You’re a car lover whose ship has finally come in or whose kids have finally graduated their pricey prep schools and Ivy League universities. You’re finally ready to treat yourself to the custom muscle car or restorod of your dreams. When shopping for an engine to breathe life into your baby, the path of least resistance and greatest selection is often the GM Performance Parts Catalogue. This bible of the biz offers a dizzying array of over 70 small-block, big-block, LS, LT, LSX, and racing engines in your choice of naturally aspirated, turbocharged, and supercharged induction. These power-dense, mostly pushrod mills tend to fit right into most engine compartments and frequently come with plug-and-play electronics to ease the build. Their drawbacks: Popularity makes them somewhat common, and they don’t all look that great.
Folks hoping to imbue their custom builds with Riddler-Award level visual pizazz as well as dyno-pegging power now have a new option to consider. It’s based on the tried-and-true GM LS Small Block architecture, and it comes from a company with historic ties to GM and motorsports. The Mercury Racing division of Mercury Marine has been developing performance engines for both marine and land-based motorsports ever since company founder Carl Kiekhaefer dominated NASCAR, winning 80 percent of the races he entered between 1955 and 1957. You might also recall that Mercury was tapped to produce the low-volume aluminum four-cam 32-valve 5.7-liter LT5 engine that powered the Corvette ZR1.
Mercury’s latest creation is called the SB4 (small-block four-valve). It shares that ZR1 engine’s all-aluminum construction, small-block bore spacing, and four-valve DOHC heads, but whereas the original was designed from scratch by Lotus, this one starts with a 428-cubic-inch GM LS7 block casting machined to Mercury Racing specifications. The cylinder heads are downsized versions of the ones topping Mercury’s mighty 9.0-liter QC4 twin-turbo engines, which are capable of producing a whopping 1,750 hp.
Down where the single camshaft used to live is a dummy cam driven by a similar primary chain from the crankshaft. Belts drive the outboard cams via pendulum-damped pulleys. The cam pairs are geared together via a “scissors gear,” where part of the input gear is split and spring loaded to remove any lash with the mating gear. The valves are actuated via finger followers with manual lash adjusters. These heads would fit any LS series engine (if fitted with pistons that provide valve relief), but only the 7.0-liter takes full advantage of the improved flow they provide, and in any case, Mercury isn’t peddling the heads by itself. There’s no variable timing or lift—this engine is optimized for performance with little regard for fuel economy or emissions (it is not CARB or EPA certified, so like many of the GM Performance crate engines, it’s for off-road racing use and is only street legal when installed in emissions-exempt vintage vehicles).
So equipped, the SB4 produces 750 horsepower at 7,500 rpm—500 rpm shy of fuel shutoff—and 545 lb-ft at 6,500 rpm. Designed exclusively for automotive use, it features a sharper power curve and a slightly less broad torque curve (off-shore, direct-drive race boats need an Ayers Rock torque curve). Fully dressed, it weighs 498 pounds. The drivability development work on this new engine was conducted with it installed in a mid-engine Ultima GTR race car, spinning through a 2004-06-vintage Ford GT transaxle. Curb weight as equipped is about 2,300 pounds. I was invited to an airfield in Mercury’s hometown of Fond du Lac, Wisconsin, to sample this mighty crate engine in this mouse of a car. I was as impressed with its light-throttle tractability as I was with its full-bore ferocity. The throttle calibration impressed with its linearity throughout the rev range—a trait that makes a race car much easier to balance on the neutral knife-edge of an optimal slip angle when negotiating a tricky set of turns, possibly in race traffic. It also makes a restomod or rodded muscle car vastly more pleasurable to drive in city-street traffic.
On the day I drove the car, the team was still making final calibration tweaks to address idle quality (it was pretty lumpy, and the engine stalled a few times) and to iron out a few bugs. During my time in the car, it suffered some spurious overheat warnings and a tendency to enter a limp mode after very brief fuel-shutoff rev excursions (which were likely due to the Ultima’s lazy tach). Any such minor foibles are quickly forgiven when you drop the hammer in first gear and hang on tight, rocketing past 70 mph in first gear with seven naturally aspirated liters of intake roar and exhaust snarl bellowing a molto forte aria just over your shoulder.
It’s not much to look at in the Ultima mule car, but the folks at SpeedKore, located just an hour away in Grafton, Wisconsin, are at work integrating an SB4 into the second-gen Camaro project car pictured above, where it will surely be a focal point of the build in much the same way as the Mercury Racing QC4 twin-turbo 9-liter 1,650-hp V-8 is in SpeedKore’s fabulous carbon-bodied 1970 Charger. That car, dubbed Tantrum, boasts a wondrous-looking exhaust system featuring four-into-two headers flowing forward to a pair of twin-scroll turbos mounted just behind the radiator support. To demonstrate the company’s commitment to making its engines look nice in a car, Mercury offers the SB4 in a choice of 12 colors or “natural” for paint-to-match applications (such as the $75,000 Alubeam Silver job done on a different Mercury Racing engine for a Mercedes-AMG/Cigarette racing boat). There’s also an optional carbon-fiber intake manifold cover.
Mercury has just started shipping SB4 crate engines priced at $28,995, which includes a Mercury Racing engine control system. That’s fairly favorable with comparable GM Performance COPO engines, which range from $25,372 for the 425-hp 427 to $35,200 for the 530-hp supercharged 350 (and those only come in red/orange and black).
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