Boss 429 Engine Build - Meet The New Boss

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Exploring the unrealized potential of the classic Boss 429 to build 670 horsepower

From the July, 2011 issue of Modified Mustangs & Fords / By Barry Kluczyk / Photography by By The Author

Like those classic Greek stories you read in school, the mythology surrounding classic muscle cars often strays quite far from reality--and there is perhaps no better example than the Boss 429 Mustang.

The mere mention of the Boss '9 and its infamous semi-hemi "Shotgun" motor evokes knowing nods of impressed enthusiasts. That perception is backed up with stellar values for original-condition models sold in the collector market. Unfortunately, the out-of-the-box performance of the cars has never measured up to the hype surrounding them.

Like numerous other engines of the muscle car era, the Boss 429 was originally developed as a racing engine and subsequently detuned for the street versions typically required for homologation. That meant compromises. Stuffing it into the tight confines of the Mustang engine compartment, for example, required saddling it with a slab of restrictive iron to serve as a low-rise intake. It effectively choked off airflow to the cavernous cylinder heads that were simply way too large for a street engine. Consequently, there was no velocity through the ports and low-rpm torque was almost non-existent. Another strike against the engine was a set of crimped exhaust manifolds that were designed to fit the Mustang's engine compartment rather than optimize airflow.

To address complaints, Ford instituted a midstream change from an admittedly "small" hydraulic cam to a hotter solid-lifter setup from the 429 Super Cobra Jet, but it did little to improve the situation. The early hydraulic engines are known by their 820-S engine code, while the later engines are known by the 820-T code (see sidebar). It's true that when wound up, the Boss 429 engine would pull strongly--until the factory rev limiter shut things down--but in most daily driving situations, the Boss 429 was a rather anemic street-performance engine. And a heavy one, at that.
On paper, however, the Shotgun Ford has the potential to deliver big power numbers if it could only capitalize on its attributes. Engine builders John Lohone and Adney Brown recently took up the challenge to see what a properly prepared Boss 429 street engine could do when its known deficiencies were addressed.

"With those mile-wide valve covers and distinctive style, the Boss 429 should be a great option for a Ford enthusiast wanting an alternative to cookie-cutter Windsor engines," said Brown, of Detroit-area Performance Crankshaft, Inc. (www.performance crankshaft.com). "We thought if we could take an engine beyond the 600-horsepower level, but with good low-speed and idle characteristics, we'd have a great, contemporary combination that's competitive with modern crate engines, but one that's going to draw some 'ooohs' and 'ahhhs' when the hood is lifted."

Brown partnered with Lohone to help develop and assemble the engine, and right off the bat, they knew using an original Boss 429 block and heads was out of the question. The specific, thin-wall casting of the Boss 429 block is all but impossible to find and, given the collector value of restored cars, makes the few out there almost as expensive as the total investment in this entire project. The same goes for a set of original heads.

Fortunately, a strong block alternative is available in the Ford Racing catalog, with the basic 460 cylinder block (PN M-6010-A460). Ford engine guru Jon Kaase casts his own Boss 429 heads--appropriately named "Boss Nine"--to fit the 460 block. That's significant, because the oiling circuits were different on wedge-head blocks and the Boss versions, resulting in different oil drain holes. Kaase's heads match the oil drain holes of production-style 429/460 blocks.

Although the project engine uses all new parts for the major components, the basic parameters are very similar to the original Boss 429. Therefore, a comparison of the original production engine and this 21st century example is appropriate. In fact, the new engine uses a host of forged, heavy-duty parts, but so did the original engine--including the crankshaft, rods, and pistons.
"The bore is a little larger, but the stroke, compression, and basic setup of the engine is very similar to the original," says Brown. "We hoped to make the most of its truly impressive specifications."

The Basics

The Ford Racing 460 block has a 10.320-inch deck height, which is the same as the original Boss 429. The bores were machined to 4.375 inches, and a 3.590-inch-stroke forged crank was used, giving the engine a 432-cubic-inch displacement. For comparison, the original Boss '9 engine used 4.360-inch bores and the same 3.590-inch stroke. Diamond forged aluminum pistons are used on the project engine and deliver a 10.5:1 compression ratio that is significantly lower than the original engine's approximately 11.3:1 ratio.

The Kaase heads' ports mostly mimic the production Boss 429 design, but with subtle improvements that help make them stronger and, of course, fit the 460 block. The biggest difference is the combustion chamber design. The original Ford head got its "semi-hemi" nickname from a chamber configuration that was based on a true hemispherical design, but with filled-in sides that provided better quench. The Kaase head design eliminates the semi-hemi combustion chamber and replaces it with a more conventional, fast-burn-style chamber that exhibits more efficient and faster burn characteristics. It also is designed to use "regular," non-Boss 429/460-style head gaskets. The original heads used O-ring-style gaskets around each cylinder.

