| History of the HEMI 
Introduction and History Introduction Chrysler's Hemi is considerd by many to be the finest engine ever made. I have created this website to be a (hopefully) useful resource for those interested in the Hemi, and for those always looking to learn a bit more. Thanks, enjoy the website and please spread the word. Scott Moseman (scottm@scotech.com) > Early Hemi R&D The years following World War II brought upon new designs for performance. Chrysler had to find a way to increase power without increasing compression, which would require higher octane fuel. The key was thought to be in the better designed cylinder head, perhaps hemispherical with conventional valve-in-head. Using hemi heads would increase thermal and volumetric efficiency, as well as provide a low surface-to-volume ratio (thus minimizing losses due to combustion-chamber deposits). Why was this design not used earlier in autos, see as it has been around since at least 1904? Complexity and high costs did not allow easy mass production, and the rugged engine loved high octane gas. Chrysler teams researched every engine available, and the hemi head design proved to be the most powerful and efficient model they could find. 
Early Hemi R&D The years following World War II brought upon new designs for performance. Chrysler had to find a way to increase power without increasing compression, which would require higher octane fuel. The key was thought to be in the better designed cylinder head, perhaps hemispherical with conventional valve-in-head. Using hemi heads would increase thermal and volumetric efficiency, as well as provide a low surface-to-volume ratio (thus minimizing losses due to combustion-chamber deposits). Why was this design not used earlier in autos, see as it has been around since at least 1904? Complexity and high costs did not allow easy mass production, and the rugged engine loved high octane gas. Chrysler teams researched every engine available, and the hemi head design proved to be the most powerful and efficient model they could find. 
Hemi Head Durability The 426 Hemi engine was designed and built with a very short deadline, but that did not stop it from being the most well-built engine ever made. The engine had a max vertical separating load of 18,800 lb @ 7200 rpm along the crankshaft center line. Frank Bialk had the job of designing the new main bearing caps, which he took advantage of the deep skirt walls by adding cross bolts through the block into the caps. With this, the engine block skirt helped the main bearing caps in resisting the loads. Bolting the heads to the block is crucial to get max power. The hemi heads were designed with an extra bolt, the fifth head bolt, to give it the extra strength. The position of that bolt was going to be directly in the way of the pushrods and intak port, which would restrict power. Again, Frank Bialk came up with a design to bring the bolts up from the bottom instead. The new pattern worked extremely well, and the Hemi engine was put into production.  
426 Street Hemi It did not take much engineering changes to allow the 426 Hemi to work in street production cars. Mounting lugs added to the side of the block, additional vibration damper, forged pistons to allow 10.25:1 compression, streetable cam, lower valve spring rates (to avoid premature wear and limit engine rpm), deep oil pan with baffles, larger diameter oil pump, black valve covers, air cleaner and cover, and a high speed water pump with small impeller to help keep it cool at the drags. Willem L. Weertman co-authored the SAE paper for the street Hemi with Bob Lechner in April 1966. 
Copyright © 1996-2001 Scott Moseman. All rights reserved Basics
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, as discussed in the following section. Advantages
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. 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 breath 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.
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