Lone Star Power

Though it gained its renown as a Viper tuner, Hennessey Performance has become increasingly associated with high-output Corvettes. We sample its first supercharged 2014 Stingray.

Photo: Lone Star Power 1
May 1, 2014

Some people just want more. More power. More speed. More excitement. While the 2014 Corvette Stingray certainly delivers on all these counts compared to its C6 predecessor, some people just want, well, more. That’s where tuners like Hennessey Performance Engineering (HPE) come in, and after driving an HPE700 Stingray convertible, we can assure you that it is no status-quo C7.

As one could well imagine, modifying the 2014 Stingray is no small undertaking. From stem to stern, this is a brand-new Corvette, and a significantly more complex one at that. Not only is the LT1 engine bristling with new technologies, such as direct fuel injection and cylinder deactivation, it is more thoroughly integrated with the rest of the vehicle’s electronics than ever before. In addition to the traction- and stability-control systems, the LT1 is, for example, wired to the electronic limited-slip differential on Z51 Performance Package-equipped cars.

Given this heightened sophistication, it’s understandable that Hennessey did not rush headlong into modifying the Stingray, preferring to take a steady, even-handed approach. Beginning in the fall of 2013, it began by working up the naturally aspirated ladder to increased performance. The first rung was its HPE500 engine-upgrade kit. By installing stainless-steel headers, stainless-steel mid-pipes and high-flow catalytic converters, then recalibrating the engine-management system, HPE was able to increase the LT1’s output from 460 to 507 horsepower. Hennessey then upped the ante by adding high-flow cylinder heads and a more aggressive camshaft to the mix, resulting the 603-hp HPE600 package. In both cases, Hennessey engineers found no way to improve upon the stock intake system and left well enough alone.

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You may have caught a YouTube clip of former GM engineer John Heinricy testing a Z51 coupe with the HPE600 package at Hennessey’s Sealy, Texas proving grounds; it made the rounds this past December. Heinricy came away impressed with the work done to the C7’s 6.2-liter V8. “It really made the engine come alive,” he enthuses in the video. “One of the things that’s cool is how strong it pulls all the way to redline.”

Since it built its reputation on more significant power gains, Hennessey logically turned to forced induction as the next step in its Stingray modification campaign. Its first move in this direction was to create a twin-turbo coupe. With its rear-mounted compressors requiring extensive fabrication work, the 700-hp HPE700 Twin Turbo package will cost a heady $39,500 when it’s ready for public consumption, greatly limiting its number of potential customers. Hennessey expects far more takers for the considerably less expensive supercharged kits it has developed.

Having collaborated with Edelbrock on its supercharged C6 offerings, Hennessey turned to the well-established Southern California tuning shop to co-develop a Roots-type blower for the C7. This bolt-on kit couples the highly regarded Eaton TVS2300 supercharger with a neatly packaged intercooler atop the engine. According to Hennessey’s PR man Doug Kott, this kit, in 50-state-compliant HPE600 form, will become the company’s “bread and butter” engine upgrade package when it becomes available.

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We didn’t drive the “bread and butter” Hennessey Stingray. The black-on-black convertible on these pages sports a centrifugal supercharger. This type of blower is known for developing power higher in the rev range, as opposed to the more instantaneous delivery of a Roots-type blower. As such, a centrifugal supercharger appeals to a different kind of customer, one that is interested in having an engine that delivers an intense, high-end punch and is willing to forgo the immediate satisfaction of a broader torque curve. Pick your poison. In an ideal world, we would have sampled both types of superchargers on a C7, but Edelbrock and Hennessey weren’t quite done with the Roots-type system when we went to press.

