Old post I know, but here is another build. http://www.moparmusclemagazine.com/...ive_a_318_poly_engine_more_power/viewall.html
We had several 318 Poly engines....great engines....they were definitely stronger than an LA 318. The 4 barrel 354 Poly in my 58 Chrysler, I think, has the distinction of being the highest HP of all Mopar Poly engines.
Poly is short for Polysphere. Polysphere is a made up name courtesy of Chrysler's advertising department. They were looking for a name comparable to Hemisphere. A Hemisphere has a specific meaning in geometry, it is half a sphere. The Polysphere was a modified Hemisphere. They changed the shape, and the exhaust valve location to get a more compact combustion chamber and to allow all the rocker arms to ride on a single shaft. The result was an engine that was cheaper to build, lighter and more compact than a hemi but had practically the same power. The Poly powered Chrysler Windsor would match a hemi powered New Yorker of the same displacement wheel turn for wheel turn up to 80 MPH, then the Hemi would pull away. This is why they made sure the hemi always had a displacement advantage over the Poly. The Poly engine was always overshadowed, first by the big brother hemi, later by the big block B and RB wedge motors. The 383 B motor was practically the same as the 318 in weight and overall size. There was no point in trying to hop up the 318 when you could just order the same car with a 383 413 or 426. The 290HP 318 used in 1957 Plymouth Fury was plenty hot. But it was dropped after 1958 in favor of the Golden Commando 361 and 383. From then until 1962 the hottest 318 was a single 4 barrel model, after 1962 even that was dropped and the 318 was only available as a 2 barrel low compression motor. Nobody bothered with the 318 and hardly any hop up parts were ever made. It was a good strong motor with a lot of potential that was never developed.
If you check the valve sizes of the old 318 poly you will find them half way in between a 318 wedge head and a 340. The 340 valves will fit easily and the heads can be ported. With a suitable cam headers and intake there is no reason the poly can't be bored out to 340 size and produce similar HP.
which one was the Block adaptable to a hemi? the seraded valve cover style or the w shaped valve cover style? kind of confusing when there both called Poly Blocks..
It went like this. Chrysler, DeSoto and Dodge all got their own hemis between 1951 and 1953. Then, Chrysler and Dodge got Polysphere versions for their lower priced models. Both these Polys can be turned into hemis, using the same make hemi heads. Plymouth got their own engine in 1955, this was the first Poly engine that never had a hemi counterpart. It became the well known 318. In case this isn't clear... Chrysler hemi heads will fit on a Chrysler poly block, Dodge hemi heads will fit on a Dodge block. DeSoto never got a poly of their own. The Chrysler and Dodge had a notched or scalloped lower edge to the valve covers. Plymouth had more of a saw tooth design.
DeSoto & Plymouth early Polys were built on Dodge Hemi blocks. To simplify the answer...if your Poly has a seperate intake & valley cover it's convertable. If the intake seals the valley it isn't!
When it comes to interchange bore centers pretty much tell the story Dodge hemi and Plymouth poly spin off 4.1875 Desoto hemi 4.3125 early 318 type poly and LA 4.460 Chrysler hemi 4.5625. Because if you change the bore centers you change the main bearing spacing.
I had one in a '63 Dodge Polara... I remember putting the barn-job on a few punks in 5.0 Mustangs from stop light through the intersection. The thing had balls down low! Sam
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