RECAP



This whole project started because of

“….A while back there was a thread “…..Chrysler,Dodge and Desoto early Hemi, what interchanges…..” and the discussion got around to whether Chrysler Poly (Spitfire) heads could be swapped onto a HEMI block……”

The part of that discussion that intrigued me was if a “sleeper” Poly motor could be built by swapping POLY heads onto a 392 block. That’s what started me down this road.

Doing the 331/354 instead of the tall deck 392 would still prove if it was possible or not. When I hit the show stopper of the head overhang with the 301 heads on the 354 bore and not having a pair of 354 POLY heads I decided to go ahead and built it back as the HEMI it originally was.

I only have access to the 301 heads I started with so some of this is still supposition. I still think it’s possible….with the right POLY heads.

Chrysler lists different part numbers for the 301, 331 and 354 Poly heads and based on what I ran into on the 301 heads I suspect the chambers are larger on the 331 heads and even more so on the 354 heads.

As far as just swapping a pair of Hemi Heads on a Poly or vice versa without any other changes other than the pushrods, it might be possible on a low compression engine without any problems. High compression pistons, high lift cam and/or larger valves would probably create issues and require a piston swap due to the location of the needed valve reliefs. One of the things I was going check was the feasibility of was just cutting a second pair of valve releifs into the stock 354 pistons.

As far as gaining or loosing compression with a head swap that will depend entirely on what you’re starting with and what you’re changing to. One of the things that really surprised me was that the 301 Poly heads and the triple nickel HEMI heads had virtually the same volume (about 101-102 CCs). I suspect the 331 and 354 heads have a larger chamber but without having the heads to measure that is still up in the air. The volume of the HEMI heads are also all over the place depending on the casting (for instance the 354 HEMI truck heads we measured have a larger chamber volume than the 55 passenger car heads).

On the combustion chamber overhanging the cylinder (the show stopper I ran into), I’m sure the heads were designed to accommodate a potential overbore of probably at least .060 oversize or so. So say 354 Poly heads on a .060 over 354 HEMI block should not be a problem. The issue might become using a 354 Poly head on a 392 HEMI block. The standard bore on a first gen 392 is approximately .062 larger than a first gen 354. At that point there should be no issue, but if you bore the 392 .060 over you’re now talking about a total of .122 total oversized and may run into the same combustion overhang into the bore problem.

Pushrod holes in the block going from a poly to a hemi head are a non issue. Going the other way may be. The angle of the pushrods when using poly heads required notches to be drilled/ground in into the block. 1955 was the first year of the POLY head and some HEMI blocks got the notches and some didn’t. If you’re going to the Poly heads on hemi block you will have to add these notches if they are not already there. Pictures I’ve seen of 392 HEMI blocks do not show the notches so going to the POLY heads will require they be added. On the plus side, modern composite head gaskets already are cut for either POLY or HEMI heads.

As far as pushrods, on the low deck 331/354 engines, the stock POLY pushrods are only about .065 shorter than the intake (short) pushrods on the HEMI, which is within the adjustment range of the adjustable pushrods Hot Heads sells. I suspect the same would be true for pushrods Hot Heads sells for the 392 tall deck motor.

So there are still some questions to answer that I can’t really do without having the parts in hand to check. I do believe that doing a HEMI to POLY conversion is still a possibility without having to do a lot of custom one off parts sourcing, just coming up with the right parts.

Why? I suppose just to have something a little different……and it would still be interesting to see what the difference in performance level really is…..remember the factory rated the 354 HEMI with more HP than the 354 POLY, but according to the factory manuals the POLY actually produced more Torque than the HEMI.