Mopar V-8 Engine Families--A Primer

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Mopar V-8 Engine Families--A Primer
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Mopar V-8 Engine Families: Chrysler, DeSoto, Dodge, Plymouth From 1951--A Short History

The practical application of this information is that, for example, if you wish to buy parts for your Hemi, you'd better know which Hemi you have.  If you buy a Chrysler 354 Polyspherical engine thinking it's a 354 Hemi, you're going to be quite disappointed.  Chrysler had far more engine families than most people are aware of.  There is considerable confusion because some engines of different families had the same or almost the same displacement, and recognizable trade names (Hemi, in particular) have been applied to several distinct engine families.  Since 1951 at least five other Chrysler Corporation engine families have been known as "Hemi", which is a trademarked name of the Chrysler Corporation.  There were three different series of "Old" Hemis; the more-modern 426 Hemi which in aftermarket form is still the undisputed king of top-class Drag racing; and the newest 5.7 and 6.1 liter version that's still in regular production for Chrysler Corporation cars and trucks.Often, a newer engine family was heavily based on an older family, but with significant revisions. 

Old Hemispherical-Head Engines; and Polyspherical-Head Engines

Hemispherical (Hemi) -Head engines
Chrysler introduced their first Overhead Valve (OHV) V-8 in 1951, the 331 cubic inch Hemi, as a response to Oldsmobile and Cadillac each releasing an OHV wedge-head engine in the late '40's.
  • Chrysler 331, 354, and 392.  The 392 is a raised-deck-height version of the smaller engines in this series.  All these engines have a 4.562 bore spacing. (measured from the centerline of one cylinder to the centerline of the cylinder next to it in the block)  The last year for the "old" Chrysler Hemi was 1959, as a 392.
  • DeSoto 276, 291, 330, 341, and 345.  The 330 and larger engines are a raised-deck-height version of the smaller engines in this series.  Introduced one year after the Chrysler Hemi, in 1952 as a 276.  All these engines have a 4.3125 bore spacing.  The last year for the DeSoto Hemi was 1957, in 341 and 345 versions.
  • Dodge 241, 260, 270, 315, and 325.  The 315 and 325 are raised-deck-height versions of the smaller engines in this series.  Introduced two years after the Chrysler Hemi, in 1953 as a 241.  All these engines have a 4.1875 bore spacing.  The last year for the Dodge Hemi was 1957 as a 325, although a Chrysler 354 was also available in some Dodges that year.
Although all these engines are Hemis, the bore spacing is an indication that they are actually completely different engines with very limited parts interchangeability.  A defining characteristic is that all these engines have two rocker shafts per cylinder head.

Polyspherical (Poly) -head engines
The Hemi engines were very heavy and expensive to make.  Still, Chrysler spent a lot of money advertising the superiority of the Hemi over a "conventional" wedge head.  There was a lot of corporporate pride in producing a Hemi rather than an "ordinary" wedge engine.  When Chrysler Corporation first built engines to a lower-cost point, instead of using wedge heads, they used a cross between the Hemi and a wedge heads that they called a "Polyspherical" head.  The Poly design uses only one rocker shaft per cylinder head.  Just like the three separate Hemi engine families, there were three separate Poly engine families--one based on the Chrysler Hemi, one based on the Dodge Hemi, and, later, a completely new engine family designed by Plymouth called the "A" series.  There was no Poly version of the DeSoto Hemi.

Although scorned by hot-rodders as being "less" than a Hemi, the Poly engines are just beginning to see some recognition with enthusiasts.  The Polyspherical valve layout was ahead of it's time in that it's somewhat similar to the 1965 Chevrolet 396 and 1968 Ford 429 canted-valve engines.  Not bad for a head design which in Dodge and Chrysler versions was introduced at least ten years earlier--in 1955!
  • Those Poly engines based on the Dodge Hemi were built in 241, 260, 270, 315, and 325 cubic inch versions.  The bore and stroke, and bore spacing of these Poly engines is the same as the equivalent Dodge Hemi engine.  The Dodge Poly was introduced in 1955.  Plymouth had no V-8 of their own, so 241, 260, and 270 Dodge Poly engines went under Plymouth hoods in '55 and '56.  The last year of the Dodge Poly was 1958 as a 325.
  • Those Poly engines based on the Chrysler Hemi design were built in 301, 331, and 354 cubic inch versions.  The bore and stroke, and bore spacing of these Poly engines is the same as the equivalent Chrysler Hemi-head unit.  The last year of the Chrysler Poly is also 1958.
  • In 1956, Plymouth began building their own V-8 engine, known as the "A" engine.  The "A" engine uses 4.46" bore spacing.  All "A" engines used Poly heads.  The "A" engine was built in 277, 301, 303, 313, 318, and 326 cubic inch variants.  The "A" 301 is sometimes called a 300, and it has different bore and stroke measurements than the Chrysler Hemi-based 301 Poly engine.  Note that the "A" engine was revised in the '60's and became the wedge-head "LA" small block engine.  The last year of the the "A" engine family is 1966 in America, and 1967 in Canada, as a 318.
B-RB, "426 Hemi", and LA Engines

