Mustang MonthlyHow To Engine
Boss 351 Block Buildup - Who's The Boss?
Was The Boss 351 The Best Boss Street Engine? We Put One To The Dyno Test
The crowning glory of the Boss 351 was obviously the Cleveland cylinder heads. For our Boss 351, the heads came from a '71 4V engine, which featured the smaller quench chambers but lacked the adjustable valvetrain of the original Boss 351 heads. This was easily cured by the good folks at L&R, as the non-adjustable pedestals were machined to accept the adjustable Boss rocker assembly, allowing us to run the solid-lifter Boss 351 cam. The 351C heads were also treated to new stainless steel intake and exhaust valves courtesy of Pro Comp. We replaced the factory multi-groove valves with the more common single-groove variety, milled the heads slightly to achieve the desired 63cc combustion chambers, and treated the heads and valves to a production-style valve job. No tricks, just an honest representation of what the factory would have done back in 1971. The heads were installed using Fel Pro head gaskets and ARP hardware. Fel Pro also supplied the intake gaskets as well.
What is a dyno test without a little controversy? In our case, loyal Mustang enthusiasts will naturally bemoan the induction system on this test. Since the factory Boss 351 intake proved to be elusive, we were forced to run a factory cast-iron alternative. We doubt there would be much difference between the two intakes, but use of the cast-iron intake allowed us to run the Boss 351 with a Holley carb rather than the factory Autolite. Back in the early '70s, Ford actually offered an over-the-counter version of the Boss 351 intake designed for use with a square-bore Holley carb. That we ran the '71 LT-1 and Boss 302 motors used for comparison with the came carburetor means we at least had a level playing field.
Would there be a big difference in power between the Autolite and the Holley on the Boss 351? While we'd like to run a back-to-back test, the reality is that the difference would be minimal once both carbs were tuned to provide the ideal air-fuel ratio, tuning we naturally performed on the dyno. Need more ammo? The factory distributor was also replaced with a modern electronic equivalent. Again, no real performance advantage other than eliminating misfires-something the stock distributor was perfectly capable of.
Prior to 1972, muscle car engines were rated for power in optimized conditions, meaning on the engine dyno sans accessories and tuned to perfection. Sometimes the ratings were more a function of marketing or the desire to place an engine favorably in a given class of racing. Some were underrated and some overrated to suit the situation. The 330hp rating given to the '71 Boss 351 was pre-SAE standards, meaning the power rating was given basically under the conditions we ran during our test. The only difference was that our Boss was run with a set of long-tube headers. The Boss 351 was also equipped for dyno use with a Meziere electric water pump.
After two break-in cycles (programmed by the Super-Flow engine dyno), the Boss 351 was finally run in anger. Equipped with the factory iron intake and Holley 750 HP street carburetor, the Boss 351 pumped out 383 hp at 6,100 rpm and 391 ft. lb. of torque at 4,000 rpm. Torque production exceeded 350 ft. lb from 3,000 rpm (and likely even lower) to 5,700 rpm.
Run under identical conditions, our Boss 351 easily out-powered the smaller Boss 302 and both the '70 and '71 LT-1 variants from Chevy. Run in exactly the same configuration, the Boss 302 produced 374 hp and 324 ft. lb. of torque. Down in the lower rev range, the Boss 351 offered an additional 80 ft. lb. of torque over the smaller Boss 302. When compared to the Chevys, the Boss 351 offered more power than any of the general's small-blocks. We know this because we recently ran the '65 L76 and L84 (365 and 375hp 327s), a Z/28 302, and both the '70 and '71 LT-1s for our sister magazine, Super Chevy. Even the legendary Fuelie 327 produced less power than the Boss 351.