From 1957 to 1966, Dodge made the 318 polyspherical engine, or the 318a. It was modelled on the popular Hemi engine. The Poly differs in that it is a small block and its heads are polyspherical rather than hemispherical1.
The spark plug is located at the focus of the parabola. Mathematically, any vector that starts from the focus and strikes the curve of the parabola will reflect away from the parabola in a straight line. Essentially, the shape of the Poly forces all of the power from combustion down the cylinder, rather than having the top of the heads absorb some of the energy. This increases power and cools the engine.
The main advantage of the Poly over the Hemi is weight. It has more horsepower than the famous 318LA, and per cubic inch its power is comparable to that of the Hemi - yet it is much lighter, making its power more noticeable. Commonly called the wide-block or semi-hemi by Mopar enthusiasts, it is a rare and beloved oddity of the Chrysler Corporation.
For anyone who has a 318 Poly - don't worry. Although parts aren't readily available like the other 318s, they are out there. The only factory performance parts available are a two four-barrel carburettor set-up and a mild camshaft upgrade. Fortunately, a well-tuned stock engine still produces about 270 horsepower with only a two barrel carb - with a four barrel you get 290 horsepower. The two-four set-up easily breaks 300, and with a good camshaft and exhaust you can reach 350 horses with a minimum of effort.
Chrysler is reinventing the Poly after 37 years. The new 4.7 litre engine is a smaller version of the hardworking 318 - the new Poly should be a standard engine in the years to come.
The 318 Poly is a classic and anyone who finds one should take care of it. There is nothing like a die-hard Mopar fan bending under your hood and saying, "What the hell sort of engine is that?!"