Using MP's 4-inch crank and...
Using MP's 4-inch crank and an .090 overbore yielded 402 cid. Minor block clearance mods were required, and the block was machined for race-type performance.
The plan was simple enough: build a boatload of torque with adequate horsepower to transform a 318 Poly engine into a mill capable of quickly hauling a '56 Plymouth Savoy down the boulevard. Adding such under-the hood attitude to Mopar's Rodney Dangerfield proved to be easier than you'd think. As promised in Mopar Muscle last month, we'll outline the parts, procedure, and power mods to our Poly, resulting in a 402-inch, stump-pulling stroker that gets you the respect Rodney himself never received.
The Teardown, Inspection, and Parts Selection
We started out with a '62 Poly 2-bbl engine whose price was right: free! The block was solid, but it was completely wasted with holes the size of Texas in every piston.
"Hey, fella, you want this engine for free?"
"A junky, old 318 Poly motor? You betcha!"
After gently persuading the seized pistons out of the egg-shaped cylinder bores (with a 6-pound sledgehammer and a two-by-four), the engine was completely disassembled and hot-tanked. The engine block and heads were then magna-fluxed for cracks and passed the test, a testimony to the stoutness of these motors given the severe abuse this engine must have seen. In fact, we had heard that hot-rodders of yore would bore these old Polys to .090 or more for increased displacement, meaning the stock 318 Poly bore size of 3.91 moved up to an even 4.0 size! Coupled with the brand-new Mopar Performance 4-inch stroker crank for the LA 318/340 motor (PN P5007256), which is interchangeable with the Poly 318, this combo would thus yield a 4-inch bore by 4-inch stroke for a monsterous 402ci Poly motor.
Can the block take the huge overbore? After we sonic-tested the block, it proved this could be accomplished with cylinder wall to spare. All machine work, engine assembly, and dyno-testing was meticulously performed by Ed Hale High Performance Engines in Lakeside, California, under the watchful eyes of shop foreman Gary Camp and me. The added support of head-flow testing and port work was assisted by Dr. John of Total Performance in Santee, California.
Workin' It
The block was bored and honed using deck plates to a .090 overbore, then fully decked to level all surfaces. The deck height on our '62 block spec'd out at 8.248. It also received a full-align hone for straightness and was fitted with ARP main and head studs for added rigidity and durability. In fact, Al Nichols at the ARP special-order department [(805) 525-1497] was a lifesaver in supplying the stud parts from other applications to fit our Poly specs.
The Poly 318 main caps were shot-peened for fun, although aftermarket steel four-bolt main caps, heavy-duty iron caps, engine parts, and 318 Poly tech services can be purchased through RAM Racing Engines. Bill Richardson of RAM is a 318 Poly guru when it comes to polyspherical speed needs. The stock 318 rods were also exchanged for heavier duty 360 rods, which were shot-peened, polished, and fully bushed for the added demands of stroker duty. ARP connecting rod bolts (PN ARP 144-6001) and hardened Clevite rod bearings (PN CB481H) were used for peace of mind. For the proper squeeze, JE Pistons supplied a great set of forged 10.7:1 aluminum pistons made to our Poly specs. These babies were lightweights at 473 grams each, a full 100-plus grams less than the stock cast slugs. Installing the pistons with the tops down in the hole .010 inch with a 74cc chamber yielded a final compression ratio of 10.1:1, just right for street duty on 92-octane pump gas with a healthy cam. A small .927 pin and Childs & Albert Dura Moly rings (PN 9343-35) completed the package. The rotary weight tipped the scales at a mere 1,108 grams, which should make the fat Poly stroker rev like a small-inch small-block!