For years, Chrysler has been talking about the Hemi engine and how it is superior to other engines. Yet, what is it? What benefits does it provide?
The Hemi engine has been offered by Chrysler since the 1960s and is in its third form. Chrysler liked the engine so much that they have trademarked the “Hemi” name. It is currently a big selling feature throughout their marketing materials and is a big discussion point among auto enthusiasts.
Chrysler’s Hemi History
First introduced in the 1950s, Chrysler called it the FirePower engine (1951-58). This was quickly followed with the 426 Hemi (1964-71) which really put it on the market. The 426 Hemi was famously used by Richard Petty and helped launch his career.
As the story goes, the 1964 Daytona 500 had the first three cars with the 426 Hemi in it. Petty was so dominant throughout that season that NASCAR changed their rules. The new rule stated you couldn’t run an engine without offering a street version of it. With no street version available, the 426 Hemi was sidelined during the 1965 season.
The “new HEMI” was introduced in the early 2000s. This new HEMI has a combustion chamber featuring value and twin spark plug locations markedly different from the previous 426ci version. It also has a pinched chamber that does not have the true hemispherical combustion chambers. Many say it is more closely resembles the mid-1950s polyspherical chamber that Chrysler developed as a low-cost alternative head for their V-8 engine. The polyspherical head needs less metal and is narrower due to using only one rocker shaft. Ultimately, this saved Chrysler material, space, warranty claims and allowed it to be used in smaller vehicles.
Ultimately, this new Hemi is really popular. The take rate (the average number of people who buy it versus another Chrysler engine) hovers around 50 percent and it is built at a profit. For comparison, Ford’s EcoBoost engine has around the same take rate.
What is a Hemispherical Engine?
The classic idea for a hemispherical engine is for the top of the combustion chamber to be a hemisphere. This would minimize heat loss per unit of volume. However, this gives a far too low compression ratio unless you have a very long stroke. Normally, the piston is domed so that it protrudes into the head at top dead centre. This gives it a more of a chamber shape. This design hasn’t been used in production vehicles for several decades.
Modern Hemi engines have flatter heads and are vastly more complex than the first models. The chambers are no longer truly hemispherical. They do use a coil-on-plug distributorless ignition system and two spark plugs per cylinder to shorten flame travel leading. This turns into a more consistent combustion and helps reduce emissions.
In 2009, Chrysler made various revisions to the 5.7L Hemi. They introduced a Variable Camshaft Timing (VCT) system. This is essentially variable valve timing that uses an oil control valve which controls oil flow to a unique camshaft sprocket that contains a phasing device. This device either advances or retards the camshaft timing depending on the operation of the oil control valve.
Also, the cylinder heads have been revised to increase flow. And for the first time, the intake manifold is now model specific. Notably, Ram trucks and non-Hybrid Electric Vehicle Chrysler Aspens and Dodge Durango’s use an active intake manifold with a short runner valve to optimize torque and horsepower. Basically at low RPMs, the intake valve is closed, resulting in better low-end torque. The valve is opened at high RPMs to improve horsepower. Chrysler’s passenger cars, Jeep vehicles and HEV models of the above don’t use that manifold and don’t have a short runner valve.
Lastly, Chrysler introduced “Multi Displacement System” to the larger V-8 Hemi engine. This system essentially is cylinder deactivation. When the computer recognizes that not all cylinders are needed, it will shut them down to conserve fuel. The modern engine can shutdown a cylinder in .04 seconds and this process is unnoticed by the driver. It has also been proven to be highly reliable and since its introduction in 2005, there have been zero reported issues.
All of these changes resulted in better reliability and gas mileage with more horsepower and torque.
Pros and Cons
With the many advantages of the Hemi engine, it would seem like everyone should use it. Yet, that isn’t the case. Why? It has several disadvantages.
First, let’s talk about the many advantages. The Hemi engine, by design, does a better job of burning all the fuel in the cylinder. Also, because of its design versus a flat head, the surface area is smaller and that means less heat escapes, plus peak pressure can be higher.
Also advantageous, is the size of the valves. With the Hemi engine placing the valves at the opposite sides of the head, there is more room for valves. This was better than the previous wedge-shaped engines with their combustion chambers in line with each other. The inline arrangement limits valve size. With the Hemi, the valves can be large and this leads to improved airflow throughout the engine.
The cons? Simply put, other engine designs have gotten better over time. While the valve size is an advantage, it is limited to having four valves per cylinder. This truth is that four slightly smaller valves per cylinder let the engine breathe easier than two large valves. Many new engines have a pentroof design to accommodate for more valve.
Also, working against the Hemi, is the current desire to have smaller combustion chambers. The smaller chamber reduces heat loss during combustion and also helps shorten the rod travel time.
While these other advancements have their pros and cons as well, the Hemi engine is renowned for its reliability and impressive power output through its hemispherical design. While the new Hemi engines don’t have the same hemispherical shape that its predecessors had, it still relies on that shape to deliver its power.