Herding Cones

In preparing this Grand Sport to compete in the growing niche of street-car racing, Brian Thomson elected to go with a naturally aspirated engine—a 442-cid LS7 with 740 horsepower.

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October 31, 2013

Border collies and similar breeds have herding in their blood. When they don’t get to act on that instinct regularly, they get antsy. Take them to the park and they may try to round up the children on the playground—an act that doesn’t generally sit well with today’s helicopter parents. That’s why many owners take their dogs to farms to let them herd sheep or cattle. It provides the release they need to remain well-adjusted.

It’s the same with late-model Corvettes. These cars were designed to perform more than workday commutes and the occasional Saturday-evening ice-cream run, so owners are turning to high-performance track days and amateur competitions to give their Corvettes the mechanical equivalent of an afternoon on the farm herding sheep.

Engine builder Brian Thomson is no stranger to the racetrack. In nearly 30 years as a professional racing-engine builder, he has participated in drag racing and off-road racing, while his shop, Thomson Automotive, near Detroit, has also built engines for scores of circle-track and road-racing customers, as well as competitors of just about every form of motorsports under the checkered flag. The modified 2012 Grand Sport coupe on these pages represents his first foray into the increasingly popular niche of competition centered on high-performance street cars, where the dogs are really allowed to run.
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The OPTIMA Batteries Ultimate Street Car Invitational is the Indy 500 of these events. Held each fall in Pahrump, Nevada, it’s an automotive decathlon of sorts, pitting the owners of a wide array of hardware against one another in a series of measured competitions, including autocross, road course, road rally, braking and acceleration. Sheer horsepower isn’t the most important factor determining a winner, as the road-course and autocross events require entries to have a competent chassis and the driver to be as good going around cones as they are going down the drag strip. Though generous modifications are allowed, the cars must be road-registered, roll on DOT-approved non-competition tires (with tread ratings above 200) and be raced by a non-professional driver.

In late 2012, Thomson’s Grand Sport scored its first big win, capturing first place at the Norwalk, Ohio qualifying event for the OPTIMA Invitational. With GM engineer and experienced hot shoe Dave Mikels at the wheel, the car trounced a field filled with fifth- and sixth-generation Corvettes, hopped-up Gen-5 Camaros and a slew of Pro-Touring-style muscle cars, which featured contemporary suspension and powertrain technology stuffed under their vintage bodies.

“You don’t have to be the fastest or quickest in every category, but you’ve got to be competitive in them all and outstanding in a few,” said Thomson. “That means you really have to focus on the all-around performance and balance of the car, because there are basically no rules when it comes to the powertrain or suspension. As long as it’s not a real race car or a kit car with a license plate, you can run just about anything.”

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Reigning champion Mark Stielow, who also won in 2011, can testify to that, capturing his back-to-back titles in first-generation Camaros and proving a May-December marriage of classic style and modern technology can be devastatingly effective. He is another GM engineer and one of the pioneers of the Pro-Touring movement.

Not coincidently, Stielow’s cars—a 1969 Camaro in 2011 and a 1967 Camaro for 2012—were powered by 7.0-liter V8 engines with LS9 superchargers built by Thomson Automotive. They produced 800 horsepower and 880 horsepower, respectively. When it came time to prepare his Grand Sport for the OPTIMA Invitational, Thomson determined forced induction wasn’t the way to go. “Mark’s Camaros have performed wonderfully with supercharged power,” says Thomson, “but frankly those early Camaros are heavy and the immediate power delivered by the supercharger helped them launch and build momentum immediately. In a comparatively lightweight car like a Z06, or even our Grand Sport—which weighs about 3,300 pounds—the instant power of a supercharger or the lag of a turbo can be more difficult to modulate on a track.”

Thomson’s naturally aspirated solution was an LS7 stroked from 427 cubic inches (7.0 liters) to 442 cubic inches (7.2 liters). It breathes through Australia-based Harrop Engineering’s unique Hurricane individual-runner intake manifold, which provides an approximately 40-horsepower advantage over a stock intake. Those features and a few others contribute to an output of 740 horsepower and 640 lbs-ft of torque. And while it doesn’t produce the prodigious twist of his supercharged 7.0-liter engines, the 442-inch mill delivers plenty of low-rpm grunt.

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At 3,000 rpm, for example, the engine produces nearly 450 lb.-ft. of torque, and by 3,500 rpm its twisting power increases to almost 500 lbs-ft. Likewise, the horsepower’s crescendo starts low and peaks at 6,500 rpm, where the engine is not only making 235 more horsepower than a stock LS7, it tops the standard ZR1 mill by more than 100. And if that wasn’t enough, there’s a nitrous system plumbed into the engine to give that extra shove over the cliff.

