The Comeback - Boss 340 Engine Build, Part 1

Horse Sense: Dominate means "to rule over; govern; control." In 1970, the Boss 302 Mustang did just that: Wreaking havoc on the street and dominating the race track with Parnelli Jones behind the wheel of Number 15, the Camaro killer ran away with the '70 SCCA Trans-Am title. Boss 'Stangs benefited from a big-headed, small-block Ford; a four-speed tranny; a 3,300-pound fighting weight; and one incredibly conservative horsepower rating (290 hp by insurance standards versus a suspected 350 hp). Making max power is the primary objective with the Boss engine we're building, so you can bet there won't be anything conservative about the power numbers when we take the revs way past redline.

It's really true what they say: Time flies when you're having fun. Although it feels as though only a month or two has passed since Dale Amy introduced us to Ford Racing Performance Parts' new Boss 302 engine block ("It's Good to Be Boss," Jan. '07, p. 104), in reality, an entire year has passed since we wrote about the legend's return.

Since the release of Dale's expos, we've certainly experienced many good times. We completed one full-build 'Stang project (T-top coupe) and immediately embarked on another (Fox 500). We watched Shelby GT 500s come together before our eyes at Ford's Auto Alliance International assembly plant and had the opportunity to take first looks at killer 'Stangs (such as Don Bowles' S197 Roush test car) and several cool bolt-on parts for '79-to-present Mustangs, Cobras, and Shelbys.

Last year, we also watched closely as 'Stangbangers on Mustang Web forums, and even our brethren from other magazines, began discussing and reporting on engines that were based on the new Boss 302 block (PN M-6010-BOSS302). As engine prognostications generally go, ideas for Boss-founded bullets range from mild (350 naturally aspirated horsepower) to wild (upward of 650 horses with a power adder). That, quite frankly, is the way it should be with a piece like the new Boss.

In his report, Dale explains that the new 5.0 casting is built to take anything, thanks to stout, splayed, nodular-iron, four-bolt main caps (for the second, third, and fourth journals of the crankshaft); the block's trick Siamese bore setup; and extra coolant crossover holes between the bores, which are a must for today's severe-duty, performance-engine applications. The Siamese bore setup avails a lot more block material between each cylinder than what's found on stock 302s, and it opens the door to boring big and achieving serious cubic inches on an 8.2-deck platform.

Sure, the Boss 302 is theoretically back in the form of an engine block, but we're taking its comeback a step further by using it as the foundation for an engine we call "today's Boss 302." Ours will actually displace 340 ci but size is irrelevant, as you'll learn when you read further. Our project engine will somewhat replicate the '70 Boss' general makeup, as the short-block will also be comprised of forged internals such as a crankshaft, rods, and flat-top pistons from Probe Industries.

That's enough for now as far as the textbook basics are concerned. The really big deal about our project is that we're daring to think outside the proverbial box of general small-block builds (331 or 347 strokers). We'll stay true to the Boss attitude by building and testing the engine with contemporary versions of the Boss 302's Cleveland-style/ canted-valve cylinder heads.

That's right-by the time this project is complete, we will present two flavors of a true Boss engine, not just another stroker with inline-valve heads. Our Boss 340 will be built and dyno tested two times, using one dual-purpose short-block to be assembled by Probe Industries.

In phase one of the project, we'll set up the engine with a top-half (heads/cam) package that is pump-gas friendly and perfect for the street, strip, or blasts around the road course. The first Boss 340 will feature Edelbrock's new Performer RPM 351C heads (PN 61629) and a Comp Cams rollerized valvetrain. Details on the heads and cam will be provided in future installments of the series.

For now, we optimistically anticipate roughly 450 crankshaft horsepower (naturally aspirated, using 91-octane pump gas) from this version of the Boss, which will top the original's power production by about 100 ponies.

Original Boss 302s were known for their high-rpm capability, which was no doubt achieved through the large ports, valve sizes, and valve angles of their 351 Cleveland heads. Boss 302s also benefited from high-lift/ long-duration mechanical camshafts. The combination was a major factor in making Boss 302 Mustangs virtually unstoppable on the road course in 1970.