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750-hp DOHC LS-based Mercury Racing Motor from the Roadster Shop

DOHC Déjà Vu: Mercury Marine and Roadster Shop team up for a four-cam crate engine with echoes of the original LT5 ZR-1 engine

Peter Murphy Sep 3, 2018
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Marine engine specialist Mercury is building an exotic, Chevy-based dual-overhead-camshaft V-8.

That’s the story lead you might have read just about 30 years ago when the company was tasked with assembling the C4 ZR-1’s DOHC LT5 engine. Selected for a number of reasons, including its prowess with aluminum castings and low-volume production efficiency, Mercury’s MerCruiser division handbuilt all of the first- (385hp) and second-generation (405hp) LT5 engines.

Three decades later, the company is at it again … sort of. The company’s Mercury Racing division is, indeed, building new DOHC V-8 engines, but rather than doing it for a production Chevrolet product, they’re building a standalone automotive crate engine package dubbed the SB4 7.0, which is offered through the hot rod builders at Roadster Shop.

In a nutshell, the SB4 7.0 uses a GM LS foundation (the LS7 cylinder block) with an all-new, custom-designed set of four-valve, dual-overhead-camshaft cylinder heads. The result is a naturally aspirated LS-based engine that spins to 8,000 rpm and produces 750 horsepower. Chevrolet relies on a supercharger to get the same output for the LT5 engine in the Corvette ZR1.

In fact, the SB4 7.0 clocks in at 1.75 horsepower per cubic inch, versus the original LT5’s 1.07 horses per cube. That’s 63 percent greater horsepower per displacement, but it’s not surprising for anyone who runs the waves as well as the highway because Mercury has been building its own DOHC, high-rpm marine engines for a long time. They’ve simply funneled that experience from the launch ramp to the street.

“This engine represents the best of everything in a restomod or Pro Touring crate engine,” says Phil Gerber, owner of Roadster Shop, the exclusive retailer for the SB4 7.0. “It takes LS-based performance to the next level, with a familiar foundation that is already supported in the aftermarket when it comes to engine mounts, transmissions and the like. It also offers a distinctive appearance that’s unlike anything else.”

That’s for sure. The extra-wide cam covers lend the engine a decidedly high-tech, exotic look that’s complemented by a custom intake manifold featuring a pair of throttle bodies. It would look pretty amazing in the engine compartment of a C1 or C2 restomod.

The unique cylinder heads, which are cast by Edelbrock, feature four valves per cylinder and a robust, high-rpm valvetrain. While not unique to DOHC designs, the heads are somewhat unique to performance V-8 engines because they employ internal cam-to-cam drive via straight-cut gears. One gear in each head is a scissors gear design, which allows for zero effective backlash and very quiet operation. It was designed that way to keep the heads, and the engine, as narrow as possible. Without this design feature, on a narrow head, the twin cam drive pulleys would be considerably smaller so as not to touch each other. That would, in turn, lead to higher belt loads because of the reduced driving radius.

And when it comes to spinning those cams, the SB4 sings like few other engines. From 2,500 to 6,000 rpm, the power ramps up on a 45-degree angle, crossing the 400-horsepower mark by 4,300 rpm and 500 horsepower by 5,100 rpm. After 6,000 rpm, the power curve bubbles up and races to 700 horsepower by 6,500 rpm and finally tops out at 750 horses at a stellar 8,000 rpm.

It would be natural, too, to assume all that high-rev horsepower comes at the expense of torque. It’s true that the engine isn’t a big-block torque monster, but it tops out at around 570 lb-ft at 6,000 rpm, so it’s no slouch in the grunt department.

As for Roadster Shop’s involvement, the company partnered with Mercury Racing to bring the engine from the production line to the market.

“We have made it our business to provide customers with the latest in design, chassis and performance, all packaged to save time and money, while helping ensure trouble-free enjoyment,” says Gerber. “The SB4 7.0 offers a unique opportunity for us to deliver one of the industry’s most unique engines in a turnkey package. We’ve already used it in several of our vehicle builds and the results have been spectacular.”

So, will it fit? Probably. With those DOHC heads, the SB4 7.0 is wide, no doubt about it. It stretches right around 30.3 inches (769 mm) at the front, measured across the broad timing cover, and 29 inches at the rear, from head to head. That’s roughly 6 inches wider than a conventional LS engine and about 10 inches wider than an old-school small-block Chevy, but only about 2 inches wider than a conventional big-block. Most of the extra width is hanging out over the headers, which will likely present some master cylinder interference issues on most generations of Corvettes, but resourceful builders should be able to work around it. Like a conventional LS engine, it’s also a lot lighter than a big-block, by 150-200 pounds.

Value is always in the eye of the beholder, but the prospect of 750 naturally aspirated horsepower with DOHC and an exhaust note that will send a shiver up your spine are elements that are difficult to quantify. And the familial link between it and that other DOHC V-8 Mercury built all those years ago is also an intangible and enticing factor.

The godfather himself, Zora Arkus-Duntov, pushed for a DOHC Corvette engine from the early days in his position as chief engineer. Short of robbing a C4 ZR-1 of its powertrain, this is your best shot and channeling his vision into your Vette.

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1. The SB4 7.0 uses the Chevrolet LS7 cylinder block for its foundation. Note the shaft in the camshaft position here. It takes the full-length position of the original camshaft, to ensure proper oiling throughout the engine, but has no lobes. Instead, it will mount gears to drive the camshafts.

