Anyone here a fan of trigonometry? Yeah, me neither. How about funky defunct engines of yesteryear, like Chrysler’s family of “Semi-Hemi” V8 engines? They sure seemed like a good idea at the time, but have long been considered either an amusing oddity or complete crap — unless you are entering the Engine Masters Challenge. Now that’s the sort of study session I can get behind.
To understand a polyspherical engine, and its head in particular, one must first understand that the first OHV Hemi motors were hemispherical engines. In essence, this means that a traditional Hemi head was a perfect half-sphere. The problem with this hemispherical head design, was its “double rocker” valve configuration. The design added a fair deal of cost, as well as size and weight to the valvetrain, and required more man-hours on the assembly line.
Being that the Plymouth brand was a solid performer at the time, but was considered to be the most affordable model line within the Chrysler Corporation, the need to lower costs was the primary objective at first.
However, since a V8 was still required for marketing and competitive purposes, Chrysler decided to keep the vast amount of what was affordable and working well within the Hemi’s block and cut corners in the head manufacturing department instead.
Cutting Corners With the Chrysler Corporation
By the time the mid-1950s came around, each of Chrysler’s sub-brands had received its own Hemi. While Chrysler had “FirePower” and Dodge rocked a “Red Ram,” the defunct DeSoto brand lumbered along under the power of a rebranded Hemi “Fire Dome.” The only brand that didn’t receive this venerable V8 was Plymouth, which is a whole other diatribe for another day.
Since an affordable V8 option needed to be formulated in a flurry, Chrysler’s engineers set to creating an engine that retained many of the Hemi block’s more affordable attributes, but with a far less expensive cylinder head on top. The “Semi-Hemi” was the automaker’s polyspheric solution to the pricy problem it was facing at that time, and at first glance, it seemed like the automaker had found a sustainable solution.
This is where that polyspheric-shaped head, with its valves controlled by a single rocker shaft, comes into play, along with a whole bunch of other clever weight and penny-pinching design feats.
Due to not having a full hemisphere chamber like the Hemi, the circular combustion chamber casting of the head allowed that single rocker shaft design to actuate a series of diagonally mounted valves with hydraulic lifters. Since this wasn’t a wedge chamber V8 design with inline valves, the central rocker shaft was stuffed between the valves so that rockers could actuate both the intake and exhaust on either side of the shaft. This meant intake valves were on the top side of the rocker arm and angled toward the port exit, and exhaust valves were down towards the bottom in standard alignment.
Relying upon this design meant that Chrysler’s engineers had to create Poly-engine-exclusive push rods, exhaust manifolds, and pistons. They also had to get a bit creative with the valve cover, which is where those “scalloped” casting designs come into play. Intended to work around the angled exhaust side of the valvetrain, this creative solution doubled as a way to make spark plug accessibility extremely easy.
Speaking of spark, the spark plug was the focus of the parabola on this motor, which essentially meant that the shape of the head channeled the combustion’s explosion back down the cylinder instead of allowing the head to absorb some of the impact. This clever, corner-cutting solution not only scavenged power but also kept the head from overheating.
Early Dodge and Plymouth Polyspheric Engines
Come the 1956 model year, a brand-new 100-percent original Polyspherical head engine was introduced: The “Hy-Fire” V8. With its own head and block, this Dodge design had a slightly beefier block, which left room for future over-boring experiments. The motor also came with a larger crankshaft and broader bearing surfaces, as well as bigger valves for greater flow up top.
While the Plymouth 277 cubic-inch V8 made around 187 horsepower in the Belvedere and Sport Suburban, upgrading to a “Power Pack” gave you a four-barrel carburetor and a true dual exhaust, upping the output to 200 horsepower. It was that same year, that the 303 cubic-inch Fury arrived on the scene, which used 277 heads, and gave them a four-barrel carburetor for 240 horsepower. The internals contained beefier dome pistons, a more performance-oriented camshaft, tighter valve springs, balanced connecting rods, and a more powerful distributor.
Plymouth also released a “High-Performance Package” in 1956, which was a dealer-added option. Twin four-barrel carbs, a high-induction air filter attached to an aluminum intake manifold, and an even more aggressive camshaft were all contained in the kit. This gave the 277 230 ponies and the 303 270 horsepower, respectively.
Dodge’s Semi-Hemi also saw some upgrades, but only in the displacement department. With 270 cubic inches now on tap, with an 8:1 compression ratio, power figures began to reach the 180- to 187-horsepower mark.