“The longer stroke delivers the low-rpm torque we wanted from the engine, which is great for road racing and autocrossing—especially in the qualifiers for the OPTIMA Invitational,” explains Thomson. “The linearity of the power progression through the rpm band is sweet, strong and tractable. The power builds predictably with an exceptionally strong foundation of grunt to pull you out of a corner very quickly—you can just leave it in a lower gear during turns and explode out of the corners with controllable power.”

Thomson is quick to note the “Hurricane 442”—its crate-engine name—is just as happy on the street as the racetrack. “The OPTIMA Invitational is all about street cars, not dedicated race cars,” he says. “We did the 2013 Hot Rod Power Tour with it, and it performed flawlessly for 1,200 miles driving from Texas to North Carolina. It’s a real sweetheart for the street or track. You can drive this car to work every day, and I often do.”

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While the stock drivetrain may have lived with freeway cruising as its primary role, it wasn’t going to stand up to the engine’s 640 lbs-ft of torque, so Thomson had the transaxle replaced with a beefed-up LS9-based setup built by RPM Transmissions, which includes a bulletproof Quaife differential fitted with a 4.10:1 ring-and-pinion ratio.

Likewise, the brakes were upgraded to the carbon-ceramic discs from the ZR1. “Those brakes are stunningly good,” says Thomson. “Not only do they stop the car amazingly quick, they deliver consistent, fade-free performance for a whole weekend’s worth of racing. It’s probably the best upgrade we did to the car after the engine.” The swap required the ZR1’s unique wheel-bearing hubs, as well as ZR1-sized rolling stock. To that end, Thomson shoehorned on a set of 10- x 19-inch front/12×20-inch rear Forgeline wheels shod with 285/30ZR19 and 335/25ZR20 Michelin Pilot Sport 2 tires. The chassis controller had to be recalibrated for the brakes, too, and while he was poking around electronically, Thomson added launch control.

To help the Grand Sport blow through the cones quicker and flatter, Thomson installed Chevrolet Performance’s SCCA-approved “T1” racing suspension kit, which features upper and lower front control arms and rear control arms—all with heavy-duty bushings—higher-rate springs, and thicker anti-roll bars with adjustable end links. The parts definitely contribute to quicker reflexes on the track, but don’t drastically alter boulevard drivability.

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“Make no mistake, the car and its suspension are set up for the track, but I drive this car to work, too, and while the ride is compromised to a degree, it’s not prohibitively uncomfortable,” says Thomson. “I wouldn’t recommend the T1 suspension for cars intended mostly for the street, but if you’re building a double-duty car like this, you can live with it. We did the whole Hot Rod Power Tour and didn’t end up at the chiropractor afterward.”

We weren’t able to test the effectiveness of the car’s performance on the track—its wins are a testament to that—but we were able to sample it on city streets and out on the highway, where we immediately concurred with Thomson’s description of the linear, predictable power delivery. At low rpm, the power is strong but not overwhelming or uncontrollable. As the revs climb, however, the deep-breathing LS7 heads come into their own, working with a special-grind camshaft to process air at a tremendous rate and giving the car the sort of thrust you’d expect from something catapulted off the deck of an aircraft carrier. The RPM Transmissions transaxle delivers bolt-action shifts that feel confident, quick and precise.

Thomson was also correct about the eye-popping effectiveness of the ZR1 brakes. They pull the Grand Sport down from speed so quickly that we found ourselves constantly checking the rearview mirror when we hit the pedal, worried that drivers behind us would not be able to stop as quickly. On the rare stretches of smooth pavement we could find around Thomson’s shop, the car exhibited very precise cornering and the ride was surprisingly compliant, but a few body-rattling encounters with craggy pothole patches confirmed the T1 suspension’s raison d’être is the track.

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The engine barks to life with all the subtlety of Drill Sergeant Hartman bringing his platoon to attention by banging on an empty garbage can in “Full Metal Jacket.” The extroverted exhaust note is conveyed through a 3-inch-diameter system from Corsa. As the revs increase, the staccato bark morphs into a mechanical wail that wouldn’t be out of place on the Mulsanne Straight.

Regardless of whether Brian Thomson captures the OPTIMA Invitational championship, the competition gives him a chance to exercise the innate nature of his Grand Sport and channel its capabilities. You’ve got to let the dogs run, and, without a doubt, this one does.

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Also from Issue 86

  • 1965 Restomod
  • Interview: Cindy Molnar
  • Buyer’s Guide: Best Bang for Buck
  • Tonawanda Engine Plant
  • 1990 Retromod
  • Monterey Reunion
  • 1970 Coupe
  • History: Zora Arkus-Duntov
  • acing: Baltimore and Austin
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