Mustang Monthly

Boss 351 Block Buildup - Who's The Boss?

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.