Hemi Engine Pros and Cons
More than you’d ever think you can learn about the roots of Chrysler’s 5.7, 6.4L, and 6.2L modern Hemi.
In a sea of LS swaps, the name "hemi" carries a lot of emotional baggage for those retro-minded enthusiasts among us. If you've got a hemi, your chest might well puff up with pride, and if you don't, you might see red when the four-letter word is uttered. As powerful as the word "hemi" is, emotions and actual information can run contrary to one another. To cut through the static, we'll lay out the pros and cons of the hemi here, so you can decide for yourself if one of these lofty engines should or should not be your automotive goal in life.
What Does "Hemi" Mean?
The word "hemi" is a shortening of the word "hemispherical," and as it relates to engines, means a type of engine that has a hemispherically shaped combustion chamber. It's the same thing that is meant in geography when referencing the Earth, which is split into two hemispheres, north and south. Unfortunately, right from the get-go, even the purest form of "hemi" engine really isn't a true half sphere, but something less than half—a lot less, it turns out. Technically a "semi-hemi" design, these spherically sectioned combustion chambers—such as found on Chrysler's original hemi that debuted in 1951—are the closest to being truly hemispherical.
Why Give an Engine a "Hemi" Combustion Chamber?
The job of a combustion chamber sounds simple: to contain the expansion and pressure of a combustion event so that the pressure may act on the piston to push it down in the bore. Moreover, the combustion chamber also contains the spark plug—the source of ignition for gasoline engines.
In an ongoing effort to improve power, efficiency, and emissions, the favor of different combustion chamber shapes has waxed and waned over the years. Original equipment manufacturers have behaved this way because the priorities of cost, mass, packaging, and performance have jockeyed for importance from year to year. Particularly with regards to performance, a hemispherical combustion chamber has the advantage over other shapes, while costing it in other areas. These are the pros and cons we'll be going over here.
The hemi chamber's characteristic semi-sphere shape is of particular interest, and produces a more rapid rise in combustion chamber pressure relative to a typical overhead valve design with a wedge-shaped chamber. Provided the ignition source is near the chamber's center, the reflected pressure wave coming off the chamber's walls will result in a faster, more coherent pressure peak. The act of burning all the fuel in a shorter window of time versus in a longer period of time means that the buildup of pressure within the chamber occurs earlier in the power stroke where it has more efficacy due to its mechanical advantage on the crankshaft.
Increasing the Valve Area
If the idea of higher cylinder pressure acting earlier in the combustion cycle appeals to your gearhead senses, you'll like the hemi's next advantage even more. By making the combustion chamber a domed shape, the valves are necessarily tilted so that their circumferences intersect the sphere shape of the chamber roof. By tilting the valve over on its side (why a hemi chamber is sometimes called a "pentroof"), the sum of the diameters of the intake and exhaust valves can be greater than the diameter of the cylinder bore, and the flow into and out of the chamber is impeded less by the cylinder wall.
Unshrouding the Valves
All other things being equal, a larger-diameter valve means more air and fuel can get in and more exhaust can exit. If it were possible to just increase the valve diameter by itself, we'd all have engines with bigger valves—except the valves would crash into the cylinder wall, and in many instances, each other as well. The hemi fixes this by tipping the valves sideways so they open away from the cylinder wall—an effect called unshrouding.
As a valve opens and gas flows past it, flow is limited at low lift values because the open curtain area under the valve is relatively small near the seat. As the valve opens, flow increases up to a point where no additional lift contributes to the overall flow. At or near the seat, however, the valve's proximity to the cylinder bore inhibits its flow, so anything that is done to mitigate the shrouding restriction helps a bunch, and that's what a hemi does really well.
The Downside of a Hemi
So far, everything looks rosy for the hemi combustion chamber, so why didn't every manufacturer use it in quantity? Perhaps the biggest problem initially for Chrysler—the manufacturer known for embracing it the most—was its size and mass. If you do everything that's needed to place the valves at hemi angles, to redesign the port shapes to avoid interference with the valvetrain, and design a workable valvetrain that lasts, you use up a lot of real estate, which means Hemis are larger, and as a result, heavier. By way of illustration, a Gen-II Street Hemi from the muscle car era weighs almost a hundred pounds more than a same-sized 426ci big-block Chrysler wedge, and it's several inches wider.
Beyond the issues of size and mass, hemi engines of all descriptions suffer from elevated emissions of oxides of nitrogen—gases that are created (ironically) through the Hemi's inherent efficiency. Without extra steps to mitigate these emissions, today's Gen-III Hemi would be an impossibility for a production car or truck.
