The Dodge Hemi has a lot going for it, not the least of which is arguably the most recognizable name in automotive industry. The modern, fuel-injected Hemi has carried on the tradition of performance and the latest 6.4L, 392ci Hemi offers serious power numbers that are easily on par with any engine produced during the original muscle-car era. Toss in the reliability, fuel mileage, and low emissions, and it’s easy to see why the Gen III Hemi has such a loyal following.

The one possible complaint we have about the Hemi is the intake manifold. It’s not that the Magnum, SRT8, or even truck intakes don’t provide plenty of performance—they just don’t look the part. Even the aftermarket single-plane intake we tested on our 5.7L Hemi lacked the sex appeal that only comes from an individual-runner induction system. For some, show is every bit as important as go, and nothing screams “look at my Hemi!” quite like a Stack Attack injection system. After all, if one throttle-body is good, eight must be better!

Individual-runner intake manifolds are not exactly new, even for Hemis. Those systems featured either mechanical injection or Weber downdraft carburetors. Modern systems, like this unit from Speedmaster, replicated the look of those original systems while upgrading them with the versatility of electronic fuel injection.

The introduction of sophisticated electronic fuel injection has all but eliminated one of the major problems associated with the eight-stack system, namely precise fuel metering. Properly tuned, the previous systems could provide one, or maybe two, of the following desirable traits: decent idle quality, driveability, or maximum power, but it was almost impossible to achieve all three. This was especially true of performance Hemi combinations that included aggressive camshaft timing, as neither mechanical injection nor downdraft carburetors could provide precise fuel metering through the entire rev and/or load range.

Modern fuel injection may have cured the precise fuel-metering problem, but it created a few other issues along the way. When it comes to an eight-stack injection, major concerns include synchronized throttle opening, proper manifold absolute pressure (MAP) signal, and pricing. According to Speedmaster, its new individual-runner intake system for the Dodge Hemi addresses all three of these concerns. With systems designed for Ford, Chevrolet, and now the Dodge Hemi, the new induction features precision throttle-blade machining and a centrally located, center-pivot throttle to minimize the deflection inherent in unequal-length, crossover rods. The MAP signal is sourced from 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 driveability.

Eager to test the new system, we installed Speedmaster’s Stack Attack intake system on our junkyard-sourced 5.7 Hemi, which we fitted with a forged 6.1L crank, forged rods, and flat-top pistons from Wiseco.. The short-block was further augmented with a mild Crane camshaft with 212/216 degrees duration at 0.050-inch tappet lift and 0.550-inch valve lift. Topping off the 5.7L was a set of stock heads treated to nothing more elaborate than a dedicated valve job.

Feeding the mild 5.7L powerplant was a Mopar Performance MPI single-plane intake manifold, 36-pound injectors, and FAST throttle body. The exhaust was comprised of shorty headers from JBA Performance that fed 2-1/2-inch collector extensions. We ran the engine on Holley’s HP EFI and installed a Meziere electric water pump. With this configuration tuned to optimize power production, the Hemi produced 426 hp at 6,400 rpm and 385 lb-ft of torque. Despite the mild cam timing, the single-plane intake allowed the Hemi to rev up to 6,400 rpm before making peak power. Credit also goes to the impressive head flow offered by Hemi heads, which was near 260 cfm.

After establishing our baseline, we removed the MPI induction and replaced it with the Stack Attack individual-runner system from Speedmaster. It includes a pair of intakes, each serving four cylinders. The pair are joined by a central plate that includes a vacuum port that draws from both intakes and provides a steady vacuum signal to the MAP sensor. Billet fuel rails are included and come with AN fuel fittings and a fuel pressure regulator. Each individual port also featured an idle adjustment screw to equalize airflow. We reused the 36-pound injectors from the baseline test and had the system up and running in no time.

The wide-open throttle (WOT) took just a few pulls, but as is the case with IR systems, plan on dedicating time to dial in the idle and driveability. Once tuned, the Speedmaster downdraft system pumped out 461 hp at 6,500 rpm and 412 lb-ft of torque at 5,200 rpm. As if Hemi owners won’t want to attack the stack system based on looks alone, the downdraft also improved the power output by more than 30 hp, with big torque gains consistent through the entire rev range.

 

 

  5.7L Hemi Intake Test

MP Single Plane versus Speedmaster Downdraft IR

If the good looks don’t sell you on Speedmaster’s individual-runner intake, how about an extra 30 hp? Still not convinced? How about more power everywhere through the entire rev range? Equipped with the MP single-plane intake, our 5.7L produced 426 hp and 385 lb-ft of torque, but those numbers jumped to 461 hp and 412 lb-ft after we installed the Stack Attack on the Hemi. Credit goes to the added runner length offered by the system for solid torque gains.

 

 The intake-manifold sections used on the downdraft, eight-stack system from Speedmaster feature O-ring sealing that eliminated the need for an intake gasket.

The downdraft system also features an adjustable, center-pivot throttle linkage. The key to a successful individual-runner system is to make sure all throttles blades open simultaneously.

Speedmaster also provided vacuum ports for each bank of cylinders. The mount for the center-pivot linkage also served as a common vacuum source.

This induction system includes dedicated billet-aluminum fuel rails, complete with AN fuel fittings.

An adjustable fuel pressure regulator is attached to the end of the fuel rail. We used the dyno regulator during testing, but it’s nice to know the fuel system came complete (minus injectors).

Each runner has a fully radiused air horn to ensure a smooth transition of air into the engine.

In addition to the adjustable center-pivot linkage, each runner also featured a bypass screw to equalize airflow at idle.

We ran the Speedmaster’s new intake on this mild 5.7L Hemi, which featured forged internals from Wiseco and K1, a forged 6.1L crankshaft, a mild Crane camshaft, and stock 5.7L heads.

Baseline runs were made with a 4150-flange FAST four-barrel throttle-body feeding this Mopar Performance single-plane intake. Though the intake will also accept a conventional carburetor, we ran this test with a Holley EFI controlling the 36-pound injectors.

Intake testing was performed with this set of shorty headers that fed 2-1/2-inch collector extensions from JBA.

After installation of the Speedmaster downdraft induction system, the power output jumped to 461 hp at 6,400 rpm and 412 lb-ft of torque at 5,200 rpm. The Stack Attack system not only looked amazing but improved the power output through the entire rev range.