Inside the Fabrication of a Unique 10-stack EFI Intake Manifold

Details are surfacing on the unique 10-stack intake manifold EngineLabs first featured last December when the 427 Windsor was on the R&R Performance dyno. Other than a couple of overall candids clearly showing a single ram tube and throttle body for each cylinder, plus a pair of smaller stacks in the middle, nothing else was known about this striking one-off fabrication.

Here’s how the engine looked on the dyno when it pulled 530 horsepower.

“I like to be original, and it’s tough to come up with something totally original anymore,” says Tom Dunrud, the designer and machinist who built the engine for his Factory Five ’33 hot rod.

Dunrod started with a Ford Racing Z427 crate motor that doesn’t come with induction or ignition packages. He wanted the nostalgic personality of 8-stack hardware that evolved from the original V8 Hilborn injectors. While an intake manifold with individual runners has performance advantages over a common-plenum design in certain race applications, they are much harder to tune and often suffer from low-speed drivability issues in street engines. They’re also more expensive with individual throttle blades, intricate linkage and complicated fuel systems.

Dunrud’s strategy focused on a 3-piece intake with individual runners, ram tubes and throttle plates for each cylinder, but he also integrated a central plenum between the cylinders for idle operation and to provide a signal for the EFI’s MAP sensor. Feeding that shallow plenum would be two smaller ram tubes and throttle plates.

Dunrud used his machine-shop experience to design the manifold on SolidWorks, then tested the dimensions using full-size templates on the engine.

“It’s similar to primaries and secondaries on a carb, except that you’re controlling just air and not fuel,” explains Dunrud. “Basically, you change strategies when going from primaries to secondaries. You’re going from a common plenum to individual runners.”

Owner of Accura Tool & Engineering in Staples, Minnesota, Dunrud used SolidWorks to draw out and test his ideas. He also made full-size printouts of the templates and mocked them up on the long-block assure the baseplate would fit properly.

Milling a small plenum

The bottom section supports the runner transition from the cylinder-head face to the vertical position for the throttle blades and ram tubes, and also cradles the lower half of the throttle-blade shafts. Nestled between the rows of intake ports is a wide, shallow plenum area with 7/16-inch feeder holes from the plenum to each port.

“It’s almost like a 360-degree intake on a very small scale,” says Dunrud.

The intake is made up of three separate layers. Note the shallow common plenum on the first section (top row) while the second plate (bottom left) encloses the plenum and throttle shafts. The top layer supports the ram tubes (bottom right).

The individual runners sport 2-inch throttle openings, a size Dunrud determined after examining other systems on the market. Diameter for the center stacks comes in at 1.5-inches.

“I didn’t think the center ones mattered all that much,” says Dunrud. “I didn’t know if the restriction was going to be the butterfly or in the hole from the plenum to the port. But I knew I didn’t want just one stack because the shape of the plenum is low and flat. I figured I’d get better air distribution with two, and I didn’t see any reason to go with more.”

Dunrud also designed the progressive throttle linkage and housing that supports the TPS.

The 3/8-inch diameter throttle shafts are cut from 4440 stainless steel and ride on self-lubricating bushings. The throttle blades are machined from 15-5 stainless steel.

“The hardest part was getting the throttle blades to fit right,” says Dunrud, noting it took three tries to achieve the desired results. “With 10 blades you can’t have any air going by or you’re never be able to control the idle. I ended up with a much thicker blade than traditional for more surface contact.”

Most of the aluminum was anodized black. The fuel and spark get their instructions from a Holley Dominator ECU.

Designing the progressive throttle linkage was much easier as SolidWorks allowed Dunrud to simulate every movement.

“The main concern was how much the primaries would open before the secondaries kick in,” says Dunrud, who again looked at conventional applications while researching the issue. “This was a guess and I settled on 30 degrees. I figure that’s similar to a 4-barrel carb.”

Tuning 10 stacks

All three sections are machined from 6061 billet aluminum. A little extra material was left around the ports so they could be matched to the cylinder head. Dunrud also machined the fuel rails for the fuel injectors, fabricated the ram tubes and had most of the components hard anodized in black. The air-filter elements are from C. Cook Enterprises. He added an MSD ignition and tied the fuel system to a Holley Dominator ECU.

View of the engine installed in the car.

“It ran way too rich at first,” says Dunrud. “For the first 30 degrees of throttle rotation, there’s actually very little throttle opening, so I had to really cut back on the fuel. After 30 degrees, then we open it up pretty quick. The Holley EFI is really nice with its flexibility.”

The engine dyno’d at 530 horsepower with limited time for fine tuning. Ford rated the Z427 at 535 horsepower using a carb and Edelbrock Super Victor intake.

“I felt pretty good coming within five horsepower of an established racing intake. I believe there’s some tuning left in it,” says Dunrud, noting that while running on only the primary openings the engine pulled 60 horsepower. “That’s more than enough to cruise this car at 70 or 80 mph.”

Finally, Dunrud has a goal of running his ’33 in the 10-second bracket at the dragstrip.

“First time out was an 11.13,” he says. “So it should run in the 10s easily.”

The engine powers a heavily modified Factory Five ’33 and already runs in the low 11s at the strip with plans to go even quicker.

About the author

Mike Magda

Mike Magda is a veteran automotive writer with credits in publications such as Racecar Engineering, Hot Rod, Engine Technology International, Motor Trend, Automobile, Automotive Testing Technology and Professional Motorsport World.
Read My Articles

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