The Gen III Hemi carries one of the most recognizable names in the automotive industry. The modern, fuel-injected platform has continued the performance tradition, and the latest 6.4L 392 Hemi delivers power numbers that match anything from the original muscle-car era. Combined with improved reliability, fuel economy, and emissions compliance, the Gen III Hemi has earned a dedicated following.

The Case for Individual Runners

The one area where the factory Hemi falls short is the intake manifold. The Magnum, SRT8, and truck intakes all perform well, but they lack the visual and functional presence of an individual-runner induction system. For builders where presentation matters as much as output, individual throttle bodies offer both.

Individual-runner intake manifolds are not new to the Hemi platform. Earlier systems used mechanical injection or Weber downdraft carburetors. Modern systems, like the Speedmaster Stack Attack, replicate the form of those original designs while incorporating the precision of electronic fuel injection.

Solving the Traditional IR Problems

Electronic fuel injection has eliminated one of the primary challenges of eight-stack systems: precise fuel metering across the full rev and load range. Earlier mechanical and carbureted systems could deliver acceptable idle quality, driveability, or maximum power, but rarely all three, especially with aggressive cam timing.

EFI solved the metering problem but introduced new concerns: synchronized throttle opening, a reliable manifold absolute pressure (MAP) signal, and cost. The Speedmaster system addresses each of these. Precision throttle-blade machining and a centrally located, center-pivot throttle minimize deflection from unequal-length crossover rods. The MAP signal draws from a common plenum machined into the base of the intake manifold rather than from a single runner, which stabilizes idle quality and driveability.

The Test Engine

The test platform was a junkyard-sourced 5.7L Hemi fitted with a forged 6.1L crank, forged rods, and flat-top pistons from Wiseco. The short-block received a mild Crane camshaft with 212/216 degrees duration at 0.050-inch tappet lift and 0.550-inch valve lift. The heads were stock 5.7L castings with a dedicated valve job.

Baseline: Single-Plane Intake

The baseline configuration used a Mopar Performance MPI single-plane intake manifold, 36-pound injectors, and a FAST throttle body. Exhaust was routed through JBA Performance shorty headers feeding 2-1/2-inch collector extensions. Running on Holley HP EFI with a Meziere electric water pump, the Hemi produced 426 hp at 6,400 rpm and 385 lb-ft of torque. The single-plane intake allowed the mild combination to rev to 6,400 rpm before peak power, supported by head flow near 260 cfm.

Stack Attack Installation and Results

The Speedmaster Stack Attack system consists of two intake sections, each serving four cylinders, joined by a central plate that includes a vacuum port drawing from both sides. This provides a steady vacuum signal to the MAP sensor. Billet fuel rails with AN fittings and a fuel pressure regulator are included. Each individual port features an idle adjustment screw to equalize airflow. The 36-pound injectors from the baseline test were reused.

Wide-open throttle tuning required only a few pulls. As with any IR system, dialing in idle and part-throttle driveability takes additional time. Once calibrated, the Speedmaster downdraft system produced 461 hp at 6,500 rpm and 412 lb-ft of torque at 5,200 rpm. That is a 35 hp gain over the single-plane intake, with torque improvements consistent through the entire rev range. The added runner length of the individual-runner design is responsible for the solid torque gains.

System Details

The intake-manifold sections feature O-ring sealing that eliminates the need for an intake gasket.

The adjustable, center-pivot throttle linkage ensures all throttle blades open simultaneously, which is critical to IR system performance.

Vacuum ports for each bank of cylinders are integrated into the system. The center-pivot linkage mount doubles as a common vacuum source.

Dedicated billet-aluminum fuel rails with AN fuel fittings are included with the system.

An adjustable fuel pressure regulator mounts to the end of the fuel rail. The fuel system ships complete, minus injectors.

Each runner features a fully radiused air horn for smooth airflow transition into the engine.

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

The test engine: a mild 5.7L Hemi with forged internals from Wiseco and K1, a forged 6.1L crankshaft, a mild Crane camshaft, and stock 5.7L heads.

Baseline runs used a 4150-flange FAST four-barrel throttle body on the Mopar Performance single-plane intake, controlled by Holley EFI with 36-pound injectors.

Intake testing used JBA shorty headers feeding 2-1/2-inch collector extensions.

After installation of the Speedmaster downdraft induction system, the power output reached 461 hp at 6,400 rpm and 412 lb-ft of torque at 5,200 rpm, with improvements across the entire rev range.