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 Polyspherical Head V8 Engines: 241, 259, 270, 301, 315, 325, 331, 354

with thanks to Bill Watson
and Hemi Andersen

Chrysler’s first V8 engine was the “dual rocker,” now known as the original Hemi; it was more efficient than its domestic competitors and set a new level of performance. Features of the new engine, developed from Chrysler’s aviation research, were (roughly) hemispherical heads, opposing valves, and a complex valvetrain that used dual rocker arm shafts.

powerdome V8

The Hemi was seen as a premium engine, but it was slow and expensive to build; chances are Chrysler did not think V8s would become so common so soon, especially on lower brands. Chrysler was paying more per car than GM and Ford, for efficiencies customers didn’t seem to care about, and couldn’t produce enough V8s in any case. Their first reaction was creating a cheaper and lighter version of the Hemi, using the same block but a simpler valvetrain.

The new engine, nicknamed the “poly” (due to its polyspherical heads), used rounded, circular combustion chambers, like the Hemi; but they only had one rocker arm shaft, instead of the Hemi’s two. To make the change, they put the intake valves on the top of the rocker arm and the exhaust valves on the bottom. The results were lower cost, less weight, and higher production; the cost was a slight loss of efficiency.

The bottom of the valve covers were scalloped so the spark plugs were accessible from the top — unlike Ford and Chevrolet V8s, whose owners had to reach under hot exhaust manifolds to get at the spark plugs.

powerdome engine valvesThe “single shaft” (pre-A) Poly has a 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 engines did not have the separate valley cover, since the intake performed this function. Both series were similar in appearance, with scalloped valve covers and a rear-mounted distributor, despite being from completely different engine families.

The engines had hydraulic tappets, reducing maintenance, and Chrysler claimed the rounded combustion chamber prevented carbon deposits. Most used a two-barrel carburetor, because the Hemi was the performance engine. The poly engines had better low-to-mid-speed performance, ideal for ordinary drivers.

331 engineThe Chrysler version was dubbed “Spitfire,” and the first car to get it was the 1955 Windsor (the lowest Chrysler). The 1955 Chrysler poly was a 301 with 188 hp and 275 lb-ft of torque. Dodge Truck also got a version, of just 260 cubic inches, which generated 169 hp and 243 lb-ft of torque.

In 1956, as GM and Ford kept upping their power, the 301 was replaced by the larger-bore 331; 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).

Meanwhile, Dodge cars got a A388-coded 270 cubic inch version, with 175 hp and 240 lb-ft of torque, giving the Chrysler a decent advantage.

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. The similar A-type 277 cubic inch engine — which used wedge-shaped combustion 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 Avenue plant and Dodge Main 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 [about efficiency].

After 1955, it gets a bit blurry. Dodge’s Hemi got to 315 cid, Desoto's to 341 cid, and Chrysler had a 354 cid in 1956 and 392 in 1957 and 1958 (Imperial and 300D only in 1958). In 1958, the B engine arrived as 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.”]

1957 325 Super Red Ram Dodge engine

Christopher Cortel wrote that the redesign saved money by eliminating one expensive rocker shaft (and its bracing), and spark plug tubes could be eliminated. The combustion chamber was cast, not machined as Hemi engines were. The pistons and rods were also different.

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 was much cheaper and lighter, and the company could make larger engines to replace the lost power. That made the Poly’s replacement also inevitable — since the wedge engines were “good enough” and, like the Poly, could be made larger for more power, except in the most extreme cases, where the Hemi came into play.

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 1955 Plymouth “Hyfire” V8 [a smaller version of the Dodge Poly] was available in two displacements and three horsepower ranges: the 241 produced 157 hp, and the 260 produced 167 horsepower. 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.

Plymouth V8

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 by Chrysler to be exclusive to luxury cars. Chevrolet’s new V8 brought that concept to an end, and brought 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 J.C. 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), Power Dome (Dodge Truck), and Fire Power (Chrysler). Canadian engines has details on Canadian-built engines.

Mike Peterson noted,

carter afbThe 1955 Carter carburetor on the Power Pack was a surprisingly modern design with metering rods on the primary jets and velocity valves controlling air flow to the secondaries. This is 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 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.