Another difference between centrifugal and Roots-type superchargers is size, with the former generally being larger than the latter. Since the Paxton Novi 1500 SL blower HPE elected to use wasn’t a good fit on top of the LT1, it needed to be mounted elsewhere in the engine bay. Fortunately, as a result of the Stingray’s accessories being mostly located on the passenger side of the compartment, there is a nice open space for the compressor on the driver side. It appears as if Chevy had reserved a parking spot for a supercharger in this spot, though the ABS unit does need to be moved slightly to make room. Still, the installation is no simple, bolt-on affair. While the Corvette’s front fascia need not be removed, the steering rack, radiator and air-conditioning condenser do get yanked in order to install the drive pulley. Locating the pulley, on the other hand, could not be more straightforward: It locates over the stock harmonic damper, lining up perfectly with the existing ribs—no key is necessary.

Packaging the intercooler presented a challenge, as Hennessey wanted to make the installation as straightforward as possible without blocking airflow to the stock heat exchangers—sticking an intercooler in front of the radiator is a bad aftermarket habit. HPE came up with a clever solution: After making a cutout in the undertray, it mounted the intercooler horizontally, just behind the front spoiler, thus preserving both airflow through the grille and under the car, as well as providing plenty of air to the intercooler.

Photo: Lone Star Power 4

To ensure that boost pressure is maintained throughout the system, HPE uses aluminum ducting, reinforced silicone coupler and heavy-duty T-clamps. As with Hennessey’s naturally aspirated C7 offerings, the stock airbox is retained, but in this case it did need to be slightly modified to get the ducting to fit properly. A Tial combination blow-off valve/wastegate controls the flow of compressed air.

On the engine side of the equation, Hennessey garnered a great deal of information from its twin-turbo project. It established that the stock fuel injectors are up to the task of supplying a 700-horsepower engine. On the other hand, it learned, after successive dyno pulls, that the stock direct injection couldn’t generate enough fuel pressure at such elevated output levels. Since the system is run off a triangular lobe on the camshaft, the only way to increase the fuel pressure was to install a modified camshaft. The $22,500 HPE700 centrifugal supercharger package includes such a cam, along with high-flow cylinder heads and long-tube headers; the $15,950 HPE650 setup retains the stock camshaft and forgoes the porting and polishing, as well as the exhaust mods. With both packages, the LT1’s cylinder-deactivation system is disabled.

While bolting on the various parts is a big part of an HPE build, no car is complete until it’s been dyno-tuned. The bulk of the engine-computer calibration is done during the development process, but each customer vehicle is run on a dyno to ensure the engine is achieving the peak output target and that no undesirable troughs or spikes are showing up on the power and torque curves. Minor adjustments are made as needed using EFI Live software. Hennessey has two dedicated engine-management technicians, and this laptop-wielding pair is kept busy.

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Though the centrifugal supercharger development was done in Texas, the installation work on the Stingray convertible we drove was performed at the company’s Southern California shop in Lake Forest. Officially considered a “proof of concept” vehicle and owned by Hennessey, this car is unique in a few regards. For the most part its engine represents an HPE650 build, but it also has mildly ported cylinder heads, upgraded valve springs and titanium retainers, which push its output to roughly 700 horsepower at the crank. More precisely, according to Hennessey’s Dynojet, this LT1 generates 557 horsepower and 517 lb-ft of torque at the wheels, as compared to the standard engine’s 400 hp and 400 lb-ft of torque baseline numbers.

What’s remarkable is that the centrifugal blower is only running at about 7 psi of boost, as opposed to the supercharger fitted to a ZR1’s LS9, which is cranked up 11 psi. The Paxton Novi unit that Hennessey uses has a lot of headroom, enough to support a 900-hp engine. Creating such an engine is not HPE’s goal at the moment. “We’re not about making absolute peak power,” says Kott. “We want to make good power that’s going to live.” Given the 3-year, 30,000-mile warranty that comes with the HPE650 and HPE700 packages, this is an important consideration. Kott said this particular engine might eventually receive a bit more tuning, but that the technicians were happy with what they had achieved thus far. The time had come for us to find out if we agree with them.

