Transportation Science

What's so special about the Hemi engine?
Answered by Discovery Channel
  • Discovery Channel

    Discovery Channel

  1. The top three finishers at the 1964 Daytona 500 had one important thing in common: the beast under the hood. Race winner Richard Petty and the other two leaders were in Plymouths with powerful Hemi engines. The standard combustion chambers in other engines had flat heads. But the Hemi got its name from a hemispherical combustion chamber in which the spark plug was at the top of the chamber, with the valves open on opposite sides of it (in flathead engines, the valves are in the engine block, not the head). The Hemi's smaller surface area helped retain heat and keep fuel warm, creating more pressure in the cylinder. Also, Hemis had more room for valves than their predecessors' inline arrangements allowed. Bigger valves meant improved engine airflow. All told, it was a revolutionary design that allowed much more horsepower than ever before.

    Chrysler introduced a 180-horsepower Hemi V-8 in some of its passenger models in 1951. The displacement in that engine was 331 cubic inches (5.4 liters), which is how it got the name 331 Hemi. Considering that it was 1951, 180 horsepower was revolutionary - - even if it sounds rather unimpressive today. The 331 Hemi engine became legendary, and improved over the next two decades. In 1956, Chrysler released a 354-cubic-inch engine (6-liter); in 1957, the 392-cubic-inch (6.4-liter) Hemi was born; and by 1964, Chrysler had its 426-cubic-inch (7-liter) Hemi. Why don't all cars use Hemis? Because there are better engine configurations on the market today. For instance, the current pentroof-design engines have room for four valves per cylinder. Hemi engines can't accommodate more than two large valves per cylinder, though four slightly smaller valves allow engines to breathe better. And smaller combustion chambers reduce heat loss during combustion, so the newer high-performance engines use smaller chambers than the Hemi does.

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