Richard Holdener
December 21, 2009

When you mention the word Boss around almost any Ford fanatic, chances are the first thing that comes to mind is the Boss 302, usually followed by the Boss 429. Rounding out the top three (if it comes up at all) is the Boss 351. This seems odd as the Boss 351 is without a doubt the best street engine of the Boss trio and is arguably the most powerful small-block ever offered by Ford. Sure, the later Modular engine easily topped the '71 Boss 351 for sheer output and performance, but it took nearly 30 years to topple the king.

Blessed with essentially the same cylinder heads and cam profile as the Boss 302, the Boss 351's additional displacement and compression allowed the engine to take advantage of the huge port volumes and flow potential of the Cleveland heads without resorting to stratospheric engine speeds. Let's face it, the Boss 302 was built strictly to legalize the combination for the hotly contested Trans-Am series. Although effective at high rpm for racing, it was not an ideal street combination. Ditto for the stock Boss 429. Built for NASCAR, the Boss 429 was not much of a performer when strangled by the smallish four-barrel carb and stock exhaust system.

There are a couple of reasons why the Boss 351 takes a backseat to the more popular Boss 302 and 429 models. For one, Ford spent a lot of money racing and promoting the Boss 302 in Trans-Am. Books have been written about the exploits of the smaller Boss engine in the hands of Parnelli Jones and George Follmer. Their 1970 Trans-Am championship in Bud Moore Mustangs cemented a place in automotive history for the Boss 302, not to mention a place in the hearts of blue-blooded Mustang owners. When the SCCA lifted the restriction on destroking factory engines in 1970, Chevy responded by offering the larger 350ci LT-1 in the new-for-'70 Z/28. Ford followed suit in 1971 by offering the similar-sized Boss 351.

While most enthusiasts of the muscle car era will tell you that 1970 was officially the end of the high-compression small-blocks, the '71 Boss 351 was still sporting 11.0:1 compression (Chevy dropped the compression of the '71 LT-1 to 9.0:1). Historians also consider the 1970 season to be the high-water mark of the Trans-Am series. The new styling of the '71 Mustang seemed to be a love-it or hate-it affair for Mustang enthusiasts. Combine this with ever-tightening emissions regulations, rising fuel prices, and the upcoming fuel shortages and it is easy to see why the Boss 351 takes a backseat to its more popular brethren.

Despite what amounts to a public relations problem, the Boss 351 has all the credentials of a true muscle car. That it was born in 1971 instead of 1969 means it will forever fight the uphill battle for recognition, but that doesn't make it any less of a solid performer. Compared spec to spec, the Boss 351 easily out-performed the more popular Boss 302. The two shared identical cam specs and head flow, but the Boss 351 offered higher static compression (11.0:1 vs. 10.5:1) and more sheer displacement.

Both Boss combinations offered aluminum high-rise, dual-plane intake manifolds, but the Boss 302 was topped off by a 780 Holley where the Boss 351 intake was sporting an Autolite 4300-D carburetor. The higher compression and increase in displacement was the main advantage for the Boss 351. The Cleveland heads were designed with racing in mind, but this made them less than ideal for a small-displacement street engine. The 351 was much better suited to take advantage of the port volume and ultimate flow potential. On the street, torque is king and the extra 1/2-inch stroke offered as much as 80 ft. lb. over the smaller 302 in the lower rev ranges. Even if both engines produced the same peak power, it is hard to ignore the extra 80 ft. lbs. of torque while accelerating through the usable rev range.

Just how good was the Boss 351, and more importantly how did it compare to the Boss 302 and its cross-town rival, the '71 LT-1? Short of joining Peabody and Sherman in the Way-Back Machine, it would be difficult to measure the off-the-assembly-line performance of a Boss 302, Boss 351, or LT-1. Since we lack the talent to facilitate time travel, we decided to do the next best thing and build a replica of a Boss 351 and run it on the dyno. Only the dyno could tell us whether the Boss 351 really produced the rated 330 hp.

It should be noted that production tolerances varied significantly on these engines and the power outputs followed suit. It was not uncommon for the power output of two seemingly identical production engines to vary by as much as 10 percent. Variables that affected output included (but were not limited to) ring seal, head flow (from core shift in the castings), valve jobs (affects both flow and sealing), cam timing (advanced or retarded), valve lash (tighter will improve top end at the expense of low speed power), carb tuning, ignition curve, and maximum advance, to name just a few. Naturally, all of these variables were supposed to be exactly the same, but there were varying degrees of the word "exactly" back on the production line in the early '70s.

Since the exact specifications of the Boss 351 are a matter of record, you might be tempted to think that reproducing the engine would not be terribly difficult. Unfortunately, you can no longer drop by your local Ford dealer and order up a new intake and carburetor for the '71 Boss 351 engine. These are now in the hands of collectors, and given the rarity, they command a premium price. According to the guys at Pony Carburetors, an original Autolite carburetor for the Boss 351 will fetch a cool $4,000, a pricey sum considering that the original sticker price on the Boss 351 Mustang was $4,200. Rather than go the numbers-matching route, we chose to build a suitable duplicate, meaning all of the critical components, including compression, cam timing, and head flow were replicated. This meant building an 11.0:1 Cleveland short-block, adding the proper reproduction Boss cam, and topping it off with a set of quench-chamber 351C heads. Our plan was to run the original Boss 351 (aluminum 4V spread-bore) intake and reproduction Autolite four-barrel carb courtesy of Pony Carburetors, but we were unable to locate (for loan or purchase) a suitable Boss 351 intake.

As opposed to using an original (and expensive) Boss 351 four-bolt main block, our reproduction test engine started out as a two-bolt, 4V Cleveland block before being treated to machine work courtesy of L&R Engines in Sante Fe Springs, California. The block was bored .030-over to make room for a set of Probe forged pistons, which feature small domes to help elevate the static compression to the factory 11.0:1 mark. Actually, the compression was a tad higher thanks to the .030-over bore. The new forged pistons were slung on a set of stock connecting rods, which were polished, shot-peened, and treated to new ARP rod bolts. The upgraded rods and new forged pistons were run on a cast-iron 351C crank. The std/std crank was polished and, after balancing, the entire reciprocating assembly installed using new Clevite rod and main bearings.

A reproduction Boss 351 solid-lifter cam, courtesy of Schneider cams, was installed into the short-block. Basically, it's a duplicate of the Boss 302 grind placed on the 351C cam core. Contrary to some information, the Boss 302 and 351 cams did not offer .477-inch lift; instead, they measured .515 (298 lobe lift x 1.73 rocker ratio). The .477 lift value often associated with the Boss cams is a result of multiplying the .298 lobe lift by the Windsor 1.6 rocker ratio. The 290 degrees of advertised duration resulted in 230 degrees of duration when measured at .050.

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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.

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While the Boss 351 might always take a backseat to the Boss 302 in terms of popularity, at least 351 owners can hold their heads up high knowing they are driving the most powerful factory small-block ever offered by Ford.

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