Before we set out from the Lake Forest shop, however, we spent a few moments drinking in the C7’s appearance. As we’ve said before, the Stingray convertible looks particularly good with its top down. This all-black version has a sinister beauty, thanks in part to its exterior modifications. It sports HPE’s full CarbonAero kit, which includes a large front splitter, rocker-panel extensions and a full-width rear spoiler, as well as sill plates inside the cockpit. Designed in-house, fabricated by Frank “The German” Morgenstern in Austin, Texas and costing $8,780 installed, these exposed carbon-fiber pieces give the car’s appearance a distinctive edge. The rear spoiler is significantly more aggressive than Chevy’s Z51 version.

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Our featured car had also been treated to a set of sharp-looking Hennessey H10 forged mono-block wheels. Sized the same as the standard Z51 wheels—8.5 × 19-inch front, 10 × 20-inch rear—these 10-spoke alloys cut a significant 19 pounds from the car’s unsprung weight. The stock Michelin Super Sport run-flats were retained, as were the Z51 brakes and suspension setup. According to Kott, Hennessey saw no need to improve upon them; it’s happy to focus most of its attention on the powerplant.

To sample the fruit of HPE’s labors, we headed out into the Trabuco Canyon area east of Irvine to drive the sweeping corners of Santiago Canyon Road and the tight bends of Live Oak Canyon Road. The intersection of these two roads houses a famous biker bar called Cook’s Corner, and we received a round of thumbs-up from a group of Harley riders as we got hard on the gas and rocketed down a straightaway, taking our first full swig of Hennessey’s high-proof concoction.

As expected, given the type of super charger used, the engine’s added power was not immediately felt. Around 3,000 rpm we could feel the boost pressure building, then it seriously ramped up at roughly 4,500 rpm. The engine then ripped to redline with head-spinning ferocity, catapulting the Stingray down the road with real supercar conviction. The boosted LT1 rushed so quickly to redline that we found ourselves repeatedly bouncing off the rev limiter, not able to pull the paddle on this automatic-equipped convertible quickly enough. While we preferred to use the paddles in the twisties, placing the transmission in automatic mode was better for hard acceleration runs; it clicked off perfectly timed shifts.

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While the hilly terrain of our test route, with its short straights, prevented us from fully exploiting all of this Corvette’s 700 horsepower, it was a great test of the car’s drivability. We liked the way the engine’s progressive power curve allowed us to get hard on the gas out of corners without upsetting the Stingray’s phenomenal handling balance. Sure, it was much easier to kick the tail out than on a standard Stingray, but at no point did the rear end feel overwhelmed. The C7’s sophisticated traction control deserves praise in this regard, as does the Z51 suspension. Given this Stingray’s relatively narrow 285/30ZR20 rear tires, the chassis’ ability to handle all the extra ponies is truly impressive.

The blown engine behaved itself around town and while cruising along the highway. We experienced no supercharger whine thanks to the Paxton supercharger’s helical-cut gears. However, as Kott had warned us, the combination blow-off valve/wastegate unleashed a veritable flood of whooshes; it was like having the soundtrack to Niagara Falls plumbed into the cabin. Any time revs climbed above 2,000 rpm, which was most of the time given our enthusiasm with the throttle, the whooshing would commence, and was only drowned out by the exhaust note under full acceleration. All this unwanted noise detracted from the driving experience, but we were assured that a much quieter type of blow-off valve/wastegate combo can be installed.

The 2014 Corvette Stingray is an enormously satisfying sports car to drive. From the sharpness of its steering to its incredible roadholding ability to its strong acceleration, the C7 delivers on so many counts. One of our few complaints has been the linearity of its powerplant; the stock LT1 engine just seems to be missing that extra top-end surge that adds to the excitement of piloting a high-performance sports car. Hennessey’s supercharged C7 solves that problem, providing a memorable surge of power at high revs while not detracting from the many things that are so right about the new Stingray. It’s more Corvette, for those that want it.

Also from Issue 90

  • 1967 Small-Block Convertible
  • Buyer’s Guide: Late-Model Z-Cars
  • 1994 Coupe
  • History: ’53 Styling
  • 2004 Coupe
  • Tech: C7 Brakes
  • Racing: Sebring
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