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The Hemi



If you like cars, then you have probably heard of the HEMI engine. Chrysler created the HEMI engines phenomina in the 1950s, '60s
and '70s and recreated it in 2003 by bringing it back to life in their cars. If you follow muscle cars or drag racing, you know that the 426
HEMI engine is a popular engine because of its performance. Cars such as the Hemi ‘Cuda are some of the most sought after muscle cars
today.  Chrysler has done a great job, espeically recently marketing the Hemi name making it a synonym for big, powerful engines.

HEMI history
While Hemi engines are associated with Chrysler cars, the Hemi was and is used in many others.   The HEMI engine was first installed in
cars in 1948.  It was a 6 cylinder Hemi installed in Jaguars.  1951, Chrysler introduced a 180 horsepower HEMI V-8 engine on several
models. The Chrysler HEMI engine had a displacement of 331 cubic inches, so it is known as the "331 HEMI."  Today, 180 horsepower
sounds like nothing, but in 1951, 180 horsepower was amazing.  Hence the beginning of the legend of the Hemi. Chrysler released several
versions of the HEMI from 1951 settling in 1964 on the 426 Hemi.  The 426 HEMI  won first, second and third place in the 1964
Daytona 500. The 426 street HEMI came out in 1965, producing 425 horsepower.

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       Notice the large heads with the spark plugs in the middle.  
       When you see spark plugs in the middle chances are you’re looking
       at a Hemi

The 426 block and heads are still available today from Dodge. The HEMI  is the base design for drag racing, funny cars and top fuel
cars as well as many muscle cars.  Surprisingly, hemispherical cylinder heads have been used since at least 1901.  Several car makers
including  Belgian car maker Pipe in 1905,  Peugeot Grand prix Car of 1912, the Alfa Romeo GP car of 1914, Daimler, were Hemis.

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The word Hemi comes for the  hemispherical combustion chamber design.  Intake and exhaust valves are on opposite sides of the chamber,
allowing for the combustion mixture to flow directly across the chamber.  This is called "cross-flow" heads. Because of the placement of intake
and exhust valves, an efficient, cost effective way of opening and closing valves was a challenge.  In the early days Chrysler developed a
design they called the Double Rocker head.  The evolution of the Hemi made for more reliable designs.  

Hemi Benefits  
Power in an engine is all about fuel volume and combustion chamber effeciencey.  Because the air/fuel mixture offers less restriction to fuel
entering the combustion chamber the Hemi “breaths”  better than the wedge.   The spark plugs are also placed in the center of the fuel charge
creating additional effeciencies.  Wedge-head design offers a simplified valve train design, but, it requires the air/fuel mixture to make sharp turns
en route to and from the chamber and puts the spark plug on the side of the fuel charge.  

Hemi Challenges
Hemis are more expensive to produce.  They are heavier than wedge engines since the clyinder heads are much bigger.  They are also don’t excell
in performance at low RPMs and have more compilcated valve trains.   

Hemis are, and have been, installed in many makes and models today.  They  are most widely known in Chrysler products.  Over the years the
legend of the 426 Hemi has grown.  Chrysler put 426 Hemis in several of  their 60’s and early 70’s muscle cars.  Today, some of these cars are
the holy grail of muscle cars.  Who knows, someday maybe the new computer  controlled Hemis will reach this stature.


The Hemi combustion chamber is a circular design.