Discussion of the "Nearly Hemi" Polys.
4 posts • Page 1 of 1
there's a lot of history involved with this questionPowerflite wrote:Can anyone tell me for sure; 100%, whether or not the Chrysler poly (331,354) camshaft is the same part as a Chrysler hemi (331,354) camshaft? Tell me also how you know.
the cams are identical right down to lift/duration on many poly and hemi engines up to 1956 from my knowledge- and although later years had different lift/duraton between hemi/poly, you could take a 1958 354 poly cam and put it in an early 331 hemi or poly and it will run fine. Cam interchange on these engines boiled down to deck height- if the engine had the same deck height, the cams will interchange, and are often identical. The whole purpose of the poly line was to make a cheaper engine with hemi-like performance- and use as much of the hemi stuff as possible, except for the expensive heads and rocker valvetrain. The solid 300 HP 331 hemi cam is much hotter than the hydraulic 331 poly/hemi cam- but if you put it in a 301/331/354 poly and used the solid lifters and custom adjustable pushrods, you could run the solid hemi cam in a poly as well. If you look in a vintage 1950's Chilton manual, you'll see the lift specs for 1955-56 poly and hemi hydraulic cams and spring tension are identical- reason being, it's the same darn cam- the poly loses a tiny bit of valve lift on the exhaust side due to a more extreme pushrod angle or rocker arm ratio variance. They would never cut a new cam for a poly just to reduce lift by .010", it would make no sense in production.
there's a lot of misconception about this swap on the net message boards, I've read more than once "you have to change the cam" in threads, when in fact you don't. The only time you need to change the cam, is when you go from a short deck motor, to a tall deck motor. Example: 241/270/277 Dodge poly/hemi and 315/325 Dodge poly/hemi have different cams. 301/331/354 poly/hemi and 392 hemi have different cams.
The Chrysler poly/hemi engines with interchangeable heads, were actually decades ahead of the competition for their time, in that they had both canted valve and hemi heads that interchanged on the same block. Everyone else was still at the inline valve stage for their OHV V8. Chevy didn't offer anything comparable until the 348 staggered valve arrangement in 1958, but the first true Chevy canted valve head with multiple valve angles wasn't until the BBC in 1965. The 348/409 had all the valves at the same angle into the head, just not in a line. (the Mystery Motor 1963 was a true canted valve head, but was not a production engine, it was a lab engine). Ford didn't get a comparable canted valve V8 engine until the 429 and 302 Boss of 1969, and Cleveland 1970. (again, the SOHC 427 was not a production car engine, it was over the counter special order, and rare- basically another Chrysler clone, hemi heads on the old FE427, with OHC's) When the general public thinks of canted valve engines, they think Chevy and Ford, when in fact Chrysler had them 10-15 years earlier with the polys. The problem is, there is almost no polys out driving around today, the history is lost to the average hot rodder.
engineering really was Chrysler's ace in the hole back then. The only thing these early engines lacked was cubic inches and compression, they were just starting to get with that program with the 392, when they pulled the plug on the hemi/poly engine line. Chrysler had to be bailed out financially in 1959, and to cut costs, they went to one basic big block engine across all the car lines, and dropped the DeSoto line.
GM and Ford competed the easy way by just adding valve size/porting/cubic inches to their OHV inline valve engines. Add 50 cubes to an inline valve engine, then you can run with or beat a hemi performance- the old saying there's no substitute for cubic inches. So in that way, GM/Ford took the cheap way out- their engine lines were more profitable and cheaper to produce.
how do I know ? I owned 7 of these engines, and disassembled them all, and looked at them closely. 2 hemis and 5 polys.
Last edited by oldngood on Fri Oct 16, 2009 7:43 am, edited 2 times in total.
The "new" nose drive cams were made in several different grinds for the low deck engines, the 300B hemi cam is certainly hotter than the late 331, and the 1958 Saratoga poly cam is hotter than the Windsor 301.