Richard Holdener
July 30, 2012

When Ford introduced the factory fuelie engine in 1986, it ushered in a new era of performance. Much has changed since the injected 5.0L H.O. first hit the streets, but the popularity of the small-block Ford has not diminished. The 5.0L is a favorite among Mustang owners, but is favored in swap applications, including Cobra replicas. While it is hard to argue with the performance and reliability of the factory (or the many aftermarket) fuel-injected intake systems, most have one thing in common—a single throttle body.

As effective as they are, they (like their single four-barrel carbureted counterpart), lack a certain sex appeal. For some, show is every bit as important as go and nothing screams “look at me” quite like an individual runner (eight-stack) injection system. When it comes to eye-popping induction, if one throttle body is good, eight is even better!

Individual-runner (IR) intake manifolds are not exactly new, having been run successfully on the Shelby Cobras back in the ‘60s. Those systems featured a quartet of Weber downdraft carburetors, but modern systems have replicated the look of the original systems, while upgrading them with versatility of EFI. The application of modern electronic fuel injection to replace the Weber carburetors has eliminated one of the major problems associated with the eight-stack system—namely precise fuel metering.

Properly tuned, the Webers would often provide decent idle quality or maximum (safe) power, but almost never both. This was especially true of engine combinations that include aggressive cam timing, as the carburetors simply could not provide precise fuel metering through the entire rev or load range. Obviously, stand-alone EFI cured this problem, but it created a few others along the way.

When it comes to individual-runner injection, the major concerns include synchronized throttle opening, proper manifold absolute pressure (MAP) signal, and pricing. According to Procomp Electronics, its new IR intake system has addressed all three concerns. While not cheap, the system sells for $3,499. That includes the lower intake, throttle bodies, polished stacks, throttle linkage, fittings, stand alone ECU, wiring harness, all sensors, injectors, fuel rails, adjustable fuel pressure regulator, fuel filter, distributor, and gaskets.

With systems designed for both 302 and 351W applications, the new induction features precision throttle-blade machining and a centrally located (bell crank) throttle to minimize the deflection inherent in unequal-length, crossover rods. The MAP signal was strengthened with the use of a common plenum machined into the bottom of the intake manifold. Rather than receiving an erratic signal from a single runner (which plays havoc on idle quality and drivability), the common plenum provides a balanced vacuum signal supplied evenly by every intake port. In terms of pricing, reduced labor costs and impressive buying power combine to keep the retail cost of this system from Procomp Electronics well below competitive systems. Concerned that price and performance often go hand in hand, we decided to put one of the new 351W systems to the test on a 408 stroker.

The Ford Windsor stroker was upgraded to 408 status courtesy of a 0.030 overbore and the addition of a 4.0-inch stroker kit from Scat. The Scat components included a 4.0-inch forged-steel stroker crank and 6.0-inch forged-steel rods. The crank and rods were combined with JE forged pistons. The dished pistons kept the static compression ratio in the streetable range (9.5:1) when combined with the 60cc chamber on the CNC-ported KC LH F 17 Brodix heads.

Some may question our choice of the smaller 195cc intake ports (the KLH 17 heads were available with 210cc intake ports) for our 408 street stroker, but how do you argue with nearly 300 cfm? Big flow through smaller ports is a one-way ticket to performance, and improved part-throttle driveability and mileage. The aluminum heads featured 17-degree valve angles, a 2.02/1.60 stainless steel valve package, and full CNC porting. The Brodix aluminum heads were secured using Fel Pro 1011-2 head gaskets and 1⁄2-inch ARP head studs. ARP also supplied a number of other fasteners, including a balancer bolt, oil pan studs, and an HD oil pump drive.

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Bigger (stroker) motors can get away with slightly wilder cam profiles. Looking for both power and driveability, we chose a suitable grind from the Crane cams catalog. Crane is back in the performance valvetrain business and has a wide variety of powerful cam grinds for your 5.0L and 351W Ford. Its retrofit cam profiles were designed to work in blocks (like our 351W) not originally equipped with hydraulic roller cams. The dual-pattern, hydraulic roller cam from Crane offered a 0.584/0.595-lift split, a 240/244- duration split, and a 114-degree LSA. The cam was supplied with PN 36532-16 link-bar lifters and a set of Gold 1.6-ratio roller rockers.

To establish a baseline, the Crane cam was first combined with an Edelbrock Performer RPM Air Gap intake and Holley 950 Ultra HP carburetor. This was the classic carbureted combination, designed for performance street use. Sure, a single-plane Victor Jr. might make more peak power, but nothing offers the combination of torque and power of a solid dual-plane manifold for street use. Equipped with the dual-plane intake and 950 Ultra HP carburetor, the 408 Ford produced 530 hp at 6,000 rpm and 535 lb-ft of torque at 4,200 rpm. Torque production with the effective dual-plane induction system exceeded 525 lb-ft from 3,800 to 4,800 rpm.

After running the Edelbrock and Holley, off came the carbureted induction to make way for the eight-stack system from Procomp Electronics. The IR system was run with a Holley EFI using 75-lb/hr injectors, though Procomp Electronics does offer a stand-alone management system as well. The 75-pound injectors were fed by an Aeromotive A1000 fuel and regulator, though Procomp Electronics also offers these components to complete the injection system.

Installation was straightforward, requiring only the purchase of four intake studs for the center mounting holes. Each of the four end throttle bodies was designed to accept a TPS sensor, making wiring harness hookup a snap. The central vacuum fitting was conveniently located, allowing us to run a vacuum line to our 1-bar MAP sensor. The throttle linkage for dyno use consisted of a Morse cable hooked to a short throttle cable and the central bell crank. The throttle crank can be activated as a push or pull system.

One of the common misconceptions regarding the IR system is that the power comes from increased throttle area—the thought being more flow equals more go. Though the IR system does offer unrestricted flow, it is a combination of flow, equalized runner length, and lack of common plenum that accounts for the change in power compared to a traditional induction system. Thus, the polished aluminum, radiused air horns offered show, flow, and go.

Once installed and tuned, the Procomp Electronics system not only provided the expected performance upgrade (peak power numbers jumped to 572 hp and 560 lb-ft), but also a respectable idle (given the cam) and impressive throttle transition. WOT power is actually the easiest part of tuning (taking just 6-8 pulls total), but driveability can require many more hours, especially on an eight-stack (IR) system. The rock-solid throttle linkage minimized deflection, which helped produce synchronized throttle openings.

Individual-runner intake systems almost always look cool, and this system was no exception. What really made this stack attack shine was the combination of good looks, performance and ease of operation.

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408 Stroker-Carb vs. Procomp Electronics IR

16 After tuning with the Holley EFI system, the new eight-stack injection from Procomp Electronics offered impressive power gains. The Stack Attack increased the power output of the 408 from 530 hp and 535 lb-ft of torque to 572 hp and 560 lb-ft of torque.

As is evident from the graph, the individual–runner induction system offered more than just a little power. The injection system from Procomp Electronics increased the power output of the 408 from 530 hp and 535 lb-ft to 572 hp and 560 lb-ft of torque. An extra 42 horsepower definitely makes this a worthwhile upgrade, especially when you consider the cost and cool factor. As well as your typical 5.0L EFI or carbureted system works, they just don’t have the wow factor like a trick IR setup.