Polyspherical Head V8 Engines: 241, 259, 270, 301, 315, 325, 331, 354
The first V8 engine produced by Chrysler was the original “double rocker,” to be informally named the Hemi. The engine had strong performance and efficiency, but the hemispherical head and complex valve operation added to the cost, size, and weight, and lengthened build time. As a result, the value brand, Plymouth, did not get a Hemi V8 — or any other V8, for that matter, until 1955.
To address some of these issues, the company adapted the Hemi engines by substituting more polyspherical-shaped heads, with valves activated by a single rocker shaft. Not quite a full hemisphere, it still had a rounded, circular combustion chamber that could be served by a single rocker arm. To manage this, they put the valves in a diagonal arrangement, with intake valves on the top side of the rocker arm and the exhaust valves on the bottom side. The bottom of the new engine's valve covers were uniquely scalloped so the spark plugs were accessible from the top — unlike Ford and Chevrolet V8s, where owners had to reach under hot exhaust manifolds to get at the spark plugs. The engines had hydraulic tappets, and normally used a two-barrel carburetor.
Another advantage of the poly engines was better low-to-mid-speed performance, for ordinary drivers of sedans and wagons; that niche had been filled with Jefferson Avenue’s L-head six-cylinder engines, but with Chevrolet and Ford selling low cost V8s, Chrysler was at a disadvantage. The first Chrysler car to get the new “Spitfire” engine was the 1955 Windsor; the first engine was the 301, which built up 188 hp and 275 lb-ft of torque. A single year later, the 301 was replaced by the 331, a larger bore version; in 1957, the engine was bored up again, to 354 cid, where it would stay for its final year, 1958, producing 290 hp and 385 lb-ft in the Windsor (the Saratoga used a four-barrel carburetor to produce 310 hp, 405 lb-ft of torque). At that point, the cheaper big-block B-series V8 took over, starting at 350 cid.
Meanwhile, at Dodge, the first poly V8 was the A388, which was similar to the Spitfire but dubbed “Red Ram” or “Super Red Ram;” the first version was a 270 cubic inch version, with 175 hp and 240 lb-ft of torque, giving the Chrysler a decent advantage.
Plymouth used smaller versions of the Dodge 270 (241 and 259 cubic inches) with poly heads in 1956, but soon replaced it with a new 277 engine, the first A block.
The new A engines shared little with the Hemi-based Poly-head V8; since Plymouth’s sales were far larger than Dodge or Chrysler, and pricing was lower, the A-type V8 engines needed to be made far more quickly at the new Mound Road engine plant. The Dodge and Chrysler engines, with smaller volumes, were not necessarily engineered with high-speed production in mind.
The “single shaft” (pre-A) Poly can be identified by its separate valley cover underneath the intake manifold; Randy Hicks wrote that the cylinder heads and intake manifolds were interchangeable with any Hemi engines that had the same deck height. A-series Poly engines did not have the separate valley cover underneath the intake manifold, since the intake itself performed this function. Both series of Poly engines — A-based and Hemi-based — were similar in appearance, with scalloped valve covers and a rear-mounted distributor, despite being from completely different engine families.
In 1957, the 301 Poly produced 215 gross horsepower at 4,400 rpm and 285 lb-ft of torque at 2,800 rpm, with compression of 125-165 psi. Taxable horsepower was 49 hp. The similar A-type 277 cubic inch engine — which used wedge-shaped combusion chambers — in that year produced 197 hp @ 4,400 rpm, 270 lb-ft @ 2,400 rpm. By comparison, in 1960, the A-series 318 V8 produced 230 gross hp at 4,440 rpm, and 340 lb-ft of torque at 2,400 rpm, fed by a two-barrel carb and using a 9:1 compression ratio.
Engineer Pete Hagenbuch wrote:
I don't know who invented the word "polyspherical" but the design was supposed to maintain the characteristics of the true hemi with one rocker shaft and attendant cost savings. What it wasn't was a wedge chamber with inline valve stems.
The Chrysler Jefferson plant and Dodge Main (now the site of a large GM plant) each had their own poly and when the A engine, built at the brand new Mound Road Engine Plant, came out, it was a poly also, of 277 cid. What Chrysler was learning was the old Yankee creed; "There ain't no substitute for cubic inches," and at a time of ridiculously low gas prices, nobody cared.
