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"Semi-Hemi" 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. Developed from Chrysler's aviation research, the efficient powerhouse had roughly hemispherical heads, opposing valves, and a complex valvetrain that used dual rocker arm shafts for each cylinder.

Engine Auto part Motor vehicle Automotive engine part Vehicle

The Hemi was a premium engine, well engineered but costly, because eight-cylinder engines had mainly been used in premium cars. Most cars had four or six cylinders; spending some extra time and money on building a better V8 made sense. Chrysler had not counted on just how popular the V8 would be, or how much it would drive sales.

It took little time for Chrysler to realize their mistake, and the engineers quickly got to work on dropping costs and increasing output. Their first move was to creating a cheaper and lighter head and valvetrain, for the same engine blocks. Dubbed the "poly" (due to its polyspherical heads) or "semi-hemi," the new design had 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.

Technical drawing Auto part Diagram Drawing Parallel

Poly head design on left; wedge head design on right.

The 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. Chrysler claimed the rounded combustion chamber prevented carbon deposits.

The 1958 354 engine was available in both Hemi and Poly form; with four barrel carburetors, the Hemi produced 350 horsepower, and the Poly just 310. Still, the Poly was much cheaper and lighter, and the company could make larger engines to replace the lost power.

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.

Randy Hicks wrote that the cylinder heads and intake manifolds were interchangeable with any Hemi engines that had the same deck height. However, the new design meant they didn't need spark plug tubes; pistons and rods were less expensive; and the heads were cast rather than being machined. Most used a two-barrel carburetor, because the Hemi was the performance engine.

Auto part Automotive engine part Machine Engine Vehicle

As Hemi Andersen wrote, the Poly headed engines were an interim solution, not requiring a new block:

You have to remember that this was happening in 1953-1954, as they planned for a V8 for the '55 Plymouth. They still had six or seven years to go with the archaic flathead 6 which came out in the early 1930s, so they weren't thinking very new; they were looking for a cheaper version of the existing Hemi.

This is why the 1955 Chevy V8 turned out to be so far advanced. Chevrolet came out with a whole new design, which Chrysler sort of finally arrived at with the wedge-head 273 in 1964, nine years late. The Y block Ford overhead-valve V8 was rubbish when it came out in 1954, but it improved, and the 312 was a pretty good engine in a lot of old stock cars.
The 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 horsepower and 275 pound-feet of torque. Dodge Truck's version displaced 260 cubic inches, generating 169 hp and 243 lb-ft of torque; it had various measures to increase durability, including added coatings, different metals, and valve rotators.

The 1956 cars saw a larger-bore version of the 301, displacing 331 cubic inches - but that wasn't the end; it was bored again, to 354 cid, for the 1957 cars, reaching 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).

Dodge cars got a A388-coded 270 cubic inch version, with 175 hp and 240 lb-ft of torque.

1955 Plymouth241157217
1955 Plymouth259167231
1955 Dodge270175240
1955 Chrysler301188275
1957Dodge (4bbl)325265355
1956 Chrysler331225310
1968 Chrysler354310405

* Gross horsepower; modern horsepower readings would be roughly 20-40 hp lower. The industry switched in 1971. Only two used four-barrel carburetors: the Chrysler 354 and one version of the 1957 Dodge 325.

The next generation of Chrysler V8 engines were designed to have lower costs and much higher production; they had the same heads, but numerous changes to allow for more automation in the factory. They, too, had polyspherical heads - but while the blocks were an evolution of the Hemi V8 design, they were not the same. These were informally, and later formally, called the "A" engines. With their lower cost and higher production volume, the A engines quickly replaced the older, Hemi-based Poly "semi-Hemis." The 1958 Dodges would be the last cars to carry them; in 1959, Dodge and Plymouth both used the new A-engines. Chrysler and DeSoto had already moved to the new B-engines.

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 A-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 (a B-engine) in the U.S. and the 354 poly in Canada. The 361 V8 B-engine was optional across the board in the U.S. The 350 B-engine was also used in the 1958 DeSoto Firesweep while other models used the 371 and the Canadian-build Firedome used Chrysler's 354 Poly."]
Engine Auto part Motor vehicle Automotive engine part Vehicle

As Hemi Andersen wrote,

Every division in Chrysler Corporation was fighting to have their own version of the "cheap hemi." When I worked on a 330 DeSoto Hemi V8in a Facel Vega, no parts were interchangeable with a 331 Chrysler Hemi, not even a valve cover gasket. How foolish was that?

I've had an A engine and a late LA 318 upside down; except for the core plugs and the engine mount tabs, they are virtually identical. The "A" engine is just another step away from the original Hemi, in the big scheme of things. The first A engine, the 1956 277, was a heavily modified 270 Poly, but it was still closely related to the original 331 Hemi. They got the next engine series, the LA, all the way to 340 cid before they had to change the block to create the 360.
The early, semi-Hemi Poly has a separate valley cover underneath the intake manifold; A-series engines did not have the separate valley cover, since the intake performed this function. Both series were similar in outward appearance, despite being from rather different engine families.

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.

Car Vehicle Table Emblem

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,

The 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.

