HEMI History Lesson

[LadyNilla]

So there I was online and I found myself on a site that described in great length about the HEMI engine. For many of you enthusiants this is common knowledge, but since I learned something I thought it would be worth sharing:

If you like cars, then you have probably heard of the HEMI engine. If you were born in the 1960s or before, you remember the phenomenon created by Chrysler's HEMI engines in the 1950s, '60s and '70s. If you follow muscle cars or drag racing, you know that the 426 HEMI engine is a popular engine because of its performance. You've probably also heard of the HEMI engines that Chrysler began using in 2003 Dodge trucks.


Photo courtesy DaimlerChrysler
2003 Dodge Ram with 5.7-liter HEMI Magnum V-8

But even if you know little or nothing about cars and engines, the word "HEMI" might still mean something to you. The word has become a synonym for big, powerful engines.

Photo courtesy DaimlerChrysler
5.7-liter HEMI Magnum V-8 engine

In this article, you'll learn about the HEMI engine and find out what make engines using the HEMI design such awesome machines.

The HEMI engine for automobiles was born in 1948 -- Harry Westlake and several others developed a Hemi 6-cylinder engine for Jaguar. A few years later, in 1951, Chrysler introduced a 180-horsepower HEMI V-8 engine on several models. The Chrysler HEMI engine had a displacement of 331 cubic inches (5.4 liters), so it is known as the "331 HEMI."
These days, 180 horsepower sounds like nothing. But in 1951, 180 horsepower was unheard of. It was an amazing amount of power for the day, and it fueled the "HEMI legend."

Photo courtesy DaimlerChrysler
Dual Ghia powered by a 392 HEMI

Chrysler continued improving the HEMI design, releasing a 354-cubic-inch design in 1956, a 392 cubic-inch design in 1957, and ultimately a 426-cubic-inch (7-liter) version in 1964. The 426 engine set the HEMI legend in stone when it won first, second and third place in the 1964 Daytona 500 NASCAR race. The 426 street HEMI came out in 1965, producing 425 horsepower.
The 426 block and heads are still available today from Dodge. The 426 HEMI is a popular power plant for drag racing, funny cars and muscle cars. Click here to see a picture of a 1966 Dodge HEMI Charger engine.
The thing that allowed the 1951 Chrysler HEMI to produce so much more power than other engines of the day was the efficiency of the combustion chamber.


In a HEMI engine, the top of the combustion chamber is hemispherical, as seen in the image above. The combustion area in the head is shaped like half of a sphere. An engine like this is said to have "hemispherical heads." In a HEMI head, the spark plug is normally located at the top of the combustion chamber, and the valves open on opposite sides of the combustion chamber.
Most cars prior to the 1950s used what was known as a flat head, and many lawn mower engines still use the flathead design today because it is less expensive to manufacture. In a flathead engine, the valves are in the block, rather than in the head, and they open in a chamber beside the piston.


The head in a flathead engine is extremely simple -- it is a solid metal casting with a hole drilled in it to accept the spark plug. The camshaft in the block pushes directly on the valve stems to open the valves, eliminating the need for pushrods and rocker arms. Everything is simpler in the flathead design. The problem with a flathead engine is its thermal efficiency, which we'll discuss next.

There are many different parts of an engine's design that control the amount of power you can extract from each combustion stroke. For example:

• You want to burn all of the gas in the cylinder. If the design leaves any of the gas unburned, that is untapped energy.
• You want the maximum cylinder pressure to occur when the crankshaft is at the right angle, so that you extract all of the energy from the pressure.
• You want to waste as little of the engine's energy as possible sucking air and fuel into the combustion chamber and pushing exhaust out.
• You want to lose as little heat as possible to the heads and the cylinder walls. Heat is one of the things creating pressure in the cylinder, so lost heat means lower peak pressures.

The last item in the list is one of the key advantages of the HEMI head versus the flathead engine. Surface area causes heat loss. Fuel that is near the head walls may be so cool that it does not burn efficiently. With a flat head, the amount of surface area relative to volume of the combustion chamber is large. In a HEMI engine, the surface area is much smaller than in a flat head, so less heat escapes and peak pressure can be higher.
Another factor with a HEMI head is the size of the valves. Since the valves are on opposite sides of the head, there is more room for valves. The engine design that preceded the HEMI was a wedge-shaped combustion chamber with the valves in line with each other. The inline arrangement limited valve size. In a HEMI engine, valves can be large so the airflow through the engine is improved.
The Dodge HEMI Magnum
The Dodge HEMI engine builds off the tradition of HEMI power to deliver a 345 cubic inch (5.7 liter) V-8 engine with hemispherical heads.

