In 1951, Chrysler engineers were seeking fuel economy and engine durability rather than high output when they designed a new V-8 motor known as the HEMI, because the combustion chamber was machined into the shape of a half dome or hemisphere. This was the first HEMI engine in an American car. The intake and exhaust valves were located on opposite sides of the combustion chamber at 90 degrees to the cylinder bore axis, so separate individual rocker shaft assemblies, two on each head, were necessary to operate and control all sixteen intake and exhaust valves.
The performance advantage of the Hemi is its ability to burn fuel evenly. With the spark plug centered in the cylinder the flame of combustion travels at an even pace, burning all the fuel and pushing down on the piston with even pressure. Hemis run cooler because of having greater surface for heat transfer, along with large cooling passages. This also allowed for higher compression ratios, due to its larger volume. HEMI engines have two rocker shafts per head and a complicated, expensive valve train, which proved to be costly and eventually exceeded production costs, causing the HEMI to be phased out in favor of a much cheaper, wedge head design.
The Chrysler Hemi’s major differences are the adjustable pushrods and offset rockers to create a Hemi engine that was financially feasible as a production engine, and that allowed more valve lift for greater performance. Milder versions came with hydraulic lifters; higher output engines had solid lifters. Chrysler’s original FirePower Hemi is referred to as Type 1. DeSoto’s FireDome and Dodge’s Red Ram soon followed with versions referred to as Type 2 and 3 respectively. Type 1, 2 and 3 Hemis all have different displacements, as well as low and raised block configurations. In 1964, Dodge introduced a new style of Hemi, the Type 4. The new Chrysler 426 HEMI engine was so fast that in 1965 it was banned from NASCAR. Nothing is interchangeable with earlier Type 1, 2 and 3 Hemi engines and the Type 4, but they are related.
The Chrysler Hemi was first introduced as a 331 cubic inch V-8, weighing in at almost a thousand pounds, with an unbelievable 180 horsepower at 4000 rpm. It came with a forged steel crank and big 1.81 intake and 1.5 exhaust valves, giving it unbeatable flow. Unusual to the first 331ci Hemis were the cast in the block bell housing flange, which was discontinued in 1954. This was only the start, as the displacement was raised to 354, followed by the 392 Hemi, which put out 390 horsepower straight from the dealer. The HEMI engine was popular in production cars during the ‘50s up until the early ‘70s, and also in drag racing as one of the fastest engines ever made.
If you’re interested in learning more about different engines, you can download our free e-book, The ATC Diesel Emissions Checklist.