Modified Mustangs & FordsHow To Engine
Boss 429 Engine Build - Meet The New Boss
Exploring the unrealized potential of the classic Boss 429 to build 670 horsepower
The BasicsThe 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 PortsBrown 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."