Just one detail before I go on: prior to the 1956 model year, which means 1955 models only, Dodge was the source of Plymouth V-8 engines, a 270 cid poly. The next few years it gets a bit blurry. Dodge's hemi got to 315 cid, Desoto's to 341 cid, and as I'm sure you know, Chrysler had 354 cid in 1956 and 392 in 1957 and 1958 (Imperial and 300D only in 58). In 1958 the B Engine arrived in two sizes; a 350 cid and 361 cid. Other Dodges got the 325 cid engine from Mound Road. [Bill Watson added: “While 1958 Dodge Coronet and Royal models used Dodge's 325 poly, the Custom Royal used the Ram-Fire 350-cid V8 in the U.S. and the 354 poly in Canada. The 361 V8 was optional across the board in the U.S. The 350 was also used in the 1958 DeSoto Firesweep while other models used the 371 and the Canadian-build Firedome used Chrysler’s 354 Poly.”]
Christopher Cortel noted that by switching to a single rocker shaft per head and mounting spark plugs on the outside of the valve cover, an expensive rocker shaft, bracing for the second rocker arm shaft, and spark plug tubes could be eliminated, saving a good deal of money and weight. The combustion chamber was cast, not machined as Hemi engines were. The pistons and rods were also different.
From the outside, the poly can be easily identified by a wavy-edge valve cover.
As an example of the difference in efficiency, the 1958 354 engine was available in both forms. The Hemi produced 350 horsepower, and the Poly a healthy 310 horsepower. But the Poly is much cheaper and lighter, and the company could just make larger engines.
The polyspherical heads were ahead of their time, as were the hemis. The "Spitfire" heads had canted valves in a cross-flow arrangement, gaining some of the advantage of the hemis, but much less expensive to make. This design would make a comeback in the 4.7 liter V8. They also had low friction valve locks to allow for valve rotation, extending their life.
Lanny Knutson added:
The new Plymouth "Hyfire" V8 was available in two displacement and three horsepower ranges. The 157 hp engine was of 241 cubic inches, while a 260 cubic inch engine produced 167 horsepower, and later, a mid-year addition of a power package (four-barrel carburetor and dual exhaust) increased the 260 to 177 horsepower. The latter engine was not part of the original plan.
J.C. Zeder, Director of Engineering, claimed "we are not seeking to develop higher speeds and greater power than anyone else. The increased speeds and torque of the 1955 Plymouth, when combined with the PowerFlite transmission, results in improved performance in low and middle ranges, plus greater economy." In other words, Plymouth's new V8 was considered to be no more than a higher-powered extension of the traditional and reliable Plymouth flathead six. The horsepower race, at the time, was considered to be the exclusive property of luxury cars. But Chevrolet's new V8 brought that concept to an end and the horsepower race to the low-priced field. Plymouth had to respond, and they did — with the 1955 power package and later with the 1956 Fury.
The overhead V8 was another facet of the latest automotive fashion. Everyone had to have one if they wished to keep selling cars. So Plymouth had one. If people like Zeder had their way, the familiar flathead six would still be Plymouth's sole powerplant.
The poly head engines were made under a variety of different names, including Fire Dome (DeSoto) and Fire Power (Chrysler). Hemi Central has information on the hemi-head versions, while Canadian engines has details on Canadian-built engines.
Mike Peterson noted,
The power package is a very rare option. The 1955 Carter carburetor was a surprisingly modern design with metering rods on the primary jets and velocity valves controlling air flow to the secondaries. This is very similar to the Carter AFB, the carburetor used on the 426 Hemis. Dual exhausts were also part of the power package. No other modifications were made to the engine when the power package was installed.
