Few enthusiasts would accuse the C7 Corvette Z06 of being underpowered. Packing a 6.2-liter supercharged and direct injected LT4 V8 that makes 650 horsepower and 650 lb-ft of torque, it stood as the most powerful factory-produced vehicle that General Motors had ever produced when the final numbers were formally announced in June of 2014.
At first glance the Callaway SC757 Corvette doesn’t look much different from that of a garden-variety C7 Z06. But closer inspection reveals some subtle hints as to its performance credentials, such as the factory-like Callaway badging and supercharger housing peeking out of the hood.
Its performance from the factory is equally impressive, dispatching the sprint to 60 mph from rest in just 2.95 seconds when equipped with the 8-speed automatic and 3.2 seconds for those who prefer to row their own with the seven-speed manual transmission.
But getting those numbers out in the real world can be a bit tricky. Putting the power to the pavement through the rear tires is a delicate balance between not bogging the motor and not sending the tires up in smoke, and that’s to say nothing of the patience required getting on the throttle coming out of slow corners or in less-than-ideal weather conditions. So it’s fair to say that in factory-stock form, the C7 Corvette Z06 can be a bit of a handful when going all out.
Still, there are those among us who’ve grown accustomed to this level of performance and still crave for more. It’s for those brave souls that vehicles like the Callaway Cars SC757 Corvette Z06 exist. But don’t think for a second that the SC757 is simply a pulley and badges affair.
The Callaway Origin Story
Callaway has been tuning Corvettes for jaw-dropping performance for more than thirty years. The company’s origin dates all the way back to 1977 though, when Bondurant Racing School instructor Reeves Callaway started adding turbocharged performance to the then-new BMW 320i to great acclaim.
But the company would truly begin to register on enthusiasts’ radar in 1987 when they unleashed their one-off Sledgehammer Corvette, a twin-turbocharged, 900 horsepower monster that would set an astonishing street legal production car top speed record of 254.76 mph at the Transportation Research Center in East Liberty, Ohio on October 19th, 1988 after a 700 mile drive to the facility.
The Sledgehammer project got underway after Callaway Cars’ successful outing at the Car & Driver Magazine “Gathering of Eagles” top speed test event in 1987, where Reeves Callaway piloted a twin-turbocharged C4 Corvette to a top speed of 231 mph. That Corvette won top honors at the event and set the stage for an even more ambitious goal – 250 miles per hour. But beyond just posting a jaw-dropping top speed, Callaway wanted this new flagship model to be a rolling showcase for what the company could accomplish, and therefore it needed to have everyday drivability, along with all of the creature comforts of a typical factory-produced Corvette.
The Sledgehammer was powered by a NASCAR-spec four-bolt “Bowtie Block” with Brodix cylinder heads, Mahle pistons on forged connecting rods, and a unique camshaft that would provide low-speed drivability as well as top-end pull. Forced induction came by way of a pair of Turbonetics TO4B turbochargers. All in, the combination was good for 898 horsepower @ 6200 rpm and 772 pound-feet of torque at 5250 rpm, making it perhaps the most powerful street-legal Corvette ever built at the time. Image: Mecum
It’s a record that would remain in place for decades until the Sledgehammer was eventually dethroned by the $1.7 million Bugatti Veyron Super Sport in 2010.
So it’s safe to assume that this company knows a thing or two about wringing extra performance out of Chevrolet’s sports car. In the years since that record-setting event, Callaway Cars has become synonymous with General Motors tuned performance.
Their current lineup of performance packages applied not only to the Corvette Z06 and Stingray, but the sixth generation Camaro SS as well as GM’s full size trucks and SUVs. Yet amongst the buffet of horsepower they have on offer, the SC757 sits at the top of the totem pole.
SC757 Corvette Z06
You’d be forgiven for assuming the SC757 is simply an exercise in horsepower one-upmanship. Dishing out 757 horsepower and 777 pound-feet of torque, the SC757’s addition of 107 horsepower and 127 lb-ft to a platform that can struggle to put the power down in stock form seems like a sure-fire recipe for an even more unruly driving experience, but Callaway’s modifications provide enhanced capability that goes well beyond sheer output statistics. However, in order to fully understand those benefits, we’ll need to go back a few years in time to the C6 ZR1.
When the C6 Z06 debuted in 2009, it made headlines for its 638 horsepower supercharged LS9 motor, which was the most powerful production engine in General Motors’ history at the time. A substantial element of how the mill made such an impressive amount of power came down to the supercharger, a 2.3-liter unit that maxed out at around 15,000 rpm. Corvette aficionados may recall the ZR1’s impressive track capability, which earned it production car lap records at both the Nurhburgring and Mazda Raceway Laguna Seca.
With a 0-60 mph time of under three seconds according to Chevrolet's own testing, few could reasonably consider the C7 Corvette Z06 to be slow. But not long after the C7 Z06 got into the hands of journalists and the public at large it became clear that the sports car's supercharged LT4 motor had some difficulty keeping engine temperatures down during repeated lapping on road courses. It's been speculated that Chevrolet's decision to downsize the LT4's supercharger displacement versus that of the LS9 came down to federal crash safety standards compliance. Whatever the reason, it's enough for many Z06 owners who track their cars regularly to start looking for solutions to the problem. Callaway's SC757 package not only gives the Z06 even more power, it also addresses some of the car's cooling issues while doing so.
