Chevrolet all but reinvented small-block performance with the introduction of the LS engine family. A potent and worthy successor to the original small-block, the LS has proven its power potential time and time again. The one possible complaint about the LS was that the factory fuel-injected combinations left out the early carbureted contingent, after all, the LS quickly became the swap candidate for trucks, street rods, and early muscle cars (and everything else). Companies like Edelbrock, Holley, and MSD soon stepped in to provide both carbureted intake manifolds and ignition controllers to allow the carburetor crowd to take advantage of everything the LS had to offer. This test is for those who have an even greater need for old-school speed, as we apply boost to a carbureted combo. Nothing screams performance like a big 6-71 supercharger sticking out of the hood. As we found out, adding a dual-quad 6-71 supercharger to the already efficient LS made not only an impressive visual statement, but plenty of power as well.
Before adding boost, we obviously needed something to apply it to. While it is certainly possible to installed the 6-71 kit from Speedmaster onto a stock LS (they have intakes for both cathedral and rectangle port applications), we decided to the kick things up a notch. Remember, any modifications you make to a naturally aspirated combination will pay even greater dividends under boost. To that end, we decided to go the 383 stroker route, starting with a 4.8L iron block. (The 383 stroker crank can also be applied to the 5.3L, as they share the same block. Installation of the 4.000-inch crank to a standard-bore 6.0L will produce a 402, or a 408 at 0.030-over block.) To produce the 383 cubic-inch displacement, it was necessary to bore the 4.8L block out to 3.905-inches (from 3.780- inches) before adding the stroker crank. With the machining complete, it was time for the stroker kit and additional modifications.
The extra inches came courtesy of a 4.000-inch stroker crank from Speedmaster, combined with a set of 6.125-inch rods from K1 and a set of flat-top, forged pistons from JE. Sealing for our boosted stroker was provided by a set of Total Seal piston rings. In addition to being plenty strong, the JE forged slugs featured the necessary valve reliefs to allow the installation of our Comp hydraulic roller cam (PN 54-459-11). The Comp cam 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 a set of 7.35-inch hardened pushrods. The short-block was completed with a new Sealed Power oil pump, stock truck oil pan and windage tray, and a set of Fel-Pro MLS heads gaskets secured by a set of ARP head studs. Topping the 383 stroker was a set of Trick Flow GenX 225 heads. The GenX 225 heads featured revised valve angles (from 15 to 13.5 degrees), full CNC porting, and altered spark plug locations, along with stainless 2.055/1.460 valves and a dual spring package. Offering over 330 cfm, the GenX 225 heads were more cylinder head than our 383 could even hope to use, even under boost.
The only thing missing on the 383 stroker was the induction system. To establish a baseline on the dyno in naturally aspirated trim, and to clearly illustrate what the 6-71 blower kit from Speedmaster had to offer, we configured the 383 with an Edelbrock Victor Jr. intake and Holley 950 XP carburetor. An MSD controller handled the ignition chores for the carbureted LS application. The controller plugged right into the factory coil packs and cam and crank sensors and allowed us to dial in the desired timing curve to optimize power production. Finishing off the 383 was an ATI Super Damper, a set of 1 7/8-inch Hooker headers, and a Meziere electric water pump. After adding a break-in blend from Lucas Oil, we performed a few break-in cycles then ran the stroker in anger. After dialing in the air/fuel ratio and timing curves, the 383 sroker responded with peak numbers of 527 hp and 496 lb-ft of torque. While most of us would be plenty happy with a 500-plus horsepower street engine, we were just getting started here. With the naturally aspirated testing out of the way, it was time for some boost, baby!
The 6-71 supercharger kit supplied by Speedmaster featured a billet aluminum intake designed to bolt to cathedral-port cylinder heads. (They also offer a version for LS3 (rectangle-port) heads.) The CNC-machined intake was impressive and featured a spring-loaded, pop-off valve; provisions for fuel injectors; and the aluminum (break-away) blower mounting studs. We liked the fact that the blower manifold also featured captured O-rings for sealing (like the factory). In addition to the dedicated blower intake, the kit also featured a blower snout, tensioner, and an assortment of pulleys to adjust the boost and power level. Additional components included a dual-quad carburetor adapter designed to accept 4150 carburetors, a crank adapter to facilitate mounting the 8mm cog pulley to the ATI Super Damper (not included in the kit). The blower pulley simply bolted to the blower snout using the supplied hardware. The final component was the massive, polished 6-71 supercharger supplied by The Blower Shop. Capable of supporting considerably more power than we intended to run on our stroker, who wouldn’t be proud to have this bad boy sticking out of the hood?
