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Go inside Mercury Racing for a look at their 750hp DOHC LS7!

Mercury Racing and Roadster Shop turn up the heat with a DOHC, 32-valve LS7

Nov 20, 2018
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In the interest of full disclosure, we’re pretty biased here when it comes to GM’s line of LS engines. That said, it would be hard to argue that they aren’t one of the greatest internal combustion engines ever devised. We can remember when people scoffed at the idea that an antiquated pushrod engine could ever be a real performer in the 21st century. They pointed at the smaller-displacement, high-revving performance of Ford’s Coyote DOHC engine as the real future. But, the LS kicked the competition in the teeth thanks to a rock solid design, parts interchangeability, and good old fashioned displacement. And while the competition’s DOHC engines made great upper rpm power, the LS held its own while still delivering torque and power down low.

But what if you could combine the best of both worlds? Well, that’s the idea that sparked in the minds of the gearheads over at Mercury Racing. What gelled in their grey matter was to take one of GM’s more successful LS engines—the 427-inch LS7—and give it the high-rpm DOHC treatment. And, thanks to the 7L displacement, the SB4 still has that low rpm grunt that makes a car fun to drive. It’s called the SB4 7.0 and it’s glorious. It’s an engine that spins up to 8,000 rpm and produces 750 hp well before that redline number. To put that into context, the high-tech LT5 engine in Chevrolet’s hottest Corvette needs a supercharger to nail that same output. Of course, this isn’t Mercury’s first foray into cutting-edge, uber-high-performance GM engines. Back in the early ’90s they were tasked with building the revolutionary aluminum DOHC V-8 in the Corvette ZR-1 (the King-of-the-Hill). In 1990, the LT5-powered ZR-1 broke a 50-year-old FIA endurance record by averaging 175.9 mph over 24 hours! So yeah, they know how to make engines live happily in the land of high rpm.

“This engine represents the best of everything in a hot rod 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 street rodding world when it comes to engine mounts, transmissions, and the like. It also offers a distinctive appearance that’s unlike anything else.” And he’s right, when you spy one under the hood, it has a high-tech yet somewhat familiar look to it. The wide cam covers tell you this isn’t your typical Chevy crate engine, and that’s because it’s not. The short-block is, for the most part, a forged LS7. But, it’s the top end where the magic really happens. You see, Mercury Racing is a division of Mercury Marine and one thing that marine engines have to do is rev, sometimes for hours. The brutal conditions that marine engines have to live in has imparted a lot of knowledge to Mercury about how to make engines live long lives at rpm operating ranges that would turn lesser mills into yard art.

The unique heads, designed in-house by Mercury Racing and cast in the US of A, feature four valves per cylinder and a corresponding valvetrain that’s more than capable of handling the high-rpm task at hand. Internally, the heads utilize a cam-to-cam drive accomplished through the use of straight-cut scissor gears. The scissor gear design yields a zero effective backlash, which makes for a quiet and reliable valvetrain. It also helps lower the belt loads and keeps the engine as narrow as possible.


002 SB4 Mercury Racing LS7 DOHC 2/42

It all starts with a GM LS7 block. The first clue that something’s up is the extension coming off of the camshaft, which will mount gears to drive the four cams that will be joining the party. The “cam” that rides in the factory spot, and that has the extension, is merely there for proper oil control and has no lobes.


As Mercury Racing’s chief engineer, Steve Wynveen, explained, “The trade-offs between cam center distance, valve angles, and pulley diameter is an interesting one. On one hand, you want to make the angle between intake and exhaust valves large. It improves airflow, as the turning angle between the port and the valve seat gets less severe. On the other hand, this drives up the width of the cylinder head, as the cams move apart to stay above the valve stem tip. Big, wide heads don’t fit in the same small space as the compact LS pushrod heads. Reducing the valve angle and pushing the cams together, to create a compact head, has its limits, too. Eventually the cam drive pulleys touch, or they get so small that the force to turn them is too much for a belt to reasonably handle. Mercury Racing’s solution, here, is a single large-diameter belt pulley on the exhaust cams, with the intake cam driven by the aforementioned scissor gears.”

The end result of all this is that the SB4 loves to rev. If you look at a graph you would see that from 2,500 rpm to over 6,000 rpm the power travels up at a 45-degree angle. By 4,300 rpm it’s over 400 hp and rocketing up to over 500 hp by 5,100 rpm. At 6,500 rpm it’s over 700 hp and still rising. But, unlike small-displacement DOHC engines, the SB4 makes torque, even down low in the powerband. It spins out a whopping 570 lb-ft at 6,000 rpm. But more importantly, by 2,700 rpm it’s already making 400 lb-ft and it makes over 500 lb-ft from around 4,500 rpm all the way to redline. Big numbers from a warrantied mill with a sweet idle and excellent street manners.

To get this new mill into the Chevys we love, Mercury Racing partnered with Roadster Shop to make it happen. “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.” The SB4 comes ready to run with a pre-programed PCM (powertrain control module, Mercury-speak for an ECU) and all the wiring bits to make it happen. Now, the engine is quite a bit bigger than your typical LS, or even a big-block for that matter. But Roadster Shop has already grafted it into more than a couple of classis Chevys, so with a little effort it’s an easy fit. Since the short-block is an LS7, mounting to the frame is a snap and the top is roughly 6 inches wider than conventional LS engine. And when it’s nestled between the fenders it just looks like it belongs there.

