In this week’s technical article I’ll describe the Hemi engine and in particular Chrysler’s design that has been in production for over 50 years. I’ll also cover some of the other manufacturers that have used this type of motor.
The hemi engine has a hemispherical combustion chamber at the top of the cylinder. This allows the engine to produce more efficient power by firstly having larger valves with straighter pipes for the input and output plus the shape of the chamber aids the combustion of the fuel/air mix. With a hemi, the spark plug is typically in the middle of the cylinder, which means that the spark has a greater chance of igniting more of the mix. These engines have been around for over a hundred years and even earlier than that, equipment that relied on ignition such as armaments used a circular combustion chamber.
Most engines have a near flat piston and cylinder head with a small recess to allow the valves to do their job without hitting the piston – although saying that, I have twice experienced a valve spring failure that resulted firstly in a hole in the piston and no more valve and secondly a bent valve stem and a gouging of the piston. So, because the hemi engine has a hemispherical cylinder head it needs something that fits to compress the mixture. Therefore to match, the piston has a domed head.
Now let’s talk specifically about the Chrysler Hemi engine. Chrysler appears to be the main company that made a noise about their engine – mostly a marketing message to try and differentiate their cars from General Motors and Ford. They have built 3 generations of hemi engines – and have even trademarked the “Hemi” moniker in relation to engines.
The first of the hemi’s was released in 1951 and was in production for seven years. It was called the Chrysler FirePower and was their first V8 and went into the Imperial, New Yorker and Saratoga models. It was a 5.4 litre or 331 cubic inch motor making 180hp. Later in its life there were 5.8 and 6.4 litre versions too.
Other Chrysler brands used this motor – Dodge called it the Red Ram and De Soto called it the FireDome. The FireDome came in versions starting at 4.6 litres and ending with 5.7 litres and 345hp. These motors used the same block and head but had different bore and stroke measurements. The Dodge motors were the smallest ranging from 3.9 to 5.3 litres. This first generation was very popular with drag racers because of the power outputs and Cunningham used them successfully in their 1950s Le Mans sports racers.
In 1964, the second generation was released and the first to be trademarked as the “Hemi”. It was a low volume motor for road cars but was a huge success in NASCAR and called the Elephant due to its size – both physically and its displacement of 426 cubic inches or 7 litres. This motor produced extra power due to the design of the head, which meant even larger valves than normal – thus more fuel/air mix could be sucked in and ignited. The street versions were rated at 425hp however the drag racers slapped on a Rootes Supercharger for more mumbo.
For consumer cars it was only sold for about 6 years and it went into the Dodge Coronet, Charger, Challenger, Super Bee and Dart, plus the Plymouth GTX, Barracuda, Superbird and RoadRunner models. It even went into a couple of prototypes from Monteverdi (a low volume Swiss manufacturer of the 1960s and 1970s).
Generation 3 of the Hemi appeared in 2003 as a 5.7 litre motor ranging in power from 357 to 400 horses depending on application in the Chrysler 300C, Dodge Ram and Jeep Grand Cherokee models amongst others. It replaced the Magnum V8 and the 8 litre V10 in the Dodge Ram. The motor is also available as a 6.1 litre in the SRT models with 425 horses and was available for a while as an aftermarket 6.4 litre. The SRT models have more technology than the base version to provide more reliable power delivery.
Other manufacturers to build hemi engines include BMW (later “borrowed” by Bristol), Ford, Aston Martin and Jaguar, although long before they were stablemates – Aston’s late 1960s V8 and the Jaguar XK engine were great examples of very successful hemi’s. Even Porsche’s flat six that went into the 911 for over 30 years used a hemi design.
Today there has been a resurgence in muscle cars – with Dodge at the forefront. They have produced the Charger Super Bee and Charger Scat Package with the 5.7 litre hemi motor as a base. The Dodge Charger SRT Hellcat uses a 6.2 litre shared with the Challenger Hellcat and Challenger SRT Demon. On the other side, Jeep use the 6.4 litre hemi in the Grand Cherokee SRT and a supercharged 6.2 litre in the Grand Cherokee Trackhawk.
The question now is what will happen with this motor as new regulations come in to ban petrol engines – will it evolve into a clean biofuel powerplant? One hopes that sense prevails and governments around the world rethink the blanket bans that they are desiring and FIAT Chrysler have the opportunity to develop an evolution.