Installation of the kit was no more difficult than your average intake swap. We made sure to properly hone the ATI damper prior to installation and took note that our aftermarket stroker crank featured a crank key. If you plan to run a blower on a stock LS crank, make sure to get the ATI crank pin installation kit. To ensure unrestricted air and fuel flow to the supercharger, we enlisted the aid of Holley, who supplied a pair of 950 XP carburetors and the necessary dual-quad linkage. Not looking for maximum boost or power, we configured the 6-71 with a 50-tooth crank pulley and 55-tooth blower pulley using the supplied 8mm cog belt. This underdriven pulley combination produced a peak boost pressure of 7.3 psi at our self-imposed shut-off point of 6,600 rpm. Using the MSD controller to retard timing under boost (compared to the NA combination), the dual-quad 6-71 383 produced 753 hp and 625 lb-ft of torque. Torque production exceeded 600 lb-ft from 4,500 rpm past 6,500 rpm, so there would never be any shortage of grunt from this supercharged stroker combo. The power output was still climbing rapidly on this combo and we had plenty of boost left with additional pulley changes, so if you feel the need, this Speedmaster kit offers plenty of speed along with a way to get some timeless good looks.
1. Starting with a 4.8L iron block, the 383 stroker included a 4.000-inch stroker crank from Speedmaster, 6.125-inch rods from K1, and a set of JE forged pistons. To accept the new forged slugs, the block was bored out from 3.780 to 3.905 inches then fitted with Total Seal rings. Also present were a Comp cam, Fel-Pro MLS head gaskets, and ARP head studs.
2. Topping the 383 was a set of TFS GenX 225 heads fed by an Edelbrock Victor Jr. intake and 950 XP Holley carb.
3. Run in anger on the engine dyno at Westech Performance with an ATI Super Damper (required for the blower) and 1 7/8-inch Hooker headers, the naturally aspirated 383 stroker produced 527 hp at 6,300 rpm and 496 lb-ft of torque at 5,300 rpm.
4. After establishing a baseline with the Victor Jr. it was removed to make room for the blower kit from Speedmaster.
5. The components supplied by Speedmaster included this billet aluminum blower manifold. The detail work on the CNC intake was impressive.
6. The blower kit included the necessary O-ring gaskets to properly seal the intake to the cylinder heads.
7. The intake also featured this spring-loaded blow-off valve in case of a backfire.
8. The Speedmaster intake was installed onto the awaiting 383 stroker. Note the use of the supplied blower gasket and mounting studs. The injector holes (the system can be run in EFI configuration) were simply plugged using a set of FAST injectors and fuel rails.
9. Next up on the install list was the polished The Blower Shop 6-71 supercharger. Nothing screams performance like a blower sticking out of the hood! The Speedmaster kit also included the necessary blower snout to properly align the blower and crank pulleys on the LS application.
10. Additionally, the kit included this crank pulley adapter that bolted directly to the ATI Super Damper using the supplied hardware.
11. We then bolted the 50-tooth, crank pulley to the adapter. Speedmaster offered a variety of different pulleys to adjust the boost to your application and power needs.
12. Driving the pulleys was a dedicated cog system that required this tensioner. The tensioner slid over the blower snout and clamped in placed.
13. We then installed the blower pulley onto the snout and tightened the blower belt, making sure that we didn’t overtighten it.
14. Speedmaster supplied the necessary dual-quad carb adapter for use with the supercharger.
15. Overkill for this boost and power level, we nonetheless relied on a pair of Holley 950 Ultra XP carbs for this blower test.
16. Both the carbureted NA and blower combinations were run with this MSD ignition controller.
17. Run with just 7.3 psi of boost, the supercharged LS stroker thumped out 753 hp at 6,600 rpm and 625 lb-ft of torque at 5,300 rpm. That’s a gain of 225 hp and 129 lb-ft of torque. The gain wasn’t just at peak numbers either, since the blown LS made more power across the entire pull. That’s a healthy performance gain to go along with the old-school-cool looks of the 6-71 blower.
383 Stroker—NA vs. Speedmaster 6-71 Blower (7.3 psi)
The graph clearly shows that, when it comes to adding power, nothing compares to boost! Run in carbureted form, the 383 LS stroker was no slouch, producing 527 hp at 6,300 rpm and 496 lb-ft of torque at 5,300 rpm, but things got serious after the installation of the 6-71 supercharger. Using the intake, components, and a 50/55 pulley combination from Speedmaster, The Blower Shop 6-71 supercharger pumped out a peak of 7.3 psi. The supplied boost pushed the peak power numbers to 753 hp at 6,600 rpm and 625 lb-ft of torque. The power curve was still climbing rapidly at our 6,600-rpm shut-off point, but maximum power was not the goal for this test. Imagine your early C10 truck, muscle car, or street rod with 750 hp!