To get a closer look at the DOHC 7L SB4, Mercury Racing invited us to their high-tech build facility in Fond du Lac, Wisconsin, to see how one is hand-assembled and witness one get wrung out on the dyno.

003 SB4 Mercury Racing LS7 DOHC 3/42

The LS7 block is already ahead of the game in terms of being ready for big power with features such as steel six-bolt main caps. As you can see by the markings, the engine builders at Mercury Racing check every bearing and clearance tolerance.

004 SB4 Mercury Racing LS7 DOHC 4/42

The next piece of the puzzle is a forged 4-inch stroke crank that’s internally balanced and much like any other high-end LS crank.

005 SB4 Mercury Racing LS7 DOHC 5/42

The 7L SB4 mill utilizes 6.125-inch forged H-beam rods. At this point, the build is pretty close to any other high-horsepower LS build.

006 SB4 Mercury Racing LS7 DOHC 6/42

Mercury Racing sourced specific forged, fully coated pistons from Mahle Motorsports for the SB4 short-block.

007 SB4 Mercury Racing LS7 DOHC 7/42

Again, the build so far seems pretty familiar. Since the “cam” installed in the block has no lobes it doesn’t manipulate pushrods. Given this, there’s no need for lifters, but Mercury Racing did design pieces to fit where the lifter would have been. These were important to the engine’s oil system. We would show them to you but they fall into the secret category. Trust us, they are pretty cool.

008 SB4 Mercury Racing LS7 DOHC 8/42

Since the SB4 is a wet-sump engine, and not dry-sump as is typical of LS7 engines, it uses a standard GM high-volume oil pump. The additional capacity keeps all the parts in the DOHC heads lubed. The pump is spaced away from the block to clear the double-roller timing chain. This is where any similarity to a typical LS engine build ends.

009 SB4 Mercury Racing LS7 DOHC 9/42

The crank turns the cam blank, which in turn will turn the four cams housed in the heads. A lot of R&D was invested by Mercury Racing into the exhaust cam drive pulley. Internal to each pulley are four weights, which are free to swing back and forth, like a pendulum. If the cams are going too fast, the pendulum absorbs energy and slows them down. If the cams are too slow, the pendulums return energy and speed them up. The overall effect is to reduce peak loads on the timing belt and improve the belt’s life. It’s a technology called a centrifugal pendulum absorber (CPA). CPAs have been around since World War II, but have been seeing a resurgence in the automotive world, particularly in flywheels and torque converters.

010 SB4 Mercury Racing LS7 DOHC 10/42

The valley cover of the SB4 is similar to a typical LS engine, but has been custom designed by Mercury Racing to better control oil and separate it from the breather gas at 8,000 rpm.

011 SB4 Mercury Racing LS7 DOHC 11/42

And here’s where things get really different. The DOHC cylinder heads were designed in-house by Mercury Racing using computational fluid dynamics. This makes for ports that are optimized for high-tumble airflow. Cast to Mercury’s specifications by Edelbrock, the intake airflow is near 380 cfm at 0.700-inch lift and the exhaust flow hovers near 240 cfm. But the real story with a four-valve head is mid-lift flow, where the valve spends a lot of its time. At 0.200 inches of lift, the intake outflows benchmark two-valve heads by 33 percent and the exhaust outflows them by 39 percent.

012 SB4 Mercury Racing LS7 DOHC 12/42

More valves means more air, more power, and hence, more fun. The 71.2cc combustion chambers house a pair of 1.70-inch intake and a corresponding pair of 1.38-inch exhaust valves. To reduce the head’s overall width a bit, the exhaust valves are angled fairly shallow from the cylinder bore axis; just 4.5 degrees. And while the chamber size may sound a bit large they still support a compression ratio of 11.7:1 with a flat-top piston.

013 SB4 Mercury Racing LS7 DOHC 13/42

Besides moving air, the heads also house Mercury’s very cool valvetrain.

014 SB4 Mercury Racing LS7 DOHC 14/42

Made of billet steel, the four camshafts (two per head) are carburized, which is a type of case hardening. The cams are also REM surface-finished and polished to reduce any friction that could lead to increased wear.

015 SB4 Mercury Racing LS7 DOHC 15/42

A shaft-mounted, end-pivot finger-follower design is key to the SB4’s valvetrain. The fingers are laterally held in place by coil springs on the rocker shafts and there are adjustment shims between each finger and its respective valve stem tip. Mercury Racing designed the fingers and they are investment cast steel, which is then case hardened, ground, REM surface-finished and then given a super-hard DLC coating.

016 SB4 Mercury Racing LS7 DOHC 16/42

As is typical with DOHC engines, exhaust and intake valve control is split up between two camshafts, which gives far more control over valve events.