One of the advantages of a wedge-shaped combustion chamber is that its design allows for a quench pad, which squishes the air/fuel mixture into the open portion of the combustion chamber just as the piston completes its upward trajectory at TDC. This has a cooling effect on the burning charge, reducing combustion temperatures and the formation of oxides of nitrogen. This is the evil side of the hemi coin. Remember how we said the hemi shape promotes a faster pressure rise time? Elevated NOX (shorthand for oxides of nitrogen) are a byproduct of that.
Hemi Compression Ratios
Due to the shape of the hemispherical combustion chamber, it's difficult to raise the compression ratio high enough to fully take advantage of its superior shape, irrespective of the brand on the valve cover. Raising the compression ratio usually involves machining the block's fire deck, milling the cylinder head deck, substituting a higher-compression piston, or a combination of these.
With a hemispherical combustion chamber, however, raising compression with a domed, or pop-up piston has a practical limit, and a down side. Changing the shape of the piston so that the combustion volume is reduced also has the effect of disrupting the speed and timing of the combustion event itself. Over the past half century, the road to the perfect hemi is littered with odd pistons, domes, coatings, and flame slots—all designed to restore the missing performance promised by the hemi's shape. For this reason, it pays to stay on the modest side when raising the compression of a hemi-headed engine on a naturally aspirated combo. To the hemi's benefit, this is also the reason why hemi-headed engines are so well adapted to boosted applications, such as Top Fuel and Pro Mod racing—this is the environment where the hemi chamber really shines.
How Many Kinds of Hemis Are There?
So far, we've talked about the hemi-headed engine in generic terms without talking about the different variants. Besides the obvious line of Chrysler Gen-I and Gen-II Hemis (for the differences between early Chrysler, Dodge, and DeSoto Hemis, read Steve Magnante's fascinating story here), there were also Ford Hemis, and even some from GM, though none made production. (You can check out Oldsmobile's four-valve hemi engine here.) Both the FE-based 427ci Single Overhead Cam (SOHC) and the 385-Series big-block "Blue Crescent" Boss 429 Ford big-blocks were Hemi-headed designs with winning histories, and Chevrolet experimented with a 302ci small-block Hemi with Smokey Yunick for Trans Am racing. Likewise, Nick Arias, Jr. made history (among other accomplishments) by designing race-winning big-block Chevy heads with hemispherical combustion chambers. In more recent years, Hammerhead Performance has developed hemi cylinder heads for small-block Fords and even the Chevrolet LS. Arias also developed a small-block Ford hemi head that works well.
And yet, one "hemi" cylinder head—the Chrysler Gen-III Hemi—is not a hemispherical cylinder head in the classic sense. Its combustion chamber is actually taken from a section of a flattened sphere, and that means it more correctly would be called a semi-oblate spheroid. If those words don't register, think of a semi-oblate spheroid as an ellipse that has been rotated around one of its two axes. Cool geometry trivia: A sphere is a special case of an oblate spheroid, the only difference being that it looks the same when the ellipse (in this case, a circle) is rotated along either of its two axes.
Chrysler's Gen-III Hemi
If the 2003-present Gen-III Hemi isn't a real hemi engine, then why do we call it that? With good reason, Chrysler (now FCA) wanted to extend the "Hemi" branding to its third-generation Hemi V-8 because of the instant recognition and status it carries. In this case, we can give FCA a pass because a flattened sphere (a semi-oblate spheroid) has all the positive traits of a real hemi (valve size, port flow, flow quality, burn time/efficiency) with far fewer of its problems. And like all hemis, FCA's third-generation unit responds in classic fashion to the application of boost in the Jeep Trackhawk, Hellcat, and Demon Hemi variants—currently the most potent domestic production car engines ever.
Combustion efficiency of the Gen-III Hemi is excellent, with its shape enhancing the combustion environment with a fast rise time just like a "real" hemi. It does this through the same mechanism as a hemi—through a surface shaped to focus the expanding burn front into the shortest practical event. Unlike a true "hemi," the Gen-III has quench pads on the side to aid squish and help lower the NOX emissions, while dual plugs on a waste-spark ignition reduce the remaining NOX to EPA-approved levels. So for those on a Hemi-hating binge, you're on the hook for calling it a "semi-oblate spheroid!"
What's in a Name?
This is the kind of thing that keeps editors, lawyers, ad agencies, and historians up at night: There are at least three ways to spell "hemi," and they mean different things to different people. Here's the lowdown. Within this story and others, you'll see the word "hemi" and "Hemi" used almost interchangeably, but they're different. The word "hemi" refers generically to any engine with a hemispherical combustion chamber. When you see the word "Hemi" with a capital "H," it refers specifically to engines manufactured by Chrysler, FCA, or one of its divisions (past and present). When you see the word "HEMI" with all caps, it's going to be in Dodge, FCA, Mopar, Ram, or Chrysler promotional material (often with a trademark registration), because, well, if you designed it, wouldn't you leave the cap-lock button on, too?