It’s said couples who play together stay together, and that adage couldn’t be more true with Craig and Rebecca Sperry. They boast “his and hers” Sanger V-drive Mini Day Cruisers. Craig’s boat is supercharged, but the focus of this Homegrown Horsepower is Rebecca’s boat  – the Loose Screw – and how it came to be. 

“We sort of fell across this boat. It was for sale due to the fact the previous owner destroyed the engine and didn’t want to have it rebuilt, so we bought it for a very reasonable price without an engine, and that is what led us to this build. My wife wanted it to be a fairly fast boat, but wanted everything to be under the engine cover so it would appear stock – not like its supercharged brother. We had no room under the cover for a turbo system and certainly no room for a supercharger,” says Craig.

Due to these restrictions, Craig was forced to take the naturally-aspirated route, which required a few months of research to determine what parts would best support the goal of 600 horsepower – which Craig says was the most difficult part of the build.

They enlisted Wade Cook of Al Hubbard Machine in Hayward, California to perform the machine work on the early ’70s 4-bolt-main LS6 engine block and assorted parts. Parts were sourced from both Summit Racing and Morgan’s Machine and Marine in Walnut Creek, California, where Chris handed all of the marine-specific and special-order parts. 

The LS6 block houses a forged-steel crankshaft and connecting rods, swinging SRP pistons to displace 496 cubic inches. The compression ratio checks in at a pump-fuel-friendly 10.8:1 when combined with the 119cc combustion chambers in the rectangular-port Merlin cylinder heads. Manley 2.300-inch stainless intake and 1.880-inch Inconel exhaust valves process the airflow controlled by a Howards custom-ground hydraulic-roller camshaft and matching valvesprings.

The rest of the valvetrain includes 1.7:1-ratio roller rocker arms and pushrods from Crane and a set of retrofit hydraulic roller lifters from COMP Cams. The intake manifold is the well-regarded Edelbrock Performer RPM Air Gap. AED Performance supplied the 1,000 cfm 4150-style carburetor, and the fuel is lit afire by a complete assortment of parts from MSD, including a 6AL ignition box and Blaster coil. Copper head gaskets are from SCE, the rest of the engine uses Fel-Pro sealing products, and ARP  fasteners are used throughout. A Professional Products PowerForce harmonic damper controls the engine’s vibrations.

“The boat runs great! It appears stock until it starts and only then do people want to see what is going on under the engine hatch. It is nice for a change to have an engine that doesn’t attract too much attention until people hear it or see it go by,” explains Craig.

The results of the pair’s hard work also showed up on the dyno, with a stout horsepower reading of 664 at 6,300 rpm and 620 lb-ft of torque at 4,900 rpm – plenty of motivation to get the boat moving on the water.

“My wife and I did all the blueprinting and assembly work ourselves, as we always do. We pretty much do everything as a team. We worked on it after work and on the weekends. The entire build, after I had made the final parts list, took about three weeks. We lead busy professional work schedules so we could not work on it as often as we would have liked and the time we spend together is never taken for granted,” says Craig.

From the results of this sweet engine build, it looks like that time was spent wisely. Thanks for sharing it with us, Craig and Rebecca!

If you have a homebuilt engine you’d like to share with EngineLabs readers, send the info and photos to [email protected].