The Pluses and Minuses of a Hemi Engine

The Pluses and Minuses of a Hemi Engine by Westech Performance Group

At the very least, among those who are mechanically inclined, the term “hemi” conveys a lot of connotations. Having a home is something to be proud of; not having one may cause you to blush when the four-letter word is spoken. Even if the term “hemi” has a lot of force, emotions and fundamental knowledge might be at odds. To cut through the hype, we’ve compiled a list of the home’s advantages and disadvantages so you can make an informed decision about whether owning one of these powerful engines is something you aspire to.

What Is the Meaning of “Hemi”?

“Hemi” is short for “hemispherical,” and the term “hemi” refers to a particular type of engine with a combustion chamber that is formed in this way. As in geography, north and south relate to the Earth’s two hemispheres, which the Equator separates. A genuine “Hemi” engine is less than half of a sphere from the get-go, even in its purest form; it turns out to be significantly less than half. Spiritually sectioned combustion chamber designs, such as those used in early Chrysler hemis (such as the 1951 model), are the closest to being wholly hemispherical and are technically known as “semichemi” designs.

A “Hemi” Combustion Chamber: What’s the Point?

One would assume that the purpose of a combustion chamber is to confine the pressure and expand- a result of the burning process- to drive the piston down into the bore. The spark plug is likewise located in the combustion chamber for gasoline engines.

The popularity of different combustion chamber shapes has fluctuated throughout time to enhance power, efficiency, and emissions. Because of the shifting goals of cost, mass, packaging, and performance, original equipment makers have operated in this fashion. It is advantageous in terms of performance to have a hemispherical combustion chamber, but it has drawbacks in other areas. This is where we’ll go over the pros and downsides.

Hemi chambers are of relevance because of their semisphere form, which results in a faster increase of combustion chamber pressure than a standard overhead valve with a wedge-shaped section. For an efficient and consistent pressure peak, the ignition source must be located close to the chamber’s centre and the reflected pressure wave originating from the chamber’s walls. An engine’s power stroke is more efficient when it can build up pressure sooner in the stroke because of its mechanical advantage over a crankshaft that is farther along.

Increasing The Area of the Valves

Hemi’s next benefit is considerably more appealing to gearheads who prefer the concept of a higher cylinder pressure sooner in the combustion cycle. Valve placement in the combustion chamber is dictated by the domed form of the room’s ceiling, which forces valves to be slanted. A hemi chamber is frequently referred to as a “pentroof” because the intake and exhaust valve sizes can be more significant than the diameter of the cylinder bore, allowing for more efficient flow into and out of the chamber.

Valve Unshrouding

More air and fuel may enter the vehicle through a larger-diameter valve, allowing more exhaust to depart. In theory, we could all have engines with larger valves if it were feasible to increase the valve diameter. However, the valves would smash into the cylinder wall and, in many cases, each other. Unshrouding is a term used to describe the impact of the hemi shifting the windows sideways to open away from the cylinder wall.

Pushing gas through a closed valve with low lift values is challenging due to the narrow open curtain region under the valve near the seat. To alleviate the shrouding limitation, which a Hemi effectively clears, the valve must be moved closer to the cylinder bore near the center. The flow increases when the valve is fully opened until there is no more lift to add to the total.

A Hemi’s Negatives

Hammerhead Performance cylinder heads on a Ford small block Hemi engine. With hemi cylinder heads, even the most miniature engines appear absurdly huge.

So far, but good for the Hemi combustion chamber, so why didn’t every manufacturer employ it in bulk?? The size and weight of the vehicle may have been Chrysler’s most significant challenge at first. To eliminate valvetrain interference, arrange the valves at hemi angles, modify the port forms, and create a stable valvetrain, a lot of real estates is required, resulting in bigger and heavier Hemis. On the other hand, a 426ci Chrysler wedge from the ’60s has greater weight and width than a Gen II Street Hemi from the muscle car period.


Oxides of nitrogen—gases formed (ironically) due to the Hemi’s inherent efficiency—are a problem for Hemi engines of all sorts. The current Gen-III Chrysler Hemi would be unusable in a production vehicle without additional measures to reduce these emissions.

Having a quench pad, which squishes the air/fuel mixture into the open area of the combustion chamber as soon as the piston reaches the top dead centre (TDC), is one of the benefits of a wedge-shaped combustion chamber design. Cooling the burning charge reduces combustion temperatures and the generation of nitrogen oxides. As a result, elevated levels of NOX (shorthand for nitrogen oxides) are produced. Hemi’s lousy side: recall when we stated it encourages faster pressure rise time because of its hemi shape?

Ratios of Hemi Compression

Because of the hemispherical form of the combustion chamber, no matter what brand of valve cover you choose, it’s impossible to boost the compression ratio high enough to take full benefit of its better design. Cutting the fire deck, grinding the head deck, or installing a higher-compression piston are the most common methods for increasing compression ratios.

Different pistons, domes, coatings, and flame slits have been tried and tested over the last half-century to achieve the promised performance gains from the home’s shape. However, increasing compression with a domed or pop-up piston has a practical limit and a disadvantage in a hemispherical combustion chamber. Reducing the combustion volume by altering the piston’s shape also changes the rate and timing of the combustion event. Keeping your expectations low pays off in the long run.