The Inside Scoop On Sonny Leonard’s New 820ci Marine Engine

Sonny Leonard’s expansion into the marine market is currently being showcased by a pair of 820ci naturally aspirated mountain motors that team up for nearly 3,000 horsepower on pump gas. As we reported a couple years ago, Leonard worked with CN Blocks to develop a billet block with generous water passages and 5.3-inch bore spacing that will allow a number of large-displacement options. This package from Sonny’s Racing Engines features a 5.046-inch bore and a 5.125-inch stroke crankshaft for 820ci, or 13.4 liters.

Sonny Leonard is leveraging his experience building mountain motor drag race engines to the marine market with this torque-laden 820ci V8 that runs on pump gas.

“To make 1,450-plus horsepower naturally aspirated on unleaded gas, it was easy with the large displacement,” says Leonard. “We don’t have to strain the engine.”

The engines were designed to run at 6,000 to 6,500 rpm for extended periods. Key to that performance objective is consistent torque. On the dyno, this engine formula delivers at least 1,100 lb-ft from 5,100 rpm up through the operating range.

“Even at part throttle it produces a lot of torque,” adds Leonard.

The foundation is a SAR billet-aluminum block with 5.300-inch bore spacing that is machined to allow for full water jackets. The deck height is 12.125 inch.

Sonny’s was a pioneer of the mountain motor formula, first developing large-bore engines with a custom 5.000-inch bore space block. Then when customers wanted more than 900 cubic inches, he engineered billet blocks and cylinder heads with a 5.300-inch bore spacing. The large bores not only help increase displacement but also allow huge valves in the cylinder head to improve airflow.

Note the water jacket cover plate and center water inlet. The rotating assembly includes a Bryant Racing billet crankshaft and Carrillo connecting rods. The pistons are from Wiseco. The dished piston is for a lower compression version that uses unleaded pump gas. The other piston is for higher compression ratio using 110-octane gas for poker runs.

Here’s a topside view of the short-block. Note the bushed lifter bores and ARP head studs.

The marine block features a 12.125-inch deck height and 70mm cam bearings. The rotating assembly comprises a Bryant Racing billet-steel crankshaft, ATI damper, Carrillo steel connecting rods (8.232-inch long) and Wiseco dished, full-skirt pistons that provide an 11:1 compression ratio. Higher compression pistons are available but would require 110-octane fuel. Leonard estimates the race fuel and extra CR will add about 200 horsepower at 7,200 rpm.

“We worked out the rod length and deck height to get the ideal angularity,” says Leonard. “Also, at 6,400 to 6,600 rpm the piston speed was more ideal. In turn that all gives you more endurance for the engine.”

The SAR cylinder heads feature a hemispherical valve arrangement with a single spark plug. Note the individual stands for the shaft-mounted rocker arms and the contoured oil feed line to lubricate the valve springs.

Here's the in-house designed and fabricated intake manifold that supports 96-pound Precision injectors. The dual 4-barrel throttle bodies feature huge 2.750-inch bores.

Lubrication is provided by a Dailey Engineering 5-stage oil pump scavenging from a Moroso steel oil pan. The valvetrain starts with a Sonny’s belt drive that turns a SAR (Sonny’s Automotive Racing)/Comp Cams camshaft. The exact specs are not being released, but we know there’s a 119-degree lobe separation angle and at least 1-inch of valve lift. Jesel supplies the 1.062-inch lifters and Trend builds the 9/16-inch pushrods that measure 10.625-inch on the intake side and 12.350-inch on the exhaust.

“The valvetrain we have will run at 6,500 with no problem,” says Leonard. “We use that same setup up to 8,200 and 8,400 rpm [in drag race engines].”

The valvetrain includes a SAR camshaft, Jesel lifters, Trend pushrods and T&D Machine 1.75:1 rocker arms.

The fully dressed front end shows the Waterman fuel pump, ATI balance, Dailey oil pump and Jesel distributor.

The SAR 5.3 GM-style hemispherical cylinder heads will flow over 725 cfm on the intake side with 1.2-inch of valve lift, but that’s for high-rpm Pro Stock competition. For marine applications, the heads are configured for a broader torque band and reliability with pump gas. They’re still pretty healthy with 2.750-inch Trick Titanium intake valves that flow 635 cfm at 1.000-inch of lift, and 2.030-inch Inconel exhaust that flow 440 cfm at 1.000-inch of lift. The valves are secured with PSI Racing triple valve springs and titanium retainers. Finally, the 1.75:1 aluminum rocker arms are from T&D Machine. The heads are installed with Cometic gaskets and torqued down with ARP hardware.

Sonny’s designed and fabricated the billet/sheet-metal intake manifold that is topped off with a pair of Accufab 4-barrel throttle bodies that feature huge 2.750-inch bores. Fuel is delivered through Precision Turbo 95-pound injectors and controlled with a Big Stuff 3 ECU.

“We could put another set of injectors in the manifold if the guy wanted to run E85,” notes Leonard. “With racing gas we’d run a different camshaft and more static compression.”

On the dyno with 11:1 compression ratio and 93 octane fuel, the engine pulled 1,450 horsepower at 6,500 rpm. It also delivered more than 1,100 lb-ft of torque from 5,000 rpm up through 6,900 rpm.

On the dyno with 93-octane fuel, the engine hit 1,000 horsepower at 5,000 rpm and is still climbing at over 1,500 horsepower at 6,900 rpm when the test run was shut down. At least 1,000 lb-ft of torque was recorded from 4,800 up through 6,900 rpm. Sonny’s actually built two engines for the customer’s boat that will be used for pleasure on both fresh and salt water.

“We ran both on the dyno, and I have never had two engines that ran as close together as they did,” says Leonard, noting that’s it’s much easier to run and build identical engines when they’re not boosted. “They were within one half of one percent.”

Two matched engines are needed for many large offshore powerboats. Unlike the old days when one engine had be set up for reverse rotation, today’s modern drive systems allow for both engines to have conventional rotation.

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About the author

Mike Magda

Mike Magda is a veteran automotive writer with credits in publications such as Racecar Engineering, Hot Rod, Engine Technology International, Motor Trend, Automobile, Automotive Testing Technology and Professional Motorsport World.
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