The weakest part of the V8 engine was (at least in early years) the crankshaft. Even though the cranks are forgings, they are prone to breakage. Mine broke between the number four main bearing journal and the number seven and eight connecting rod journal. I know of at least seven other 1955 Dodge owners who have experienced similar problems. My car, however, gave me a warning of impending trouble with low oil pressure. There were no "funny" noises until it cut loose, and then there were plenty of new audio sensations. The crank looks structurally sound; I believe that the trouble is in the Dodge bearing materials. A good aftermarket bearing should be used during a rebuild. The cranks can be welded back together and made into interesting lamps.
| Specifications for 1958 Coronet , Royal and Custom Royal
Red Ram (325) Poly V8 and Super Red Ram (350, 361) B-Engines
|Super Red Ram
|Type||90 deg V8||90 deg V8|
|Valve Arrangement||In Head Single Rocker Shaft|
|Piston Displacement (cu. inch)||325||350
(361 in D500)
|Taxable Horsepower (AMA)||43.9||52.81|
|Compression Ratio||8.5 to 1||10 to 1|
|Compression Pressure (minimum 150 rpm,
plugs removed, wide open throttle)
|90 (min) - 155 (max)||150 (min) - 180 (max)|
|Maximum Variation Between Cylinders||15 lbs||25 lbs|
|Cylinder numbering (from drivers seat,
front to rear)
|1-3-5-7 left, 2-4-6-8 right
|Crankcase Capacity (qts)||5||4|
|Oil Pump||Rotary Full Pressure, Camshaft Drive|
|Minimum Pump Pressure at 500 rpm||15 psi||15 psi|
|Operating Pressure at 40 to 50 mph 1500 RPM||50-65 lbs||45-70 lbs|
|Oil Filter||Shunt; Replaceable Element||Replaceable Full Flow|
|Cylinder Bore (std)||3.6875-3.6895"||4.0625-4.0845"|
|Cylinder Bore Out-of Round
(max. before reconditioning)
|Cylinder Bore taper (max. before reconditioning)||.020"||.010"|
|Max allowable over bore||.060"||.040"|
|Number of links||68||50|
|Camshaft Journals Diameter and Length||No.1 1.998-1.99 x 7/8"|
|No.2 1.982-1.983 x 3/4"|
|No.3 1.967-1.968 x 3/4"|
|No.4 1.951-1.952 x 3/4"|
|No.5 1.4355-1.4365 x 15/16"|
|Type||Fully Counter Balanced|
|Bearings||Steel Backed Babbitt|
|Thrust taken by||No.3 main bearing|
|Finish at rear Oil Seal Surface||Diagonal Knurling|
|Main bearing Size|
|Diameter and length||No.1 2.50 x.73"|
|No.2 2.50 x.73"|
|No.3 2.50 x.72"|
|No.4 2.50 x.73"|
|No.5 2.50 x 1.19"|
|Main bearing Journals|
|Max Allowable Out of round||.001"||.001"|
|Max Allowable Taper||.001"||.001"|
|Center Bearing Run-Out
(total indicator reading)
when supported at front and rear main bearing
|Max Allowable Out of round||.001"||.001"|
|Max Allowable Taper||.001"||.001"|
|Length (center to center)||6.62||6.358|
|Weight (less bearing shell)||22.5||28.6|
|Bearings||Steel Backed Babbitt||Steel Backed Babbitt|
|Diameter and Length||2 1/4 x 13 /16"||2.375 x .927"|
|Connecting Rod Bushing|
|Type||Steel Backed Bronze||none|
|Diameter and Length||1.110-1.125-.9217-.9220||____|
|Type||Conformatic with Steel Strut /
Horizontal Slot with Steel Band
|Material||Aluminium alloy tin coated|
|Land Clearance (in Bore)||.027-.033"||.042-.047"|
|Clearance (top of Skirt)||.0005-.0015"||.0005-.0015"|
|Weight (Standard through all oversize)||18.6 oz||705 gram|
|Ring Groove Width (upper)||.032"||.032"|
|Stem to Guide Clearance||.002"||.002"|
|Face angle||45 deg||45 deg|
|Stem to Guide Clearance||.003"||.003"|
|Face angle||45 deg||45 deg|
|Type||Cast in Head||Cast in head|
|Pressure when compressed (Valve Closed)||1.69"-72 lbs||1.86"-75 to 85 lbs|
|Pressure when compressed (Valve Open)||1.31"-166lbs||1.47"-173 to 187 lbs|
|Valve spring installed height
(spring seat to retainer)
|1 5/8 - 1 11/16"||1 55/64"|