Vintage Engines: Mopar Polyspheres

The original Hemi was only in production from 1951-58, but the original poly variants were used for an even shorter period, 1955-58. We say variants because the polys used in the lower-line Chrysler models (Windsor and Saratoga) and the Dodge polys were built differently from the poly engines used in the other car lines, in keeping with the crazy variations between the 12 different models of Hemis. The Plymouth line never had its own Hemi in this period, and used polys based on the Dodge engines. From 1956-66, a newer variant of the polys was developed that was even less expensive to produce, and these are called the "A" engines. The confusion comes in when new '67-and-later non-poly engines were also called A engines. Most people decided to call that last group the "LA" series, for "late A" or "light A," adding to the mystery. We're told poly engines were used as late as the late 1960s in Canadian applications, overlapping with the "other" 318 engines. Poly engines were also used in many Dodge trucks (Power Wagons) of the 1956-66 period, even after most passenger-car use had ended. Their decent low-end torque worked well with low-geared applications, and they even did duty in some early motorhomes.

There are several terms thrown about concerning the polysphere blocks, such as low-deck and high-deck (terms also used in describing Hemi variations), but the '50s poly blocks were actually all of the same height/width. It was the variation in cylinder heads that made some appear taller or wider. The polysphere is often fondly called the "semi-Hemi" by its supporters, and it's an appropriate label in some ways. In the Hemi design, the intake and exhaust valves are stacked vertically (looking at the head from the exhaust side), and the pistons for a Hemi have their valve clearance notches similarly aligned. In the polyspheres, the valves are not stacked exactly the same because of the quite different spark plug location, but it is still what we now call a "canted-valve" design. The later LA engines had their valves aligned side by side in a "wedge-shaped" chamber, which became the defacto design for pushrod OHV American engines for the next four decades.

The poly engine was introduced with the '55 Chrysler Windsor in a 301-inch displacement offering 188 hp to those buyers who couldn't step up to the higher-level Chryslers with the Hemi. That same year, in typical Mopar intra-corporate rivalry, Plymouths could be ordered with their own (Dodge-based) 241-inch poly engine, and Dodge offered a 270-inch version. Chrysler lettered its poly engines with "Spitfire" on the valve covers, treading on the cach that name gained in the immediate postwar period when it referred to its most powerful Flathead inline engines. The only branch of the family during the 1950s without its own poly was DeSoto, which used Dodge polys for its lesser models.

Within the corporation, flagship Chrysler had the most powerful versions of the poly design. It may have been designed to save money, but even a Windsor Chrysler had to have more scoot than any Plymouth or Dodge. In 1956, they stroked the Spitfire to 331 inches, then to 354 in 1957, staying at that size for 1958-the last year for these engines in Chryslers. The most powerful motor offered was the optional 310hp version in the '58 Saratoga models. Polys continued in the other brands, with the lineage enlarged to 325 inches and 252 hp in '58 Dodges.

The '55-58 Chrysler and Dodge polysphere engines were "semi-Hemi" in another way. They were actually completely Hemi from the waist down, i.e., a 331 or 354 poly was a 331/354 Hemi with poly pistons and heads. When drag racers learned this in the early days of fuel racing, they could pull one of these polys from the wrecking yard, get back to the track, and slap on their cam, pushrods, race pistons, and ported Hemi heads and be good to go. The same may be the fate of stock polys in the current craze over early Hemis for street rods. Today's trad-rodders are learning what the fuel racers knew back then: Original Hemis in the underbrush are an endangered species, escalating in price all the time. With their value for conversion to collectible Hemis, stock poly motors today might be only a little easier to source out than finding a First Generation Toyota Land Cruiser that still has a Toyota engine!

Poly Potential
If you can find a gem-in-the-rough polysphere engine and you want something really different to power your next project, go right ahead and take "the road less traveled." If you have any Mopar friends, check with them first, since the polys are often replaced with the LA or other later-model engines when those early cars are rebuilt. Someone you know may have their old poly motor tarped up out behind the garage. They're excellent engines, and while parts aren't SBC-easy, all you'll need to rebuild one is available. Gaskets sets, bearings, rings, replacement pistons, and other standard rebuild parts are still offered by vintage engine supply companies such as Egge Machine and Kanter Auto. Any good machine shop can do the boring, honing, balancing, or decking, just as they would for any engine. Your best bet is to pick up the excellent book by SRM Senior Editor Ron Ceridono, The Complete Chrysler Hemi Engine Manual, published by Tex Smith's Hot Rod Library. It's a must for any Hemi fan, of course, but also has a rebuild story on a 318 poly engine, plus lots of history and parts resources.

