When it comes to engines, we don’t care if they are built to operate on the land, sea, or in the air. But, you catch our attention when you take an engine originally designed to operate on land, and then modify it to pull duty on the open sea. In this case, it’s a big-block Chevrolet that is being outfitted to operate in a 22-foot Avanti off-shore powerboat built for the open ocean.
Thankfully, our friend Jeff Huneycutt over at The Horsepower Monster caught wind of the unique beast being built at Prestige Motorsports and made sure to document the build, as this is no ordinary big-block Chevrolet engine. As Huneycutt explains, the project has three major design goals beyond fitting the engine into the inboard hull of a boat: To make power; to run on questionable marine 93-octane pump gas; and to be ultimately reliable, since this is going to be running out on the open sea.
Basic Building Blocks
Since this powerplant needs to be ultimately reliable under significantly elevated cylinder pressures, the starting point for this engine is a World Products Merlin IV cast-iron block (which we’ve discussed previously here). It’s cast with thicker cylinder walls and main webs, and comes with splayed four-bolt main caps (unsplayed on mains 1 and 5). This variant has a 9.800-inch deck height and comes pre-notched for stroker crank clearance.
The crankshaft is a forged piece from Eagle Specialty Products, with a 4.250-inch stroke, and dual keyways machined into the snout for the blower drive. It rides in King XP coated main bearings and is secured into the block by ARP main studs. Bolted to the crankshaft are a set of Eagle forged 4340 steel H-beam connecting rods measuring 6.385 inches, center-to-center.
Custom JE forged pistons, measuring 4.500-inches in diameter attach to the rods via a .990-inch diameter wrist pin. The short 1.290 compression height means oil support rails will be used, and the 14.9cc dish will keep compression in the pump-gas-friendly zone. The JE piston rings feature a PVD-coated top ring with a nitride second ring
A Melling M77HV high-volume oil pump, fitted with a Hardin Marine heavy-duty oil pickup, provides the engine its lifeblood. The Hardin pickup is built to specifically fit the 14-quart steel oil pan, which features an integrated windage tray and is built to handle the extra stroke. An Innovator’s West harmonic balancer completes the short-block
Boosted Valvetrain and Top End
The brain of the engine is the camshaft, and for this combination, Prestige worked with Comp Cams to sped out a Hydraulic Roller bumpstick. They ended up with a .388-lift lobe (which works out to .660-inch of lift at the valve) for both the intake and exhaust lobes, with 265 degrees of intake duration and 268 degrees on the exhaust — both at .050-inch of lift. The lobes are on a 116-degree lobe separation angle designed to complement the pressure from the supercharger.
Topping off the short-block are a set of AFR’s rectangle-port 24-degree cylinder heads, with as-cast 325cc intake ports and raised 132cc exhaust ports. The chambers are the 121cc fully CNC’d design and house a set of stainless steel 2.300-inch intake valves and Manley 1.880-inch Inconel exhaust valves, to handle the extreme exhaust temperatures generated by the roots blower. Both the intake and exhaust valves have 11/32-inch stems. They are sealed to the block with Cometic MLS gaskets, as well as a set of ARP head bolts over studs due to the clearance issue inside the boat’s hull.
A set of forged Johnson hydraulic roller lifters were chosen to translate the camshaft’s rotational movement into vertical movement. Rather than use a tie-bar setup, these lifters are kept from rotating through an OEM-style dogbone retainer and spider setup. To keep the valves under control, Prestige opted to swap out the included valvesprings on the AFR heads for something a little better suited to the camshaft profile chosen. They went with a double spring offering a spring pressure of 150 pounds and 450 pounds of open pressure.
Topping off the long block is a cast-aluminum intake manifold, with a bolt pattern designed to fit popular Mercury Marine superchargers, and uses an O-ring to seal the supercharger outlet to the manifold inlet. The TBS 250 cubic-inch low-profile supercharger is a trick piece modeled after the Mercury Marine unit, but in a much more compact design, measuring less than six inches tall. While designed to accept dual 4150-pattern carburetors, this build will be fueled by a single 950cfm carb.
With the pulley configuration installed, the unit should make a touch over 7psi on the dyno. Speaking of the dyno, a pair of big-block Chevy dyno headers are used in place of the fancy water-cooled marine units for clearance purposes on the dyno. Additionally, an MSD 6BTM boost retard unit was wired in and set to pull one degree of base timing per pound of boost seen by the unit.
Once the final jetting was reached with the carburetor, the engine was happiest at about a 12.5:1 air-fuel ratio. A little leaner than your average supercharged setup, but what this combo seemed to like. The results on the dyno were stout. 704 horsepower at 5,800 rpm and 706 lb-ft of torque at 4,400 — all on pump 93-octane gasoline. More impressive than the power peaks were the power curves – nice and flat across the operating range of the engine.
When looking at this build, remember what it’s designed to do, and its design restrictions, and you start to realize just how impressive this big-block build is.