VIDEO: Inside A Roots-Blown 427 Cubic-Inch LS3 Built For An Airboat

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.

The Dart LS Next iron block has a host of features that make it ideal for this insane engine build.

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 BoostLine rods are designed specifically for high cylinder pressures encountered in forced-induction applications. The DSS Racing pistons feature a 22cc dish to bring the compression ratio down to 9.5:1 for the boost.

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.

Prestige Motorsports’ 11-degree LS cylinder head is fully CNC’d and houses a 2.165-inch stainless intake valve and a 1.595-inch Inconel exhaust valve.

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.

The Jesel Sportsman aluminum shaft rocker system adds rigidity and stability to the valvetrain.

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.

The billet 8-71 supercharger from Blower Drive Service is fitted with a pair of Holley 4150 EFI throttle bodies, and a fuel injector plate from The Blower Shop, fitted with eight 66 lb/hr Holley fuel injector.

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.

You’ll notice that the dyno test doesn’t go past 6,000 rpm, even though power is still climbing. That’s because, in the real world, this engine will never go past that mark. The Prestige team feels that running it to its full potential would yield north of 900 horsepower on 93-octane.

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.

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About the author

Greg Acosta

Greg has spent fifteen years and counting in automotive publishing, with most of his work having a very technical focus. Always interested in how things work, he enjoys sharing his passion for automotive technology with the reader.
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