B-RB engines
The Hemi-head engines were discontinued after the 1959 model year.  In an effort to both modernize engine offerings, and at the same time cut costs, the "B" series engine was introduced as a 350 in 1958 DeSotos and Dodges.  In 1959, a raised-deck-height version called the "Raised B" or "RB" was used on Chrysler-branded cars, introduced as a 383.  This was in keeping with old Hemi tradition, where each family had a low-deck-height and a raised-deck-height version within each family.

The B and RB engines are a much more modern and less-complex design than the old Hemis, having lighter weight and using in-line valves opening into a wedge-shaped combustion chamber.  Chrysler Corporation increased the bore spacing from previous engines; on the B/RB it is 4.80".  Unlike all previous Hemi and Poly engines, the distributor is at the front of the B/RB.
  • The B engine was offered as a 350, 361, 383, and 400.  All B engines use a crankshaft stroke of 3.375".  The last year of the B engines was 1978 as a 400.
  • The RB engine was offered as a 383, 413, 426 and 440.  All RB engines use a crankshaft stroke of 3.75".  The last year of the RB was 1978, as a 440.
The B 383 and the RB 383 use different bore and stroke measurements.  The B and RB engines were used in all Chrysler Corporation car lines.

426 Hemi
Chevrolet scared the pants off of Ford and Chrysler in the qualifying races for the 1963 Daytona 500.  Chevrolet provided several race teams with a special, prototype 427 engine which was the forerunner to the '65 396.  The new Chevy engine won both qualifying races, setting a new record in the process.  Ford protested the engine.  Chrysler just built a better one.  The existing 426 wedge castings were revised and developed to accept a new version of the Hemi-style head.  The "New Hemi" went on to win the '64 Daytona 500.  "Race Hemis" were produced in limited numbers for '64 and '65, the "Street Hemi" was optional in certain '66 passenger cars.  The last year of the Street Hemi was 1971.  Bore and stroke, and bore spacing of the 426 Hemi was the same as the 426 RB wedge engine.

LA Engines
The original Plymouth-designed "A" engine that debuted in 1956 was simplified and lightened to produce the "Light A" or "LA" engine first installed in 1964 Dodges and Plymouths as a 273.  The LA has a shorter deck height, and uses light "thinwall" casting methods.  The cylinder head uses in-line valves.  A few parts interchange with the older "A" engine, and the two families share the same bore spacing of 4.46".  The LA was offered in 273, 318, 340, and 360 cubic inch versions.  The LA 318 shares it's bore and stroke measurements with the earlier "A" 318.

The LA engine underwent further development and became the "Magnum" series; it was also the foundation for the Viper V-10.

Current Hemi
In 2002 (trucks) and 2005 (passenger cars) Chrysler introduced a new engine family designed to compete with the General Motors LS-series of pushrod V-8s.  The new Chrysler engine, predictably, has Hemi cylinder heads.  Everything old is new again.  The current Hemi is available in 5.7 and 6.1 liter versions, with a 6.4 liter version sold over-the-parts-counter as a crate performance engine.

4.7 liter OHC section
Deck height 9.09
Bore spacing 4.09
Bore 3.66"  Stroke 3.405"  Rod length 6.12"

Other engine-related eBay Guides I've written are:

GM Big Block 455 Engine Families--A Primer
GM Small Block Engine Families--A Primer
Ford V-8 engine families since 1932
GM HEI Ignitions--A Primer

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Entire contents copyright (C) 2007, 2008  Camino3X2    Feel free to LINK to this Guide in your auctions.
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