We’ve previously brought you the story of a large-displacement big-block Chevy making almost 1,000 horsepower naturally-aspirated that was built for an airboat, and this time, we have something even more outrageous, again for an airboat: An 8-71-blown, 427-cube LS3 which stands over six feet tall when mounted to the engine stand.
By itself, a Roots-supercharged LS isn’t the most common thing to see. But when you add in that it’s for an airboat, you have a genuinely unique take on what is probably the most popular engine in the aftermarket right now. As Jeff Huneycutt points out, airboats don’t need lots of engine speed, but absolutely love torque. When you put those two requirements together, Roots superchargers should be the first thing that comes to your mind.
A Next-Gen Foundation
Since this project was going to be making lots of cylinder pressure and lots of torque, along with having a significant amount of displacement, there was no choice but to source the block from the aftermarket. For this project, Prestige Motorsports started with a Dart LS Next block, both for its inherent strength, as well as all of the neat aftermarket features included in the block.
Starting with the thick-wall siamese bores, a 4.125-inch bore doesn’t even come close to minimum wall thickness for a supercharged application. Priority main oiling ensures everything stays well lubricated during extended periods of elevated RPM operation. The cylinders are also lengthened to provide additional stability with longer strokes and there are additional head bolt holes in order to provide more clamping force to fight extreme cylinder pressures.
Additionally, the LS Next block features billet-steel main caps, which eschew the usual LS six-bolt design and instead opt for a more traditional splayed four-bolt configuration seen on its predecessors. Bolted into the mains is a 4.00-inch stroke forged steel crankshaft from K1 Technologies. It comes fitted with a Gen-IV 58x reluctor wheel, and has been double-keyed on the snout for the supercharger. The crankshaft is riding in a set of King Racing XP-series tri-metal bearings, which should handle the loads well.
Attached to the crank’s rod journals is a set of BoostLine connecting rods. Made from forged 4340 steel and using ARP2000 rod bolts, the unique rod design almost looks like an I-beam rod, except that its three distinctive pockets are strategically located and sized to add additional bending strength without adding excess weight. Measuring at 6.125 inches long, they are the typical aftermarket LS length, but are a bit beefier, at 700 grams apiece.
Filling the bores are a set of DSS Racing forged pistons. With DSS’ signature X-Groove skirt and a 22cc dish, the 1.115-inch compression height means that the 1.2mm/1.2mm/3.0mm ring pack just ever so slightly intersects the wrist pin hole, necessitating the use of oil support rails. An aluminum Innovator’s West harmonic dampener keeps the bad vibrations in check, and a low-profile Moroso fabricated aluminum drag/road race oil pan.
The Tall Top End
Starting with the valvetrain, Prestige spec’d out a hydraulic roller camshaft from COMP Cams with .627 inch of valve lift on the intake side and .615 inch on the exhaust. Duration comes in at 240 degrees at .050-inch of lift on the intake side, and 250 degrees at .050 on the exhaust. The camshaft is tied to the crank via a GM timing set and an IWIS timing chain.
A set of performance-upgrade hydraulic roller lifters from Gaterman Products translate the cam’s rotation to vertical movement and are kept from spinning, themselves, with a set of low-profile lifter guides designed specifically to work with aftermarket blocks. Jesel Sportsman series shaft-mounted aluminum roller-rocker arms with a 1.7:1 actuate the valves. A set of PAC valvesprings with 155 pounds of seat pressure and 400 pounds of open pressure control the valves, while a set of steel locks and retainers keep the springs in place.
Topping off the combination is a set of Prestige Motorsports own 11-degree LS3 cylinder heads. The heads are completely CNC ported — intake, exhaust, and combustion chambers. The 255cc intake ports feature massive flow numbers, while the 69cc chambers combine with the 22cc dish on the piston for a 9.5:1 compression ratio. 2.165-inch stainless-steel intake valves and 1.590-inch Inconel exhaust valves — both with 8mm stems — reside in the combustion chamber. A set of 0.51 inch-thick SCE MLS gaskets and a set of ARP head studs designed specifically for the LS Next block combine to seal the cylinder heads.
The intake manifold is a rectangle-port 8-71 blower base from Blower Drive Service, which required some pretty significant finessing from Prestige to bring up to their fitment standards. From there, the blower adapter plate is bolted to the plenum. A marine-grade air-to-water intercooler plate from The Blower Shop is fitted between the supercharger and the mounting plate.
Atop the intercooler is the piece de resistance, the billet BDS 8-71 supercharger. Moving 436 cubic inches of air per revolution, it has more displacement than the engine it sits atop. The 16-inch long rotors are hard-anodized and all the billet case components have been polished to a high shine. Feeding the combination is a system with the same retro-modern theme, using a pair of Holley 4150 throttle bodies.
The pair sits atop a billet aluminum fuel injector plate from The Blower Shop, which incorporates eight Holley EFI 66 lb/hr fuel injectors. The pre-compressor location of the fuel injectors is designed to keep the blower cool through the fuel spray, as well as aiding in lubrication. A set of Holley Smart Coils are mounted to the valve covers and keep the candles lit in all conditions, thanks to the Holley Terminator X ECU.
Spinning it on the Pump
Once together, and after a quick break-in session, the engine was configured with a 1:1 blower drive ratio and had the coals put to it. In that configuration, it easily surpassed the owner’s power goals, with 813.75 horsepower at 6,000 rpm and 764.2 lb-ft of torque at 4,600 rpm, all while staying well under 10 psi from the blower, on 93-octane pump gas.
Of course, when you have a bigger drive pulley just sitting on the bench, you can’t not put it on. With a slightly larger crank pulley (57 tooth vs. 53 tooth) the blower would now have a 13.25-percent overdrive. Even with slightly reduced ignition timing, the new configuration made another 40 horsepower, peaking at 853 at 6,000 rpm and 797 lb-ft of torque at 4,500 rpm, with boost levels still not quite touching the 10psi mark.