Large, 2.300-inch intake and 1.900-inch exhaust valves are used in the Kaase heads--the intakes are only 0.02-inch larger in diameter than the ones used in the OE heads, while the exhaust valve size is the same. Brown and Lohone used a flat tappet camshaft, like later Boss 429 engines, to actuate the valves. They specified a Comp Cams grind that delivers a whopping 0.650-inch lift on both sides, with 251 degrees of duration, also on both sides. That's a huge difference from the 0.478-inch/0.505-inch cam used on the later-style, solid-lifter production engines. As the builders would quickly find out, bigger wasn't necessarily better.

Really Big Ports

Brown and Lohone discovered the engine's displacement wasn't enough to satisfy the capability of the cylinder heads.
"The heads are modified when compared with original Ford heads, but they're very similar in design--especially in the intake ports--and we found they're just too darn big for an engine of this displacement," says John Lohone. "To build low-rpm power, we cut down the intake runners' volume by about 35 percent, and they still were too big, flowing more than 400 cfm." Sure, it would have been relatively easy to stretch the bore and stroke to accommodate the heads' capabilities, but the project's aim was to build power within the range of the original engine's size. Their experience demonstrated why the factory versions left much to be desired on the street.
"You just can't adequately fill the ports at low rpm with those big heads," says Brown. "What you really need is about another 100 cubic inches of displacement to process what the heads are capable of flowing."

Despite the challenging combination, the builders nonetheless achieved eyebrow-raising results after experimenting with a couple of different camshafts and the aforementioned squeeze-down of the heads' intake ports. They topped the engine with a Jon Kaase single-plane, spider-type high-rise intake manifold (with welded-in "wings" to effectively lengthen the interior runners), a 1-inch double-tapered spacer and a 1,050-cfm Quick Fuel-built, Dominator-style carburetor. The factory Boss 429, of course, used a lower-rise, dual-plane intake and a much smaller, 735-cfm carburetor.
"We even looked at the original-style, NASCAR-style intake and while it appears impressive, it's totally wrong for a street engine," says Lohone. "It just doesn't flow air at low speed, period." The velocity afforded by the high-rise intake absolutely benefited the engine at higher rpm, but like the production engine, low-rpm power was relatively weak. During testing, the engine didn't produce 300 horsepower until 3,500 rpm, although torque was better than 430 lb-ft at only 2,800 rpm.
"There's no getting around those big heads," says Lohone. "Without larger displacement, there's a limit to producing low-rpm power that is still streetable."

Nevertheless, in the mid- and upper ranges of the rpm band, this new-century Boss engine performed admirably. Brown and Lohone experimented with camshafts, header designs, and more on the dyno at Jim Kid Motorsports (www.jimkidmotorsports.com), in a give-and-take learning session that saw peak horsepower and torque numbers vary widely. The best result they saw delivered 670 peak horsepower at 6,400 rpm and 556 lb-ft of torque at 5,600 rpm, with 32 degrees of total timing.

"It's a hell of a street engine, no question about it," said Lohone. "From 3,500 rpm to 6,500 rpm--right where you want a street engine to perform--it pulls strong and smooth. It ought to put a '69 Mustang easily into the 10s."
Despite their engine's more-than-respectable performance, Lohone and Brown were left wanting more from it.

"Ford had a great design with the Boss 429 heads, but they were never used anywhere near their potential on the street," says Brown. "As the dyno results show, our procedures helped bring out more of the power, but there's still a lot left in there--and a roller cam would have easily pushed horsepower past the 800 level." The builders proved their point with this project, realizing much of the pent-up potential the original engine packed under its distinct valve covers. Next step: The Boss 529!

Quick History: Early vs. Late Boss 429s

There were two versions of the Boss 429 production engine, both rated at 375 horsepower. An updated engine was introduced during the '69 model-year run with changes designed to boost its anemic feel. Starting around the assembly of car No. 0280--still within the '69 model year--a higher-lift, solid-lifter cam from the 429 SCJ engine replaced the smaller, hydraulic camshaft that was installed at the start of production. The rod fasteners were also changed to ARP cap-style bolts.
The update delivered slightly better low-speed characteristics, but didn't radically alter the car's performance. Ford also specified a shorter, 3.90 axle ratio that also improved low-speed acceleration. The early, hydraulic-cammed engines are recognized by an 820-S engine code and magnesium valve covers, while the later engines (the vast majority of production models) had an 820-T code and aluminum valve covers.

A Badass Boss Project

In the nondescript shop for his concrete business, enthusiast Tom Marcucci is working on an interesting Boss 429-based project--he's shoehorning the engine into a classic '72 Gran Torino fastback. Purists may cringe to learn the project car is a virgin, 20,000-mile car in beautiful, unrestored condition, but that clearly doesn't bother Marcucci, who's plowing ahead with the swap. Although the assembly isn't an original Shotgun engine, the heads are original Boss '9 parts. The plan is to back the engine with a T-56 six-speed. Not surprisingly, Marcucci cites the need for scratch-built headers among his biggest challenges. We plan to check the progress of the project and grab some shots when his Boss Torino hits the street.

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