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.
Feeding The Beast
Getting enough air and fuel into a 331 cubic-inch engine would normally require a overly healthy, choppy idle camshaft that might give away the true secret of this stock-looking engine. But with a cylinder block that's set up to use a roller camshaft, J Bittle American's use of a Ford Racing E303 hydraulic roller camshaft and lifters will keep the engine well fed while maintaining a smooth yet performance-like idle. The cam features 220/220 degrees of duration at .050 lift and uses .498/.498-inch of valve lift on a 110-degree lobe separation angle.
The AFR aluminum cylinder heads also received some light porting and port matching, but J Bittle American engine expert J.R. Twedt says that aside from the extra porting, the AFR cylinder head is one of the best performing street heads in the industry and is perfect for this application. Tony Mamo of Air Flow Research concurs and adds that the 165cc heads work well in this application, flowing 250 cfm to make them more than capable of producing power in the mid to high 400 horsepower range. Mamo also suggested that if we wanted to squeeze a bit more out of the engine, the 185cc head might be worth another 30 or more horsepower.
In designing this engine combination, Twedt also decided to use an Edelbrock RPM Air-Gap intake manifold, as it has excellent fuel distribution and runs cooler to provide a more dense air charge with its raised runners and plenum. But while it might be great to show off a set of aluminum aftermarket pieces like these, the goal here was stealth. In this case, the names on both the cylinder heads and intake were machined away. To further add to the engine's stealth, after it was assembled, the team at JBA painted everything Ford black to match the correct engine color for Peck's '65 Mustang Convertible.
Big Power, Small Package
Because the engine was to remain as stock-looking as possible, the factory Autolite 4100 carburetor was used during the engine's break-in and initial dyno tests. The Autolite four-barrel carburetor is Peck's original equipment on the '65 Mustang and it was in perfect working condition. Even though the carb is estimated to flow about 450 cfm, it allowed the 331 to crank out nearly 400 horsepower on JBA's DTS engine dyno.
By adding 36 degrees of total advanced timing from the PerTronix electronic distributor and outfitting the Autolite carburetor with 68/78 jets and a 6.5 power valve, the engine managed to achieve 418.5 corrected horsepower at 6,300 rpm and 392.9 corrected lbs. ft, of torque at 4,500 rpms. This stock-looking 289, which originally made 225 horsepower, now packs a big punch that will leave many people wondering how they got beat by a "restored" 289 cubic-inch small-block in a '65 Mustang.