Auto part Engine Automotive engine part Automotive super charger part Transfer case part
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. As V8 demand kept increasing, the A engines could not keep up with the need for more power; larger displacements were needed. The company designed a series of new large engines ("B engines") that could be made more cheaply and more quickly; even the 392 Hemi was matched by a new 413 cubic inch B engine. Chrysler engineers discovered, while developing this series, that the wedge-head engines were actually more efficient, and much cheaper and faster to build, than the poly design; and the Hemi advantages were overcome with sheer size. 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.
The Poly engines lasted from the 1955 cars to the 1958 cars - a fairly short lifespan, but they were always an interim solution while work proceeded on the A and B engines. DeSoto used the Dodge 325 for just one year (1957); Plymouth used four different displacements of the same engine series from their 1956 to 1958 lines; Dodge and Chrysler each used three displacements, from the 1955 to 1958 cars (though not the same three displacements). It would not take much longer for Chrysler Corporation to stop having different displacements for each brand (and in each successive year), which was a nightmare for dealer parts departments.

[A enginesB enginesLA engines]

1958 Red Ram distributor
Advance ControlAutomatic
Point Gap.015 to .018"
Breaker Arm Spring Tension17 to 20
Contact Dwell (degrees)29 to 32 deg
Automatic Advance curve (Distributor speed)
290 to 410None
4100 to 2°
6504 to 6°
16508 to 10°
Vacuum advance:
6.1 to 7.3 inches of mercury, manifold vacuum
0 Deg
... at 10 inches 4.6 to 6.6 Deg
.. at 14 inches 10 to 12 deg
Condenser capacity (microfarads).25 to .285
Timing mark location (marks 2° apart)Fan Drive Pulley
Engine Idle Speed (RPM)475 to 500

Specifications for 1958 Coronet, Royal and Custom Royal

Red Ram (325) Poly V8 and Super Red Ram (350, 361) B-Engines
SpecRed Ram (Semi-Hemi)Super Red Ram (B)
Type90 degree V890 degree V8
Valve ArrangementIn Head Single Rocker Shaft
Piston displacement 325 cid350
(361 in D500)
Taxable Horsepower (AMA)43.952.81
Compression Ratio8.5 to 110 to 1
Compression pressure
(min 150 rpm,
90 (min) - 155 (max)150 (min) - 180 (max)
Max variation betw. cylinders15 lbs25 lbs
Firing order1-8-4-3-6-5-7-21-8-4-3-6-5-7-2

Cylinder numbering

1-3-5-7 left, 2-4-6-8 right
(from driver's seat,
front to rear)
Crankcase Capacity (qts)54
Oil Pump Rotary Full Pressure, Camshaft Drive
Min Pump Pressure at 500 rpm15 psi15 psi
Pressure at
40-50 mph, 1,500 RPM
50-65 lbs45-70 lbs
Oil FilterShunt;
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"
End Paly.002-.006"002-.006"
Max allowable.010".010"
Radial Clearance.001-.003".001-.003"
Max allowable.010".005"
Camshaft chain
Number of links6850
Width1 1/8".88"
Cam journals: diam x length#1: 1.998-1.99 x 7/8"
#2 1.982-1.983 x 3/4"
#31.967-1.968 x 3/4"
#41.951-1.952 x 3/4"
#51.4355-1.4365 x 15/16"
Crankshaft TypeFully Counter Balanced
BearingsSteel Backed Babbitt
Thrust taken byNo.3 main bearing
End Play.002-.007"
Max allowable.010"
Radial Clearance.0005-.0015"
Max allowable.0025"
Finish at rear oil seal surfaceDiagonal Knurling
Main bearing Size
Diameter and lengthNo.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 reading) when supported
at front and rear main bearing
Crankpin Journals
Max Allowable Out of round.001".001"
Max Allowable Taper.001".001"
Connecting Rods
Length (center to center)6.626.358
Weight (less bearing shell)22.528.6
BearingsSteel Backed BabbittSteel Backed Babbitt
Diameter and Length2 1/4 x 13 /16"2.375 x .927"
Max allowable.0025".0025"
Side Clearance.009-.017".009-.017"
Connecting Rod Bushing
TypeSteel Backed Bronzenone
Diameter and Length1.110-1.125-.9217-.9220____
TypeConformatic with Steel Strut /
Horizontal Slot with Steel Band
MaterialAluminium 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 oz705 gram
Ring Groove Width (upper).032".032"
Valves (intake)
Head Diameter1.84"1.95"
Length (overall)4.31"4.81"
Stem Diameter.37".37"
Stem to Guide Clearance.002".002"
Max. allowable.004".004"
Face angle45 deg45 deg
Valves (Exhaust)
Head Diameter1.47"1.60"
Length (overall)4.31"4.81"
Stem Diameter.37".37"
Stem to Guide Clearance.003".003"
Max. allowable.006".006"
Face angle45 deg45 deg
Valve guides
TypeCast in HeadCast in head
Valve Springs
Pressure, compressed
(valve closed)
1.69"-72 lbs1.86"-75 to 85 lbs
Pressure, compressed
(Valve Open)
1.31"-166lbs1.47"-173 to 187 lbs
Valve spring installed height
(spring seat to retainer)
1 5/8 - 1 11/16"1 55/64"

Chrysler 1904-2018

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