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2. The LS7 block makes an excellent foundation for the engine, as it features strong steel main caps and large 4.125-inch bores. Other 6.2L GM blocks feature smaller, 4.065-inch bores. The LS7 block is also deck plate-honed from the factory, for more precise cylinder sealing when the heads are clamped in place.

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3. An internally balanced, forged steel crankshaft with a 4.000-inch stroke complements the block’s 4.125-inch bores to give the engine its 7.0-liter (428 cubic inches) displacement.

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4. Lightweight hypereutectic pistons supplied by Mahle Motorsports enable the engine to rev quickly to 8,000 rpm. The unique head profile includes a pair of reliefs for the two intake valves at each cylinder.

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5. The SB4 7.0 uses forged H-beam connecting rods measuring 6.125 inches in length. Their strength and comparative low weight is key to the engine’s high-rpm durability.

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6. The engine is set up for a conventional wet-sump oiling system, including a GM high-volume pump, which differs from the LS7’s production dry-sump system. Note, too, the double-row timing chain behind it.

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7. Mercury Racing designed the four-valve DOHC cylinder head in-house, using computational fluid dynamics to optimize the ports for high-tumble airflow. Edelbrock handles the aluminum heads’ casting. Intake airflow is around 380 cfm at 0.700-inch lift. Exhaust port flow is around 240 cfm.

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8. The 71.2cc combustion chambers process the airflow drawn in and out by 1.70-inch intake and 1.38-inch exhaust valves. The exhaust valves are angled a little steeper to reduce the heads’ overall width. The comparatively large chambers still support an 11.7:1 compression ratio that enables the engine to run on premium pump gas.

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9. The camshafts are made of billet steel and carburized (case-hardened) on all running surfaces for exceptional wear resistance. They are also polished and REM surface-finished to help reduce friction.

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10. On each head, the exhaust cam drives the intake cam with a scissor-type action that ensures the pair are phased properly at all times and engine speeds. The design also helps keep the heads relatively narrow.

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11. The cam drive system has reduction in two stages to keep belt loads as low as possible. Together, they yield the required 2:1 overall ratio. The chain primary drive has a Mercury-designed hydraulic tensing system to maintain stability at 8,000 rpm, while the cam-drive belt pulleys (on the exhaust cams) have integrated pendulum absorbers to keep the timing drive stable at high engine speeds and high valve accelerations.

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12. The valvetrain relies on a shaft-mounted, end-pivot finger-follower design, with adjustment shims between each finger and the respective valve stem tip. The fingers are held in place, laterally, by coil springs on the rocker shafts. The fingers themselves are Mercury’s own design and are made of investment cast steel, then case-hardened, ground REM surface-finished and DLC-coated (diamond-like coating).

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13. The cylinder head has a bedplate design, meaning it does not have individual cam caps. Instead, the cam caps and outer head structure are all one bedplate (also known as a ladder frame). Because the bedplate does not have enough fasteners to hold it down against flexing from valvespring force, when the top cover is removed, the top cover’s bolts do double duty: they hold the top cover on and clamp the cylinder head’s bedplate to the head. It makes setting lash a bit harder, but it saves weight because of the reduced fastener count. The steel girdle seen atop the head here is used when setting valve lash.

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14. The port-injected engine uses 60-lb/hr fuel injectors.

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15. Like the heads, the cast-aluminum intake manifold was designed with computational fluid dynamics for optimal airflow. A pair of 80mm electronically controlled throttle bodies is one of the engine’s distinctive design elements.

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16. The curved, L-shaped component at the right of the photo is an oil drain-back tube and there’s one at the rear of each cylinder head. The engine also uses a Holley oil pan.

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17. A Mercury Racing-developed controller is included with the crate engine kit.

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18. On the dyno, the SB4 7.0 winds up to 8,000 rpm to produce 750 horsepower on pump gas. Peak torque is a strong 570 lb-ft, making this high-winding, LS-based powerhouse a strong performer across the lofty rpm band.

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19. Dimensionally, the engine measures 30.3 inches at its widest, in the front, and is 27.1 inches long and 17.1 inches tall (from the crankshaft centerline to the top of the throttle bodies). As for weight, it tips the scales at 498 pounds, without an accessory drive system. And the black-and-silver finish is standard. Custom colors and a carbon fiber engine cover are optional.

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Mercury and the Building of the Original LT5

The original LT5 DOHC V-8 of the C4 ZR-1 was a technical marvel from every point of view and perspective. It also represented a fascinating departure from General Motors’ characteristically cloistered engine development culture. The company reached out to Britain-based Lotus Engineering for their overhead-cam expertise and tapped Mercury Marine’s MerCruiser division to assemble the low-production engine. MerCruiser was selected for, among other things, its decades-long experience with aluminum-intense engines and its relative efficiency with low-volume production. More than simply bolting the engine together, the MerCruiser facility performed a good deal of CNC-based machine work on the aluminum components, adhering to Chevrolet’s strict tolerances of as little as seven microns on most of them. That required designing some all-new tooling, including a $750,000-investment in a custom cam tunnel boring machine. Much of the engines’ assembly was performed by hand, before each and every one was dyno-tested. The LT5’s balance and precise assembly made for such a smooth-running engine that MerCruiser operations director Bud Agner suggested a nickel would remain standing on end while the engine was running. He was right. Search YouTube for a look at MotorWeek’s visit of the Mercury plant, wherein you’ll learn that, for all the high-tech, CNC machinery that went into the assembly, workers used rattle-can barbeque grille paint for the black lettering on top of the intake manifold. No kidding.

Photography by Steven Rupp

Sources

Roadster Shop
Mundelein, IL
847-949-7637
www.roadstershop.com

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