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.
“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.
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.
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.”
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 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 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.”