017 SB4 Mercury Racing LS7 DOHC 17/42

The exhaust cam drives the intake cam and the two are kept in perfect timing (phasing) by a pair (per head) of anti-backlash, ground, scissor-type gears. The scissor gear ensures zero lash, which minimizes engine noise, vibration, and harshness (NVH).

018 SB4 Mercury Racing LS7 DOHC 18/42
019 SB4 Mercury Racing LS7 DOHC 19/42

The belt from the cam blank turns a drive pulley that spins the exhaust cam, which in turn spins the intake cam, so a failure anywhere is a failure everywhere. To keep belt loads low (increasing belt life) the cam drive system has a reduction set up in two stages to get to the needed 2:1 overall ratio. There’s a Mercury Racing designed hydraulic tensioner to maintain stability over 8,000 rpm on the chain portion of the drive. Meanwhile, the belt drive pulley for the exhaust cams have built-in pendulum absorbers to ensure the timing drive stays happy at high engine speeds.

020 SB4 Mercury Racing LS7 DOHC 20/42

The Mercury heads utilize a bedplate design. This means the cams don’t have individual caps but instead the outer head structure and cam caps are all one bedplate, sometimes referred to as a ladder frame. This bedplate doesn’t have enough bolts to keep it down against the force of the flexing valvesprings without the top cover in place. This means the bolts for the top covers hold the covers on as well as secure the bedplates to the head. To set valve lash, a steel girdle is used to keep the bedplate secured without the top cover.

021 SB4 Mercury Racing LS7 DOHC 21/42

While hard to see, and harder to photograph, the perimeter of the bedplates have oil sprayer holes to further ensure the cam lobes stay properly lubricated.

022 SB4 Mercury Racing LS7 DOHC 22/42

To feed fuel to the port injected engine, Mercury uses 60-lb/hr fuel injectors mounted in rails of their own design. We got a kick out of how many parts were labeled in terms of port and starboard instead of left and right, a nod to Mercury’s marine roots.

023 SB4 Mercury Racing LS7 DOHC 23/42

The SB4 uses a coil-on-plug arrangement typical of DOHC engines.

024 SB4 Mercury Racing LS7 DOHC 24/42

Much like the heads, Mercury Racing put a lot of effort into optimizing the SB4’s cast-aluminum intake manifold. To optimize airflow, Mercury used computational fluid dynamics so ensure the air moved as efficiently as possible to all eight cylinders. Coupled to this intake is a pair of 80mm electronic throttle bodies. As a bonus, they look badass.

025 SB4 Mercury Racing LS7 DOHC 25/42

Here’s how the SB4 ships from Mercury, all dressed up and ready to play hard. The drive belts for the cams are protected by covers and the 427 is fitted with a complete accessory drive system. Due to a proprietary timing cover, the SB4 is able to utilize an LS-style water pump.

026 SB4 Mercury Racing LS7 DOHC 26/42

The 7L SB4 uses a modified wet-sump oil pan (6.5 quarts of 15W-50) sourced from Holley (PN 302-1). The large L-shaped tube is an oil drain-back channel. There’s one at the rear of each head to help get all the oil from the heads back into the pan.

027 SB4 Mercury Racing LS7 DOHC 27/42

The SB4 comes with everything needed to get it running in your classic Chevy, including a preprogramed ECU and even the throttle-by-wire gas pedal.

028 SB4 Mercury Racing LS7 DOHC 28/42

Mercury Racing had an SB4 on their dyno so we could experience some power pulls. Of course, they weren’t like any dyno pulls we’ve seen before. While we’re used to running engines up to peak rpm and them bringing them right back down; Mercury took the SB4 to 8,000 rpm and held it there for what seemed like forever. They explained that this is common for how they validate their engines and comes from how marine engines are forced to operate. The result is a bulletproof engine that loves to rev and one that can easily handle what your car might throw at it.

029 SB4 Mercury Racing LS7 DOHC 29/42

Of course, the end goal of all of this is power—lots of power—and the SB4 delivers. The large displacement of the 427-inch LS7 combined with the high-revving properties of the DOHC arrangement makes power high, power low, and power everywhere in between. It makes around 750 hp from 7,500 rpm all the way to its redline of 8,000 rpm and it spins out 570 lb-ft at 6,000 rpm. It makes over 500 lb-ft from around 4,500 rpm all the way to redline. Big numbers from a warrantied mill with a sweet idle and excellent street manners.

030 SB4 Mercury Racing LS7 DOHC 30/42

The big question we get is if it will fit in a classic Chevy. The Roadster Shop has already transplanted them into quite a few cars, including the ’66 Chevelle on this month’s cover. As for specs, the SB4 measures out at 27.1 inches in length (back of the block to the front of the crank pulley), 30.27 inches wide (across the timing belt cover), 29.1 inches wide (across the rear of cylinder heads), and 17.1 inches tall (from the crank centerline to the top of the throttle body motors). The SB4 tips the scales, without the accessory drive, at 498 pounds.

031 SB4 Mercury Racing LS7 DOHC 31/42

Photography by Steven Rupp

Sources

Roadster Shop
Mundelein, IL
847-949-7637
www.roadstershop.com
Mercury Marine Racing
Fond du Lac, WI
920-921-5330
http://www.mercuryracing.com/

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