In the long run, the same issues that brought about the Poly also ended their production. Chrysler engineers discovered that the wedge-head engines were actually more efficient, and much cheaper and faster to build. Pete Hagenbuch said:

..the performance improved by getting rid of the silly polysphere. A wedged chamber have some advantages... you can build in a lot of what we call squish, where the chamber is just part of the cylinder head surface and the piston has a flat area that matches up with it. Squish is why you can run 12:1 on a wedge head because without squish you would have to run 9:1. It gets the charge moving and mixed, moving through the chamber at high velocity, which means the flame travel is fast and there isn't anything left to burn by the time it gets to top dead center where you expect the detonation. Anything that reduces detonation also helps reduce pre-ignitionm which is catastrophic.

Plymouth needed vastly higher production rates than the slower-selling Dodge, DeSoto, and Chrysler cars, which spurred development of the A and B series wedge engines. [A enginesB engines]

Specifications for 1958 Coronet, Royal and Custom Royal
Red Ram (325) Poly V8 and Super Red Ram (350, 361) B-Engines
Spec Red Ram Super Red Ram
Type 90 deg V8 90 deg V8
Valve Arrangement In Head Single Rocker Shaft
Bore 3.69" 4.0625"
Stroke 3.80" 3.375"
Piston displacement 325 cid 350
(361 in D500)
Taxable Horsepower (AMA) 43.9 52.81
Compression Ratio 8.5 to 1 10 to 1
Compression pressure
(min 150 rpm, WOT)
90 (min) - 155 (max) 150 (min) - 180 (max)
Max variation betw. cylinders 15 lbs 25 lbs
Firing order 1-8-4-3-6-5-7-2 1-8-4-3-6-5-7-2

Cylinder numbering

1-3-5-7 left, 2-4-6-8 right
(from driver’s seat, front to rear)
Crankcase Capacity (qts) 5 4
Oil Pump Rotary Full Pressure, Camshaft Drive
Min Pump Pressure at 500 rpm 15 psi 15 psi
Pressure at
40-50 mph, 1,500 RPM
50-65 lbs 45-70 lbs
Oil Filter Shunt;
Replaceable Element
Full Flow
Cylinder Bore (std) 3.6875-3.6895" 4.0625-4.0845"
Bore Out-of Round (max) .005" .005"
Cylinder Bore taper (max) .020" .010"
Max allowable over bore .060" .040"
Drive Chain Chain
End Paly .002-.006" 002-.006"
Max allowable .010" .010"
Radial Clearance .001-.003" .001-.003"
Max allowable .010" .005"
Camshaft chain
Number of links 68 50
Pitch .375" .50"
Width 1 1/8" .88"
Cam journals: diam x length #1: 1.998-1.99 x 7/8"
#2 1.982-1.983 x 3/4"
#3 1.967-1.968 x 3/4"
#4 1.951-1.952 x 3/4"
#5 1.4355-1.4365 x 15/16"
Crankshaft Type Fully Counter Balanced
Bearings Steel Backed Babbitt
Thrust taken by No.3 main bearing
End Play .002-.007"
Max allowable .010"
Radial Clearance .0005-.0015"
Max allowable .0025"
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
Diameter 2.5 2.625
Max Allowable Out of round .001" .001"
Max Allowable Taper .001" .001"
Center bearing run-out
(total reading) when supported
at front and rear main bearing
.002" .002"
Crankpin Journals
Diameter 2.2495-2.2505" 2.2495-2.2505"
Max Allowable Out of round .001" .001"
Max Allowable Taper .001" .001"
Connecting Rods
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"
Clearance .0005-.0015" .0002-.0022"
Max allowable .0025" .0025"
Side Clearance .009-.017" .009-.017"
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 (std thru all oversize) 18.6 oz 705 gram
Ring Groove Width (upper) .032" .032"
(intermediate) .0790-.0800" .0790-.0800"
(lower) .1875-.1890" .1875-.1890"
Valves (intake)
Head Diameter 1.84" 1.95"
Length (overall) 4.31" 4.81"
Stem Diameter .37" .37"
Stem to Guide Clearance .002" .002"
Max. allowable .004" .004"
Face angle 45 deg 45 deg
Valves (Exhaust)
Head Diameter 1.47" 1.60"
Length (overall) 4.31" 4.81"
Stem Diameter .37" .37"
Stem to Guide Clearance .003" .003"
Max. allowable .006" .006"
Face angle 45 deg 45 deg
Valve guides
Type Cast in Head Cast in head
Size .374" .374"
Valve Springs
Pressure, compressed
(valve closed)
1.69"-72 lbs 1.86"-75 to 85 lbs
Pressure, 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"

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