Digging deep in search of hemispherical combustion chambers.
With the possible exception of FORD, HEMI is the most celebrated set of four letters in all automobiledom. At the mere mention of the word, teens drool, musclecar mavens bow, and mothers draw small children under protective arms. The actor Jon Reep launched his career by uttering, "That thing got a Hemi?" in recent Dodge Ram television ads.
After three decades of retirement, the return of the Hemi engine ranks with the capture of Sadam Hussein as righteous proof of America's unyielding spirit. At least that's what I, as a member of the 1960s Hemi-owning and admiring generation, thought upon hearing that Chrysler was resuscitating its Hemi. But, upon close inspection of the new engine's innards, I was disappointed to find that the vaunted hemispherical combustion chambers are NOT part of the deal.
The HEMI name comes from the dome-shaped ceiling atop each cylinder inside some engines, a configuration that has seen wide aircraft, motorcycle, and racing use since it was introduced by obscure Belgian car maker Pipe in 1905. The key benefit is that the hemispherical dome facilitates extra large intake and exhaust valves, a relatively uninhibited pathway for fuel and air into the cylinder, and an easy means for spent exhaust gasses to escape. The easier it is for an engine to pump its working fluids, the more power it produces.
Chrysler first experimented with Hemi designs for World War II aircraft engine use. The fruit of that research appeared under the hoods of 1951 Chrysler Saratoga and New Yorker sedans. Chrysler's first hemi-headed V-8s produced 180 horsepower versus Cadillac's 160 hp and Oldsmobile's 135 hp, lighting the fuse on the original horsepower war.
DeSoto and Dodge picked up variations on the hemi theme for 1952-58 models. In 1964, when Chrysler needed a hot engine for stock car and drag racing, a second generation V-8 was introduced and a version informally called street hemi was offered to civilian Dodge and Plymouth buyers. My 1968 Hemi Road Runner generated a mighty 425 horsepower and ran over 110 mph in the quarter mile.
To find out how Chrysler's new 5.7-liter V-8 could be labeled Hemi in spite of the fact its combustion chambers look nothing like sliced grapefruit, I met with Bob Lee, Chrysler's vice president of powertrain product engineering.
Lee assured me no deception is afoot. The engineers responsible for Chrysler's new engine had better intentions than merely dusting off Hemis in the back of their closet. Early in the research phase, they discovered a combustion chamber that Porsche used for 1965-97 air-cooled 911s offered the ideal starting point for their new design. Porsche's head happened to be a hemi.
Engineers are not about to leave well enough alone so little from the Porsche design made the long trip to what's now called the 5.7-liter Hemi Magnum V-8. Much of the bowl area of the combustion chamber was filled in to encourage the fuel-air mixture to stir itself prior to ignition. The chamber is shallower than past Hemis so the top of the piston can be nearly flat, thereby saving reciprocating weight. The one vestige from Porsche's classic design is large valves rotated 90 degrees from the most common orientation (in a plane parallel to the crankshaft), and splayed at a 34.5-degree angle in the interests of free breathing.
The natural course of engineering evolution has moved Chrysler's hero powerplant far from its Porsche roots and severed all ties to the Hemis of yore. Even though it isn't a genuine hemi, this engine still packs a powerful punch as a marketing ploy.
Half of an original Hemi cylinder head - note the dome-shaped chamber and relatively great depth.
5.7-liter Hemi Magnum V-8 cylinder head - nothing hemispherical about it. Note the irregular shape.
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