Polyspheric Engine Power Figures And Model Years
While Plymouth and Dodge shared a singular line of Poly motors, it is worth noting that they also came with the smallest of the set, with the 241 cubic-inch two-barrel carb version only producing 157 horsepower and 217 pound-feet of twist from its 4.0-liter displacement cycle. This motor was only offered during the Polyspheric engine’s launch-year of 1955 and was promptly phased out thereafter due to a lack of consumer interest.
The 260 faired a bit better, as it provided about 20 more horsepower due to being overbored, adding 0.2 liters of displacement, but it only came with a fuzz more torque. It wasn’t until you got to the Dodge 270 that things started to get lively. Thanks to an additional 0.2 liters of displacement, horsepower figures hit anywhere from 175 to 189 horsepower, depending upon the model that was selected, allowing the torque curve to crest the 240 lb-ft mark. But the American market still didn’t seem all that interested, and after just two years of sales, the 270 was retired following the 1956 model-year.
Unique to the 1956 model-year, was the Dodge 315, which had 5.1 liters on its birth certificate, and anywhere from 218 to 230 horsepower pushing its weight. Torque finally surpassed the 300 mark with 316 pound-feet on record, which surprisingly did not change when you upgraded to the manufacturer’s four-barrel carburetor.
It was not until 1957 leading into the Polyspheric “Semi-Hemi” engine’s last run in 1958, that you could get the 325-cube Dodge option. This was good for 355 pound-feet of torque and anywhere from 245 to 265 horsepower, depending upon the package option and carburetor configuration, of course.
Chrysler’s “Spitfire” Line of Affordable Polyspheric Power
Being that Chrysler was the cornerstone of the entire motor group at the time, it received its own “Spitfire” line of larger, bespoke Poly V8 engines, including a 301, a 331, and a 354 cubic-inch option. All of the other brands were handed 241, 260, 270, 315, and 325 alternatives. To this day, this engine continues to cause all kinds of confusion in the automotive community, as these Polyspheric engines received the same name as Chrysler’s inline eight-cylinder less than a decade prior.
That tidbit of info disclosed, it’s worth mentioning that the base model “Spitfire” V8 was the only engine that did not have a Hemi doppelganger. Furthermore, the 4.9-liter variant produced 188 horsepower and 275 pounds of torque but was only available in the Windsor line for the 1955 model year. This was replaced the following year by the 331 cubic-inch model, which generated a bit more power at 225 horsepower and 340 pounds of torque.
And then there was the big boy. A 354 cubic-inch “Spitfire” Polyspheric engine that unleashed anywhere from 285 to 290 ponies on average, and created a respectable 385 pound-feet of torque courtesy of 5.8 liters of displacement and a 9.25:1 compression ratio.
The Semi-Hemi Gets Beheaded Because…
Come 1958, Chrysler made the decision to discontinue this Hemi hybrid motor in favor of the “A Family” of affordable engines, which retained the Poly head design, but did away with the shared block.
Naturally, this led to the discontinuation of the polyspheric Semi-Hemi motor from that period, as it relied upon much of the Hemi block’s architecture for all of its short-lived existence.
And while the polyspheric head did go on to grace other engines after 1958, it was only this 1955-to-1958 iteration that shared the same general block architecture with the vintage Hemi.
The Poly Life Ain’t for Everyone
Since the Poly engine was born during the early throws of America’s iconic horsepower wars, some surmise that a lack of performance support may have caused the “Semi-Hemi” to prematurely go extinct. Chrysler never really offered factory performance parts for the original Poly engine line on the same scale as the regular Hemi. Plymouth received that “V-800” Fury 318 V8 we mentioned earlier, and there were some “High-Performance Pack” options, but they were nothing compared to the love that the Hemi received.
We also have to remember that during this period (and for quite a while prior and thereafter) Chrysler was subjected to ridicule for inflating (and deflating) its power figures. Whether this was due to antiquated dyno designs from the time, operator oversights, fully-lead fueling mechanical miscalculations, or sheer dishonesty, no one really knows. So consumer mistrust may have been a factor as well.
What is certain though is that all of the power figures listed within this article should be taken with a grain (or shaker) of salt and be eyed as more of a “ballpark average” than exact stats. Oh, and if you happen to have a scalloped Semi-Hemi head lying around somewhere be sure to send us a shot of it, because we’d love to see what you’ve got.