Posted by: Mark Polk | 2013

Making 300 HP on 318 Poly Engine Rebuild

disassembled 318 poly

disassembled 318 poly

I dropped the engine off at the machine shop and was waiting to get measurements so I could order the pistons, rings and bearings for the 318 Poly engine rebuild. When they contacted me they told me the cylinders had already been bored at some point in time and would need to be bored to .060 over. That could be bad news for many engines since .040 is about the most you would want to bore the cylinders, but these old 318 poly engines can go to .060 with little concern. Hopefully this is the last time this old engine will need to be rebuilt!

 With the news from the machine shop I was curious about what the new displacement would be for the engine. Math was never my strong suit, but the formula for this is pretty straight forward. Here is the formula I used to figure the new Cubic Inch Displacement after boring the cylinders .060 over.
Pi X radius squared X stroke X # of cylinders


Pi = 3.1417

Radius squared (radius is 1/2 of the diameter, so for an engine it is 1/2 the bore size squared) = ½ the bore size X ½ the bore size

318 Poly engine stroke = 3.31 in

318 poly engine = 8 cylinders

But before I could apply the formula I needed to figure the new bore size at .060 over. A stock 318 Poly engine has a 3.910 bore size + .060 over = 3.97 new bore size

So our formula is 3.1417 X 1.985 X 1.985 X 3.31 X 8 = 327.79 or 328 Cubic Inch Displacement

Increasing the Cubic Inch Displacement (CID) by boring the cylinders does not increase the HP much at all. Granted a large CI engine has more HP than a smaller CI engine, but boring the cylinders is more about the volume displacement inside the cylinders from Top Dead Center (TDC) to Bottom Dead Center (BDC).

 Out of curiosity I wanted to see how adding 10 Cubic Inches to my engine did affect the HP. My stock 1963 318 poly engine was rated at 230 HP with a 2 bbl carburetor.
The easiest way I could determine to figure this was to take the new CID and divide it by the old CID to see what the increase was. So if the stock 318 poly engine produced 230 HP and I increase this by the change in CID I will have an idea of what the added 10 Cubic Inches equates to in HP
So, if I divide the new 328 CID by the old 318 CID I get an increase of 1.0314.  Now if I multiply the old HP rating of 230 by the 1.0314 increase I get 237 HP. So, adding 10 CI to the volume of the cylinders increases my HP by 7.
Keep in mind that my goal is to drive this truck when it’s finished. I want to lower the RPMs and improve the fuel economy. To do this my plan is to replace the old 727 3-speed transmission with an overdrive transmission, change the gear sets in the axles to 3:55 and add a 3 deuce induction set-up. This way I can run on a center 2 bbl carburetor for fuel economy, but have 4 more bbls when I want them. I would call my plans for this engine a mild street engine build.
I would like to make 300 HP with this rebuild, and I think that is a realistic number to shoot for. There are lots of variables involved with these calculations and I am not qualified to come up with exact increases in HP but I can estimate. The 262 high performance cam I am adding should gross about 20 HP, the P600 Edlebrock 3-deuce intake with 3-Rochester 2G carburetors should add 25 more, headers and a free flowing dual exhaust is another 10, and the electronic ignition will add a couple more.
Altogether these rough figures put the 318 Poly engine rebuild at 294 HP. Not bad! The only way to really know would be to have it dyno tested, but I am happy knowing it is in the 300 HP range.
Mark Polk


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