Discussion in 'OT Driven' started by dirteemac, Feb 25, 2005.
But why is a hemi engine unique?
hemispherical combustion chambers.
Whats the advantage the HEMI has over other motors of the same size exactly? Or is it more of a marketing ploy? I've always wanted to know as well
Hemi was orignally made as a gas efficient motor.
Too lazy to type so i'm doing a copy and paste job.
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. For example, you can get a 2003 Dodge Neon with a stock 2.0 liter, 4-cylinder engine that produces 150 horsepower. The 5.7 liter LS6 V-8 in the 2003 Chevy Corvette produces over 400 horsepower. But in 1951, 180 horsepower was unheard of. It was an amazing amount of power for the day, and it fueled the "HEMI legend."
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.
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.
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.
Howstuffworks.com > *
Nice, thanks a lot
Actually Chrysler was playing with Hemis during WW2 (before Jag) -- If jets hadn't come out when they did we would've seen a few Hemi-powered planes --- P47H
Chrysler built an all-aluminum inverted V16 Hemi with a goal of 2500HP --- they acheived 3500
Actually what? You're talking about air planes.
" The HEMI engine for automobiles was born in 1948 "
Yes, but the engine for the automobiles was derived from the engine used in aircraft.
The big thing (really) about the Hemi was when Chrysler mated the Hemi head to the big block (RB)
And, while other manufacturers (Toyota for one) use the design, they don't have the centrally located spark plug which is a major part of the Hemis success.
I almost forgot about this thread. Thanks to all who contributed, it's very interesting...