When Ferruccio Lamborghini purchased a Riva Aquarama speedboat in the late ’60s, he quickly swapped out the GM-sourced Crusader engines for a pair of marinized 4.0-liter Espada DOHC V12 engines. Appropriate for the legendary Italian automaker, perhaps, but not the best of practical choices.

“The torque peak on an Espada engine is 5,000 rpm,” explains Tate Casey of Carobu Engineering, which specializes in exotic engines. “On a marine engine, that’s typically where the horsepower peak is.”

Regardless of the limited speed or struggles in getting the boat up on plane in a timely manner, the choice didn’t bother Mr. Lamborghini. He kept the engines in the stylish wooden Riva until his death in 1993. The boat was then refitted with Crusader engines, and the Espada engines were sent to the Lamborghini museum. The boat itself was somewhat forgotten and put in storage, where it lingered until a Dutch auto enthusiast recently rescued it. He then took the watercraft to Riva World in Holland where it’s undergoing a show quality restoration. The new owner also contacted Carobu to build period-correct Espada engines for the boat.

“But he also wanted the engines to work properly in a boat,” says Casey. “I did an engineering study using simulation software and created a model that looked like it would work. First, we had to bore and stroke the motor to get it bigger. We were able to do that, but it required a lot of specialized work.”

The Carobu engine shop, which is based in New Mexico, couldn’t just throw in a long-arm crankshaft. The deck height had to be extended with custom 9mm plates that were sealed and fastened to the block. Longer piston sleeves were required and bored to 85mm to support the Razzo Rosso 80mm-stroke crankshaft and Razzo Rosso forged pistons. Final displacement is 5.4 liters, and the compression ratio is 10:1.

The rotating assembly consists of a stroked Razzo Rosso crankshaft and oversized Razzo Rosso pistons. Note the custom deck plate that raised the block's deck height by 9mm. All photos courtesy of Carobu Engineering.

The taller deck forced a number of accommodations, such as longer head studs, longer timing chain and modified tensioner sprocket. The cylinder heads were ported and matched to the intake, and custom-profile camshafts from Razzo Rosso were ordered. The period-correct exhaust manifolds were recreated by the same fabricator who built Lamborghini’s original marine manifolds. Finishing up the engine are six Weber 40 DCOE side-draft carbs — which had to be recallibrated to complement the new camshaft — and a Pertronics electronic ignition.

“The combination of making the engine bigger and changing the camshafts created the low-speed torque that was necessary,” says Casey. “At 3,500 rpm, the larger engine is making 150 lb-ft more torque.”

On the dyno, the first of the two new marine engines pulled 322 horsepower at 5,100 rpm with peak torque of 379 lb-ft at 3,600 rpm.

The engine shown in the dyno video is now in Holland being installed in the Riva. The second engine is currently being assembled, but now there are new challenges for master engine builder Bert Wehr.

“The trick, of course, is that the second engine has to be counter rotating,” explains Casey. “That requires changes with the oil pump, water pump and such.”

The finished boat will debut at a Lamborghini family event celebrating the 50th anniversary of the automaker, then appear at the Concorso d’Eleganza Villa d’Este, which is held on by the shores of Lake Como. That will provide a picturesque setting for the boat to show off the true potential of Lamborghini marine power.