Outsiders to the Mopar cult may deride the brand for its confusing array of part numbers and applications, but a silver lining there is that Chrysler kept making newer engines that shared many parts with their older ones, so interchangeability is a big help in building up a poly engine. From the later LA engines of 273/318/340/360 displacements, you can use distributors, valve springs, rings, bearings, rods, cranks, some oil pans, fuel and water pumps, and front covers. That also means you can find modern stroker cranks made for the LA engines to retrofit in your earlier A poly. Some of these items involve minor modifications to transfer to the older engines. Do your research first. One of the best bargains in performance ignitions is the Mopar Direct Connection electronic ignition kit (distributor and module), and you can bolt one right onto your poly motor-or your vintage Hemi for that matter, with a slightly longer extension.

Speed equipment for the poly motors is another story. New, long-tube headers are available, but no shorties. Several companies can sell you 3/8-inch-thick flange plates to make your own if you want shorties or outside headers. The intake is the real problem. Back in the day, all the major players in manifolds each had something for the polys (a small market even then), and Weiand had a nice aluminum four-barrel manifold, but discontinued it a few years ago. You'll have to hunt the swaps meets, online discussion groups, and eBay for anything other than a stock two-barrel intake. We don't have room here for all the machinations some poly fans have gone to in adapting other intakes, but one that should be mentioned is using a small-block Ford Windsor or early FE Ford Hilborn intake, since the ports are spaced very close to the poly design. Width across the engine and the flange angle aren't issues for fuel injection, and today shops around the country are converting Hilborns to EFI-an expensive route, but radically cool. If you have an early Dodge poly, things are a little easier, as these engines can use manifolds made for that size Dodge Hemi, and that might be easier to find.

One possibility that has arisen to fill a need in the vintage engine arena is Gear Drive Speed & Custom in Minnesota. Owner Matt Legare makes lakes headers and log-style multi-carb intakes for almost any vintage engine, and also offers the components in kit form for you to weld. Log manifolds are easy to adapt to engines that have a valley cover, and the width variations on engines aren't a problem because the two logs are connected by rubber hoses. You'll have to fabricate a custom valley cover to use log-type intakes on the A polys. You can "make lemonade" out of this seeming drawback by using thick aluminum and have some fins milled in.

You can even get a supercharger kit for the poly from Dick Landy Industries, but you have to supply them with an aluminum intake first, which they set up for a Weiand 174 blower. Cams are available from several old-school grinders like Isky, as well as newschoolers like Nielson Vintage Cams. Stock-compression, cast replacement pistons are readily available, and for more compression, the aftermarket companies have forged blanks that can be machined to whatever you need.

You should be able to easily produce 300 hp out of the larger-displacement poly motors with a little more bore, a little more compression, mild performance cam, headers, and a modern four-barrel intake. These engines already have excellent low-end torque, which in street use is what you really feel in the seat of your Mexican-blanket upholstery, not horsepower. For example, the '58 Dodge 325ci poly developed 355 lb-ft at just 2,800 rpm. Compare that to the Dodge 325 Hemi with 350 lb-ft at a higher 3,200 rpm. The last Chrysler Spitfire, the '58 354-incher, made 385 lb-ft at 2,400 rpm, and that's with a two-barrel carb. Rebuild one of these to stock specifications, add a modern ignition system, full-flow oiling, and adapted spin-on filter setup, side headers, and a four-barrel, and you'll make some Chevys eat your vintage dust.

Transmission availability is an important factor when choosing an alternative powerplant for street rod use. There isn't much to choose from for the poly engines, or the early Hemis for that matter, without the good folks at Wilcap, who produce kits to adapt just about any modern transmission to the poly block. Mopar favorites such as the 727 TorqueFlite can be utilized, as well as GM 350 and 400 THs, plus overdrive automatics. For the three-pedal rodders, there are kits for the Mopar A-883 as well as Ford Top Loaders, Muncie four-speeds, early Ford trannies, Cad-LaSalle vintage trannies, and the modern close-ratio Ford T5 five-speed. The early polys share the same block bolt-pattern and eight-bolt crankshaft flange as all their Hemi brothers (except for the '51-53 "extended-bellhousing" Hemi blocks), so Wilcap has these swaps down to a science.

At this point, you're asking yourself, what's been holding me back from having one of these cool and different motors? We know you've been tortured by the choice between paying dearly for a pre-'58 283 just because it doesn't have side-mounts or spending 10-large to build a 150hp Flathead. Think of a Mopar poly in your rod and the stories you can tell when people ask, "What the hell is that?" Your reply could be a "rare '50s British engine designed for channel-racing boats," or "the last Packard V-8 of the 1950s," or "a Marmon-Herrington OHV conversion for a Flathead." Just be wary of anyone who recognizes it right away. He's apt to be a member of the Mopar cult, and liable to talk your ear off with poly trivia.