In 1955 Chrysler introduced the polyspherical cylinder-head design for its Spitfire V8, a less-expensive alternative to the mighty but costly hemi.

From its introduction in 1951, The Firepower V8 was a success for Chrysler, burnishing the automaker’s image as an engineering leader. Overhead-valve V8s were a were big selling story for the Motor City in the early ’50s, and the Hemi was glamorous and powerful. But it was expensive to manufacture and thus unsuitable for the base model Chrysler Windsor, priced $800 lower than the New Yorker. The Windsor’s standard engine was an L-head six, neither glamorous nor powerful, and by 1955 it was deemed no longer competitive. Its replacement, a lower-cost alternative to the hemi Firepower, was the Spitfire V8.

One source of the Hemi’s extra cost was its elaborate valvetrain, which required two rocker shafts per cylinder bank. In fact, at Chrysler the engine was originally called the Double Rocker V8. The Spitfire required only one shaft, but with the valves arranged in an unusual way. The exhaust valve was situated parallel to the cylinder bore, while the intake valve was tipped inward. This created two shallow domes in the combustion chamber, sort of, rather than one. Thus, at some point this cylinder head design gained the name polyspherical, in contrast to hemispherical.

So here, the valves are not aligned parallel to the cylinder banks, as with most OHV V8s, or perpendicular to them in crossflow fashion as with the Hemi, but somewhere around halfway between. This, Chrysler determined, was a sound compromise in cost versus airflow, power, and efficiency. (And it also dictated the distinctive shape of the valve covers.) Some have called the layout a “semi-hemi,” and sure, why not. Hemispherical, polyspherical, and semi-hemi are not rigid technical terms in the auto industry, so call it what you want. Chrysler calls its current Gen III V8 a Hemi, try and stop them. The Spitfire is what it is in any event.

For its 1955 introduction the Spitfire V8 was blessed with a perfectly square 3.625-in bore and stroke for a declared displacement of 301 cubic inches. Thus, its stroke was identical to that of the 331 CID hemi, but with a smaller bore. And actually, from the head gaskets down the Spitfire is, with some changes, essentially the hemi V8, and hemi heads can be adapted to a Spitfire block. In its original ’55 form with an 8.0:1 compression ratio, the Spitfire was rated at 188 hp at 4,400 rpm. In 1956 the displacement was boosted to 331 CID and to 354 CID in 1957, but then both the Spitfire and the hemi were discontinued for 1959 in favor of the B-series V8.

Meanwhile, elsewhere in the corporation, Dodge was taking an identical approach in creating an inexpensive poly alternative to its Red Ram hemi V8. And in 1956, Plymouth introduced its own V8 that included the polyspherical cylinder head layout, but in an otherwise new design that was simplified and optimized for high-volume production at an advanced plant in Warren, Michigan. This A-series poly engine, as it was known, remained in the Chrysler Corporation’s engine lineup in Plymouths and Dodges through 1967.

1955 Chrysler Windsor Nassau