Old-School Boost on a Modern Platform

The LS engine family redefined small-block performance, and the aftermarket quickly followed with carbureted intake manifolds and ignition controllers from Edelbrock, Holley, and MSD. This build takes the carbureted LS concept further by applying forced induction through a traditional 6-71 supercharger, pairing the visual presence of a roots-style blower with the efficiency of the LS architecture.

The Speedmaster 6-71 supercharger kit is designed to bolt directly to cathedral-port or rectangle-port LS heads. For this test, the kit was installed on a 383 cubic-inch LS stroker, first baselined in naturally aspirated trim, then run under boost to quantify the gains.

Building the 383 Stroker

The foundation was a 4.8L iron block, bored from 3.780 to 3.905 inches to accept the 4.000-inch stroker crank from Speedmaster. The same stroker crank fits the 5.3L block (shared casting), and when applied to a standard-bore 6.0L produces 402 cubic inches, or 408 at 0.030-over.

The rotating assembly included 6.125-inch rods from K1 and flat-top forged pistons from JE, sealed with Total Seal piston rings. The JE pistons featured valve reliefs sized for the COMP hydraulic roller cam (PN 54-459-11), which offered a 0.617/0.624-inch lift split, a 231/239-degree duration split, and 113-degree LSA. COMP also supplied the hydraulic roller lifters and 7.35-inch hardened pushrods.

The short block was completed with a Sealed Power oil pump, stock truck oil pan and windage tray, and Fel-Pro MLS head gaskets secured by ARP head studs. Topping the assembly was a set of Trick Flow GenX 225 heads featuring revised valve angles (15 to 13.5 degrees), full CNC porting, stainless 2.055/1.460 valves, and a dual spring package. With over 330 cfm of intake flow, the GenX 225 heads provided more airflow capacity than the 383 could use, even under boost.

Naturally Aspirated Baseline: 527 HP

To establish a clear baseline, the 383 was configured with an Edelbrock Victor Jr. intake, Holley 950 XP carburetor, and MSD ignition controller. The controller interfaced directly with the factory coil packs and cam/crank sensors, providing full timing curve control for the carbureted application. An ATI Super Damper, 1-7/8-inch Hooker headers, and a Meziere electric water pump completed the setup.

After break-in and tuning at Westech Performance, the naturally aspirated 383 stroker produced 527 hp and 496 lb-ft of torque.

The 383 stroker: a 4.000-inch Speedmaster crank, K1 6.125-inch rods, JE forged pistons bored to 3.905 inches, Total Seal rings, COMP cam, Fel-Pro MLS head gaskets, and ARP head studs.

The 383 topped with Trick Flow GenX 225 heads, fed by an Edelbrock Victor Jr. intake and Holley 950 XP carburetor.

On the engine dyno at Westech Performance with an ATI Super Damper and 1-7/8-inch Hooker headers, the naturally aspirated 383 produced 527 hp at 6,300 rpm and 496 lb-ft of torque at 5,300 rpm.

With the baseline established, the Victor Jr. intake was removed to make room for the Speedmaster blower kit.

Speedmaster 6-71 Supercharger Kit: What Is Included

The kit centered on a billet aluminum intake manifold, CNC-machined with a spring-loaded pop-off valve, provisions for fuel injectors (enabling EFI configuration), and aluminum break-away blower mounting studs. Captured O-rings provided factory-style sealing to the cylinder heads.

Additional components included a blower snout, belt tensioner, an assortment of 8mm cog pulleys for boost adjustment, a dual-quad carburetor adapter for 4150 carburetors, and a crank adapter for mounting the cog pulley to the ATI Super Damper. The supercharger itself was a polished 6-71 unit from The Blower Shop, capable of supporting power levels well beyond the scope of this test.

The billet aluminum blower manifold from Speedmaster. The CNC-machined detail work is evident throughout.

The kit included O-ring gaskets for proper intake-to-head sealing.

A spring-loaded blow-off valve, integrated into the intake, provides backfire protection.

Installation

Installation followed the same process as a standard intake swap. The ATI damper was properly honed prior to fitment, and the aftermarket stroker crank's existing crank key simplified pulley mounting. (Note: stock LS cranks require the ATI crank pin installation kit.)

The Speedmaster intake installed on the 383 stroker with the supplied blower gasket and mounting studs. The injector ports were plugged using a set of FAST injectors and fuel rails.

The polished Blower Shop 6-71 supercharger mounted on the Speedmaster blower snout, which aligns the blower and crank pulleys for the LS application.

The crank pulley adapter bolted directly to the ATI Super Damper using the supplied hardware.

The 50-tooth crank pulley installed on the adapter. Speedmaster offers a range of pulley sizes to adjust boost levels for different applications.

The dedicated cog drive system used this tensioner, which slid over the blower snout and clamped in place.

The blower pulley installed on the snout with the belt properly tensioned.

Speedmaster supplied the dual-quad carburetor adapter for use with the supercharger.

A pair of Holley 950 Ultra XP carburetors and dual-quad linkage provided fueling for the blown combination.

Both the naturally aspirated and blown configurations used this MSD ignition controller.

Results Under Boost: 753 HP at 7.3 PSI

The 6-71 was configured with a 50-tooth crank pulley and 55-tooth blower pulley using the supplied 8mm cog belt. This underdriven combination produced a peak boost pressure of 7.3 psi at the 6,600 rpm shut-off point. With timing retarded under boost via the MSD controller, the supercharged 383 produced 753 hp and 625 lb-ft of torque.

That represents a gain of 225 hp and 129 lb-ft over the naturally aspirated baseline. Torque production exceeded 600 lb-ft from 4,500 rpm past 6,500 rpm, and the power curve was still climbing at shut-off. Additional pulley changes within the kit allow further boost increases for applications that require more output.

At 7.3 psi, the supercharged LS stroker produced 753 hp at 6,600 rpm and 625 lb-ft of torque at 5,300 rpm: a gain of 225 hp and 129 lb-ft over the naturally aspirated configuration.

Summary: 383 LS Stroker, NA vs. Speedmaster 6-71 Blower at 7.3 PSI

Naturally aspirated: 527 hp at 6,300 rpm / 496 lb-ft of torque at 5,300 rpm

Supercharged (7.3 psi): 753 hp at 6,600 rpm / 625 lb-ft of torque at 5,300 rpm

Gain: 225 hp / 129 lb-ft of torque

The Speedmaster 6-71 kit delivered a complete, bolt-on supercharger solution for the cathedral-port LS platform. With additional pulley options included in the kit and the power curve still climbing at 6,600 rpm, the system has capacity well beyond what was tested here. For C10 trucks, muscle cars, and street rods running carbureted LS combinations, this kit pairs proven roots-style forced induction with a purpose-built LS intake platform.