The C7 Z06 has been notably less successful on track though, and many in the know would point the finger squarely at the LT4 motor – not for lack of power, mind you, but for its well documented cooling issues. The problem is widespread enough that a class action lawsuit has been brought to GM by Z06 owners, and while many contend that the problem stems from multiple sources, most agree that the new supercharger is a major culprit. Rather than using a 2.3-liter unit similar to that of the LS9, General Motors downsized for the LT4, opting for a 1.7-liter blower. The decision to reduce displacement means the blower must spin faster to create a similar level of boost, which in turn generates more heat. That might not be an issue in typical daily commuting duties, but the sustained intensity of the road course driving is an entirely different matter.
Callaway says that their SC757 supercharger setup directs charge air up toward the top of the housing through a primary intercooler and then around the sides of the housing. This intercooler conducts heat from the charge air to the intercooler coolant. The coolant is circulated through a heat exchanger in front of the radiator, where the heat is radiated into the atmosphere. The cooled air exiting the intercooler then flows through the upper section of the supercharger housing. Since the housing extends through the hood and is exposed to ambient temperature, convective heat transfer removes additional heat from the charge air as it passes over the internal surface. Just before reaching the cylinder heads, the inlet air passes through a pair of auxiliary intercoolers where heat is extracted yet again.
Rather than throwing more boost at the stock components and further exaggerating the issue, the SC757 treatment swaps out the stock 1.7-liter blower with Callaway’s GenThree supercharger system. The package consists of a 2.3-liter TVS2300 roots-style blower that utilizes three discrete intercooler elements and larger capacity coolant pump to keep inlet air temperatures down.
It’s paired with a new intake manifold with a runner configuration that’s tweaked to increase mid-range torque. And the supercharger housing popping through the hood? That’s not just for cool points on cruise nights – it allows for better air flow and heat transfer through the engine bay versus the stock hood, which is another aspect of the heat management equation that the factory Z06 has struggled with.
From fifty feet away you'd be hard pressed to tell a Callaway SC757 from a factory-stock Z06, but therein lies some of the car's charm.
In terms of aesthetics, rather than wearing its performance credentials on its sleeve the Callaway SC757 opts for relative subtly, allowing the already head-turning wide-body Z06 and optional Z07 aero package to do most of the work. Truthfully, aside from the hood extractor and the factory-style badging that adorns the rear fenders and rear fascia, there isn’t much to tip off onlookers to the fact that Callaway has had a hand in enhancing the car’s performance. The interior tells a similar story, as a pair of embroidered floor mats and small badge on the center console are the primary tip offs.
Callaway also offers a few optional mechanical tweaks for the SC757 as well, including a low restriction sport exhaust system that does away with the factory’s active dual-mode system and its habit of suddenly switching from subtle rubble to deafening roar by implementing non-active system that’s tuned to eliminate cabin resonance while giving the SC757’s bark a little more character.
For owners of manual transmission equipped Z06s, Callaway also offers a short throw shifter that eliminates the rubbery feel of the factory setup and cuts down the length of shift throws while retaining the stock shift knob for a factory look.
As this dyno chart indicates, the power curve of the Callaway-tuned mill is remarkably similar to that of a standard LT4, albeit with a significant bump in horsepower and torque throughout the rev range.
The tuning company is also in the midst of developing an optional heavy duty cooling system for hardcore track junkies who require an even more comprehensive cooling solution. Callaway says that package will add a reshaped front clip to the mix for better airflow through the engine bay, as well as higher capacity cooling system and other tweaks.
While the package may be visually low key, the performance it provides certainly is not. Equipped with the factory-optioned Michelin Pilot Sport Cup 2 tires, Callaway clocked a 2.8 second sprint to 60 miles per hour in the SC757 on the way to a 10.5 second quarter mile time, putting the car’s performance on par with vehicles costing multitudes more than a Callaway-modified Z06 does.
But what’s perhaps even more impressive is the fact that the SC757 package doesn’t void the factory warranty – as it happens, these Callaway packages can actually be optioned on new Corvette orders through select Chevrolet dealerships across the country. Moreover, Callaway provides a three-year, 36,000 mile warranty of their own on top of that as well, which can optionally be extended to five years and 60,000 miles.
The AeroWagen is a polarizing design in that most folks either love it or hate it. While that's largely a subjective matter, there's no denying that it stands out compared to standard C7s.
For those looking for an even greater level of Callaway Corvette exclusivity, there’s also the AeroWagen conversion. Available for both the C7 Stingray and the Z06 (whether they have Callaway performance packages or not), this modification gives the Corvette a shooting brake-style profile by replacing the hatch with a reshaped piece, along with new glass and spoilers. The effect not only gives the AeroWagen unique presence, it also provides a degree of functionality in that it expands the car’s limited cargo space.
So what’s next for Callaway Cars and the Corvette? Well, there’s little doubt they’ll be adding more performance to the upcoming ZR1 not long after it lands in Chevrolet dealerships, and we have no doubt they’ll be looking to boast the capability of the long rumored mid-engine C8 Corvette (or whatever GM model it proves to be) if and when it finally becomes a reality.
In the meantime, it looks like we’ll just have to make do with this 757 horsepower C7 Z06. The times we live in, eh?