Posted by: Mark Polk | 2013

71 Power Wagon – Change in Plans!

A major concern I have with this project is you cannot get an overdrive transmission for a 383 big block engine. When I drove the truck home it ran about 3,000 rpms at 60 mph. It sounded like it needed to shift again, but there are only 3 speeds in the old 727 transmissions. When you add to this that the 8 3/4 rear axle is probably a 3:91 or 4:10 the problem is compounded.

I mentioned earlier that my main goal is to be able to drive the truck and not go broke at the gas pump. I need to lower the rpms and to do that I will need an overdrive transmission and/or change the gears in the rear and front axles of the truck. I researched the topic and one thing that kept coming up was that with a small block Mopar engine I could use an overdrive transmission. The problem is  I wanted a time period correct engine, and I did not want a small looking 318 LA engine sitting in the truck’s big engine bay.

I really didn’t want to get rid of the big block but I needed to be practical and focus on my end goal. I kept running across the old poly head 318 A engine during my research. It was a derivitive of the early hemi engines. It had a similar rocker arm design with the canted valves and a polyspherical shaped combustion chamber as opposed to the full hemisphere design found on hemi engines. Bcause of the poly head engine design the heads are large giving it the appearance of a big block engine. Some folks refer to the 318 poly as a wide block or even a big block 318, but it’s not, it’s just that the larger heads make it much wider.

The A Series 318 Poly Head Engine:

The A-series 318 Poly Head V8 engine gets its name because the combustion chambers are Polyspherical in design. The hemi designed engine was complex, expensive to build and heavy. To address these issues and concerns Chrysler designed and used a polyspherical shaped cumbustion chamber. What this did was allow the Poly head engine to use a single rocker shaft to operate the valves on either side. The rocker shaft was centered between the valves with the exhaust valve on a vertical plane and the intake valve canted toward the carburetor, like its larger hemi brother. This design positioned the spark plug in a prime location in the combustion chamber and allowed for larger valves and more efficient breathing. The Poly head engine is easily identified by its valve covers scalloped design to fit around the exhaust valve springs on the exhaust sides of the engine. The 318 A series Poly head engine was produced from 57 through 67 when it was replaced with the LA series 318 engine. The engine displaces 318 CI with a 3.91″ bore and the 3.31″ stroke.This head & engine design was coined Semi Hemi by Mopar enthusiasts.   
318 poly head

318 poly head

 When these engines were produced they came equipped with 2-bbl carburetors, not really taking advantage of the hemi type rocker arm design and polyspherical combustion chamber. Sadly they never viewed these poly head engines as performance engines so there is very little in the way of performance parts available for these engines. But when I saw some articles about these poly head engines being stroked to 402 CI I knew this was indeed a special engine. Further research revealed that there was a hard-to-find Edlebrock three deuce manifold set-up for these old poly engines. I really liked that too. I have always regretted selling my numbers mathcing 70 Plymouth Duster with a 340 six-pack, so here was my chance to have a 318 A poly head engine with a 6-pack.

Edlebrock P600 Intake

Edlebrock P600 Intake

Not only are the six-pack mainifolds hard to find, they are expensive when you do find one, and that’s without the carburetors, linkage and fuel block.  So now I was going from a perfectly good running 383 to finding and rebuilding a 318 poly engine, adding a hard to find and expensive six-pack set-up and finding a suitable transmission that would solve my original problem of running at high rpms and 10 mpg fuel economy. My budget would surely suffer from this new almost crazy idea, but on the other hand if you’re doing a build-up and plan to keep the vehicle why not do it the way you want it!

318 poly engine

318 poly engine

I I searched Craigslist for an old poly head engine and finally found one about 3 1/2 hours from where I live. It was a complete engine and the owner was asking $550 for it. I cut-to-the-chase and offered $350 firm. He needed the money and said to come get it. I probably over paid for the engine at $350 but not many poly engines were advertised. Next I found the rare edlebrock three deuce intake manifold and they were asking $1,200 for it with three old ford 2-bbl carbs on it. I noticed the ad had been up for quite some time so I offered $450 cash for the manifold without the carburetors. They countered at $650. I told him I had $500 cash and he said come get it. I was already $850 into this 318 poly engine and didn’t even start on the rebuild.

The thing I like about the new induction set-up is that I will be able to drive the truck on the center 2-bbl carburetor helping to increase the fuel economy, but when I wanted more power it would be there. Not to mention the wow factor when you raise the hood!

After I bought the engine and manifold I started making some phone calls about the transmission and axles. I would need a transmission that would bolt up to a small block engine, and I really want an overdrive transmission. I spoke to a knowledgeable guy from a local performance shop about the axles and transmission.  When I told him I wanted to lower the rpms and increase fuel economy, but still be able to tow the restored vintage travel trailer he said there would have to be some compromises, and I knew that. My options were to use a 518 overdrive transmission (a little pricey) or a 904 transmission with a lower 1st gear ( I believe he said it was a 998), but this was a lighter-duty transmission. My other option was to change the gears in the front and rear axles. He told me I could use 3:55 gears in the axles, which I liked for everyday driving and occasional towing, and he seemed to think the combination of the transmission and new gear ratio could lower the rpms to about 2,000 at 60-65 mph. He also said a 727 transmission  was more heavy duty, but it would not solve the problems I was trying to solve. From a budget standpoint the A518 overdrive would be costly and when you add the converter and no core to turn in the price keeps going up. I will need to think about the transmission, but I definately want to move forward with changing the gears in the axles.

This engine swap was getting more expensive by the minute, but hind-sight is always 20/20.

Join me next time when we dig deeper into the 318 poly head engine. For a list of 318 poly head engine resources click here: 318 A Poly Head Engine Resources Page

Click here to go to the next post:  71 Power Wagon Build-Up – No Turning Back!

Mark Polk

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