Photo courtesy Daimler Chrysler
5.7-liter HEMI Magnum V-8 engine from the 2003 Dodge Ram

The engine produces 345 horsepower, and compares very favorably with other gasoline engines in its class. For example [ref]:

• Dodge 5.7 liter V-8 - 345 hp @ 5400 rpm
• Ford 5.4 liter V-8 - 260 hp @ 4500 rpm
• GMC 6.0 liter V-8 - 300 hp @ 4400 rpm
• GMC 8.1 liter V-8 - 340 hp @ 4200 rpm
• Dodge 8.0 liter V-10 - 305 hp @4000 rpm
• Ford 6.8 liter V-10 - 310 hp @ 4250 rpm

The HEMI Magnum engine has two valves per cylinder as well as two spark plugs per cylinder. The two spark plugs help to solve the emission problems that plagued Chrysler's earlier HEMI engines. The two plugs initiate two flame fronts and guarantee complete combustion.
Disadvantages
If HEMI engines have all these advantages, why aren't all engines using hemispherical heads? It's because there are even better configurations available today.


One thing that a hemispherical head will never have is four valves per cylinder. The valve angles would be so crazy that the head would be nearly impossible to design. Having only two valves per cylinder is not an issue in drag racing or NASCAR because racing engines are limited to two valves per cylinder in these categories. But on the street, four slightly smaller valves let an engine breathe easier than two large valves. Modern engines use a pentroof design to accommodate four valves. Another reason most high-performance engines no longer use a HEMI design is the desire to create a smaller combustion chamber. Small chambers further reduce the heat lost during combustion, and also shorten the distance the flame front must travel during combustion. The compact pentroof design is helpful here, as well.

[ChargerFever]

Thank you for the lesson!!

[cadzilla74]

Excellent LadyNilla, I have seen this before but not here in the Forums ... the original 331 Hemi was actually offered first in a Desoto FirePower .... and you are right, in it's day that sucker scared the hell out of people with 180HP ....

Beans for a great post ...

[BlackSXT]

Great post....will get my son to read it too, he likes this stuff.... thanks for the info on the HEMI!

[viper3ez]

good work.

consider me learned

[goMANgo259]

read that same article like a year ago! thanks for posting it. it was cool learning that jaguar made the hemi... i wonder if the heads on my jag are hemis...

[MattRobertson]

Thanks LN for the informative piece. Here's another little-known fact. Check out the picture below: See how the pistons are hanging out, and part of the block is missing and the heads are all asunder?




That's Cam's engine. True story.

[Flanman]

If I remember my Chrysler Hemi history correctly, I think the first Hemi engine Chrysler made was a WWII tank engine.

[MikeEast]

If I remember my Chrysler Hemi history correctly, I think the first Hemi engine Chrysler made was a WWII tank engine.

I thought it resulted from the war department getting them to work on a more efficient airplane motor in the mid 40's. Read that somewhere, not long ago.

I my Hemi...

Mike

[Flanman]

I thought it resulted from the war department getting them to work on a more efficient airplane motor in the mid 40's. Read that somewhere, not long ago.

I my Hemi...

Mike

Ya, I think an airplane engine factored in there too. I think that some airplane engines already were using a hemispherical chamber, Chrysler did some work with them, so on..... I think the tank engine was the first Hemi engine they actually designed and produced. But I could be wrong.

[IES99]

Is this correct then: A 4 valve/cylinder pentroof V8 has 4 overhead camshafts

[Meathammer]

A little off topic, but an above post made me think of this.

Did you know that it was Chrysler that designed the M1A1 Abrams tank? It was there design submitted to the military that won the contract for a new main battle tank for the US Armed forces. (source - Discovery Channel/TLC program about the M1A1 Abrams).

[Phil Hunter]

Hi,

The tank engine and the airplane engine are the same. The first Chrysler Hemi was a supercharged 16 cylinder engine designed for use in both fighter planes and tanks. It was tested but never put into production. Here it is:

[Fargo59]

Is this correct then: A 4 valve/cylinder pentroof V8 has 4 overhead camshafts

a 4 valve pent roolf system CAN have 4 cams (total on a V type engine.)

a neon has a 4 valve pent roof design as well and has 1 cam, or 2.

a hemi is a dual rocker shaft pent roof OHV system, and the origins of the hemi date back well before the 331, or other 1st gen chrysler hemi's. chrysler did a 16 cylinder hemi headed engine for the military that put out a rediculous amount of power by using hemi heads and a supercharger, but the design went nowhere as the engine was too big, and had too many costs (always the drawback of the dual rocker design.)

[Meathammer]

Hi,

The tank engine and the airplane engine are the same. The first Chrysler Hemi was a supercharged 16 cylinder engine designed for use in both fighter planes and tanks. It was tested but never put into production. Here it is:

Imagine getting a hold of one of these and putting it in a pulling rig.....