1965 Ford Mustang Convertible Gets 331 Ford Small Block
Disguised As A Stock 289, This Stroker 331 Ford Small-Block Makes Well Over 400 Horsepower
The element of surprise is a tried and true military method of winning battles. So when retired Air Force fighter pilot Rod Peck decided to rebuild the 289 in his '65 Mustang convertible, he wanted it to look stock but have lots of hidden horsepower.
The need for having the engine remain stock in appearance negated the use of a typical crate engine, so the idea led Peck to J Bittle American Performance Engines. Bittle's AntiCrate philosophy of building custom, blueprinted engines for specific vehicle applications was a perfect match for Peck's goal. J Bittle American designed a healthy combination of parts that would turn the tired 289 into a 331 cubic-inch stroker while making the engine appear as stock as possible. Another goal for this engine was to make more than one horsepower per cubic-inch, which required JBA's Stage II level of performance engine modifications and parts selection.
Why was 331 cubic-inches the choice for this particular application? Generating more than one horsepower per cubic-inch from a 289 requires larger displacement and a longer stroke. The 331 combo works well because it incorporates a specific rod-length-to-stroke ratio that makes for a well-balanced package that's closest to the ratio of a stock 289. When you divide the rod length by the crankshaft stroke, you get the engine's rod-length-to-stroke ratio. As the length of the connecting rod and the stroke of the crankshaft increases, the rod-length-to-stroke ratios decrease, which adds more piston load on the thrust side of the cylinder wall. This increased angle of thrust leads to excessive bore wear-not good for the longevity of an engine.
In this example, the factory 289, with a 3.00-inch stroke and 5.090-inch connecting rod, has a rod-length-to-stroke ratio of 1.69. The 331 uses a 3.250-inch stroke that isn't much longer than stock, but combines it with a longer 5.400-inch rod that yields a 1.66 ratio. This is close to the factory combination and makes for a well-balanced package. While it's easy to see that this new rotating assembly combination is a very tight fit, it doesn't require additional expense in machining portions of the engine block for clearance, which also makes it a less-expensive choice.
Upgrading The Block
Aside from the longer crankshaft stroke and connecting rods, the block's cylinder bores must also be enlarged. The stock 289 features a 4.00-inch bore; Peck's 289 block was already over-bored to .040. While in most cases you can use a fac-tory 289 block, the cylinder walls in Peck's engine were already too thin. Because of this, J Bittle American opted not to use the original block and swapped it out for a late-model 302 roller camshaft block that was machined .030-over to achieve the final 4.030-inch bore. The block also features a one-piece rear main seal and now incorporates a Scat cast-steel crankshaft and I-beam connecting rods that also help to strengthen the engine's bottom end.
Because of the larger bore and the fact that the engine was expected to achieve horsepower levels approaching 400 and possibly greater, a set of Ross aluminum, forged, dish-top pistons were used in conjunction with a set of aluminum AFR 165cc CNC Outlaw Street cylinder heads. The combination of the AFR heads, which incorporate 58cc combustion chambers, along with the Ross pistons and Total Seal plasma moly rings, will yield a 9.8 to 10:1 compression ratio, making sure that this engine combination can make plenty of power on standard octane pump gas.