge Magnum, and soon will be found in the 2005 Jeep Grand Cherokee.

Like its illustrious predecessors, the new HEMI's two valves per cylinder are operated by pushrods. As in previous Hemis, intake and exhaust valves for each bank of cylinders are at an angle in the head and have their own rockers. In this era, when dual overhead cams and four-valve heads are found in economy cars, why did Chrysler go for such a seemingly atavistic design? All things considered, even though it's more complex than a typical pushrod engine with its two valves per cylinder in line with the crankshaft and using one set of rockers, it's simpler and more cost-effective than an overhead-cam engine. It produces equivalent power to an overhead cam engine, but costs less to build. Each cylinder in a 5.7-liter V8 is large enough and the revs are low enough that the two-valve hemi design works very well. Hemis can run well on lower octane fuel than some other designs, and Chrysler's newest is happy with less-expensive mid-range gasoline. And undoubtedly tradition played some part in the renaissance of the HEMI.

Don't think that the new HEMI is lacking in contemporary technology. It's the first variable-displacement engine to make it to production. The ``Multi-Displacement System'' (MDS) is standard on all HEMIs in 2005 Chrysler 300Cs, Dodge Magnums, and Jeep Grand Cherokees. Put most simply, the MDS works by deactivating the lifters in four cylinders, keeping their valves closed. No fuel is injected at that time, increasing fuel economy, and no air is pumped, decreasing energy and therefore power that would be lost pumping air. Use of electronic throttle control, electronic fuel injection, sophisticated electronically-controlled hydraulic lifters, and control algorithms makes it all work quickly and seamlessly.

It takes only 40 milliseconds (0.040 seconds) to switch between four-cylinder and eight-cylinder modes, and the process can't be felt. Serendipitously, during the week in which I had a Chrysler 300C as a test car, I attended a demonstration of the MDS system. I drove a 300C in which Chrysler engineers had attached instrumentation on a short test loop that included city streets and freeway driving, with full-throttle acceleration needed for freeway merging. I never noticed any change in the engine's response or power - there was never any lack of power - and was surprised when I saw the graph of operation after the data was downloaded to a laptop. The HEMI switched between modes constantly, not just occasionally. Chrysler estimates at ten to twenty percent increase in fuel economy, and indeed, I found the 300C to be most efficient for a car of its size and power.

And if 340 horsepower isn't enough, Chrysler has just announced the 300C SRT-8, to be available sometime next spring. Its HEMI has been enlarged to 6.1 liters and compression increased, to make 425 horsepower and 420 lb-ft of torque. A sport-tuned suspension and Brembo(r) four-piston brake calipers will be hidden under slightly-modified bodywork. With a 0-60 time in the low 5-second range and a quarter-mile time in the high 13s, this looks to be the hottest HEMI since the glory days of the muscle car era, and a far more refined car.