Mustang MonthlyHow To Engine
How to: Build a 429 Super Cobra Jet
We get 467 horsepower from an iron-head 385-series-big-block
At 375 horsepower and 450 lb-ft of torque, the 385-series 429ci big-block was the most powerful factory engine ever installed in a first generation Mustang. Ford initially used the short-block with hemi heads in '69 to conceive the limited production Boss 429 Mustang. However, the Boss 429 was not for everyone. For '71, the 429 got wedge heads for two forms of Mustang rocket propulsion—Cobra Jet and Super Cobra Jet.
The 385-series big-block was Ford's response to Chevrolet's 396, 427, and 454ci big-blocks, conceived to replace the FE-series 390, 427, and 428ci big-blocks along with the heavy and outdated 462ci MEL (Mercury, Edsel, Lincoln) V-8 in 1968. The 429 and 460ci big-blocks were a new generation of large-cube engines developed for both luxury and performance. They employed a lightweight casting technique with a no-skirt block and made huge power with poly-angle valve heads.
The '71 429 Cobra Jet hit the option sheet with an aggressive flat-tappet hydraulic cam, stamped bolt/fulcrum rockers, 715-cfm Rochester Quadrajet carburetion, dual-plane "spread-bore" iron manifold, four-bolt main block, and large-port cylinder heads to make a factory rated 370 horsepower and 450 lb-ft of torque. You might ask why Ford looked to General Motors for a carburetor when it had the Autolite 4300 and 4300D available. It boiled down to tougher emission standards and the absence of a large enough Autolite four-barrel for a 429 Cobra Jet.
The mechanical lifter 429 Super Cobra Jet was the second punch response to Chevrolet's 450-horse 454ci LS6 big-block. It added an aggressive mechanical cam, adjustable stud rockers, and 4150 Holley carburetion. Like the big Chevy, Ford's 429 SCJ was underrated at 375 horsepower. According to Ford insiders, the goal was to take this engine to 500-plus cubic-inches had the battle for street dominance not ended in 1970. Detroit turned down the wick when focus shifted from racing and performance to safety, emissions, and fuel economy.
There are few things Jim Grubbs of JGM Performance Engineering loves more than 385-series 429/460ci big-blocks. They make big low- and mid-range torque for luxury cars and trucks, plus they can be dialed in for horsepower in a Mustang. Jim wanted to know how much power could be made from a 429 Super Cobra Jet with factory iron heads and intake along with a stock Crane flat-tappet hydraulic camshaft.
Grubbs has always known the great potential of Ford's 385-series big-block and has collected a treasure trove of big-block Ford parts through the years. In his arsenal was a 429 Cobra Jet block with four-bolt mains, thick webs, 4.360-inch bores, and 10.310-inch deck height. He also had 429 Cobra Jet heads with 2.19/1.73-inch intake/exhaust valves, 72-74cc chambers, and adjustable valvetrain. Also on the shelves was a 4UA iron crank with 3.590-inch stroke along with a set of D0OE-A forged steel Cobra Jet rods.
All Jim needed were Speed Pro forged aluminum .030-inch oversize pistons, bearings, rings, and Fel-Pro gaskets to get started. Crane provided a stock flat-tappet hydraulic cam, while Comp Cams set him up with hydraulic lifters equipped with tiny .012-inch oil holes designed to keep lobes lubricated. Speed Pro provided stainless steel replacement 2.19/1.73-inch valves. With a little head port massaging, Grubbs achieved 467 horsepower and 480 lb-ft of torque.