KJ Jones Senior Technical Editor
March 1, 2008
Photos By: KJ Jones
Probe Industries owner Chris Huff (left) and one of his technicians hoist our Ford Racing Performance Parts Boss 302 block (PN M-6010-BOSS302) from its crate in preparation for our development of a modern-day version of the Boss 302 engine-one of Blue Oval's baddest small-blocks ever.

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.

Shawn Mendenhall is heading up our project at Probe Industries and will oversee assembly of our Boss 340. The build process includes production of a set of custom flat-top, forged pistons designed to work with the Edelbrock, Air Flow Dynamics, and Comp Cams cylinder-head/camshaft combinations we'll be testing.

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.

Derek Ranney Jr. sets up our block on a Rottler boring machine. Derek is following in the footsteps of his dad, who followed in the footsteps of his father, Larkin Ranney (L&R-get it?), who started the business more than 30 years ago. It's always cool to see this type of service carried on by the younger generation.

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.

Precise measurements are critical for all aspects of an engine build, and being exact matters most when it comes to machining the block. While it's easy enough to remove material, putting it back in the event of removing too much is impossible. Derek Jr. uses micrometers...

In phase two of our project, we'll uphold the OG Boss engine's racing and high-rev legacies by topping our short-block with Air Flow Dynamics' (AFD) Cleveland heads and a sinister solid-roller camshaft from Comp. Australia-based AFD is a new player in the Ford/Cleveland cylinder-head game, and owner Dave Webb is excited about our project. He's confident AFD's heads will bring the engine near the 550hp target we hope to hit when the dyno spins our Boss to the moon (7,500-7,800 rpm) in race trim.

We mentioned at the beginning of this story that today's Boss 340 will include modern-day appointments to set it light years apart from its predecessor. In addition to the aforementioned heads, our engine will sport electronic fuel injection, managed by ACCEL's new Thruster EFI system as opposed to the double-pumper carburetor that many might think is the only applicable induction alternative.

Thankfully, we aren't stuck with using only carbs. Wilson Manifolds is producing intake manifolds, fuel rails, and a four-hole/ carb-style throttle body for both sets of the Boss cylinder heads to sit atop our bullet. We're especially hyped about the manifolds, mainly because they're unequivocally the most important pieces required for building this type of engine. Think about it: When is the last time anyone has seen an EFI intake for an 8.2-deck engine with Cleveland-style heads?

...and a dial-bore gauge to measure our block's bore size. A reading is taken prior to making any cuts to confirm the holes are standard at 4.000 inches. It's done again after a test pass is made in one cylinder.

The manifolds will be available by order and they'll probably be expensive, but until Edelbrock releases its Performer RPM E-Boss manifold for 302-based engines (a 351W intake is currently available for those who wish to build Boss 351 engines), you'll have to get with Wilson to accomplish the same bullet we'll show over the next few months.

That's enough of a preview for now. Believe it or not, we've actually detailed more of our Boss effort than we originally planned. Breaking new ground in our hobby always excites us, so there's really no such things as too much information in this case.

It's time to get with the business at hand, which is preparing our FRPP Boss 302 engine block for assembly. Derek Ranney Sr. and his staff at L&R Automotive Supply in Santa Fe Springs, California, will put our project block through a battery of prep procedures that includes boring the cylinders to their proper size (Boss blocks are shipped at standard 4.000 inches), align-honing the main journals, honing the cylinders, and decking the block. Thorough block-prep is common for a new aftermarket engine block, as it usually requires fine-tuning and finishing in addition to the necessary bore resizing.

The accompanying photos and captions provide a detailed look at L&R's expert preparation or our Boss 302 engine block. Pay close attention: During the prep process, we discovered a few interesting characteristics about the Boss 302 that aren't addressed in Dale's article. Now that many people are thinking about building a 'Stang engine with the new block, its important to know as much as possible about it (see sidebar).

During our visit to L&R Automotive of Santa Fe Springs, California, we asked owner Derek Ranney Sr. for his thoughts on Ford Racing Performance Parts' Boss 302 block and what he believes is an absolute limit for boring and stroking it for maximum performance.

In a side-by-side comparison, we learned that a stock 5.0 block has longer cylinder walls (5 1/8 inches) versus shorter holes (4 3/4 inches) in the Boss. "I think this may have been done (shorter cylinder) to allow for more crankshaft stroke," he says. "However, longer-stroke cranks (3.400) may experience problems that can include pistons coming completely out of the cylinder, piston slap, or abnormal oil consumption due to poor ring seal. We recommend 3.250 as a good maximum stroke for this block."

As far as thickness is concerned, a sonic test of both blocks proved the Boss 302 is far superior to a stocker. It will easily support bore sizes as big as 4.155 to 4.185 inches, despite Ford's advertised 4.125-inch maximum bore size.