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Chevrolet Corvettes and Independent Rear Suspension - The Power Of Independence - TechDrag Racers Are Pushing Their C5/C6 Corvettes Into The 8s With Independent Rear Suspension-And We Find Out How They're Doing It From the April, 2011 issue of GM High-Tech Performance By Barry Kluczyk Photography by Barry Kluczyk
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At a glance, there's nothing that indicates Al Brodbeck's C5 Z06 is a capable of low-9-second e.t.'s. Apart from the racing wheels and tires, it looks stock and the Vortech YSI-trim blower generates little aural clue to the capability under the hood. But when this Corvette launches, time seems to stand still and all too soon the board lights up at the far end of the strip with stunning numbers. The best performance to date is a 9.04 at 159.96 mph-with what Brodbeck describes as a "lousy" short time in the 1.50-second range. More remarkable is this car squats, launches, and freight-trains down the track with little drama, all while still channeling its power through the C5's independent rear suspension (IRS). As impressive as Brodbeck's performance is, it's not an anomaly. These days, 9- and 8-second C5 and C6 Corvettes are regular sights at tracks around the country and, surprisingly, many aren't leaving chunks of half-shafts at the starting line. That's not to say these cars are performing their warp-speed e.t.'s through stock axles, of course, but the fact they're doing it through an IRS (at all) flies in the face of conventional drag racing wisdom. "Those racers are really pushing the envelope, but with the right drivetrain parts and suspension, you can get an IRS-equipped Corvette into the 7s," says Rodney Massengale, founder and president of RPM Transmissions-and he should know. His Anderson, Indiana, shop has been on the front line of enabling C5 and C6 Corvettes to go quicker without shedding parts. "It's not just about a stronger differential and axles-although they definitely play a crucial role," says Massengale. "It's the whole drivetrain that must be strengthened to support this level of performance. It's like that saying about a chain only being as strong as its weakest link." The layout of the C5 and C6... The layout of the C5 and C6 Corvettes is unique and not simply because of the independent rear suspension. The rear-mounted transmission and supporting components-including the torque tube that surrounds the driveshaft-create their own dynamics on the drivetrain. The chassis was designed for driving performance that includes corners, not 8-second e.t.'s, so making it compliant and consistent on the dragstrip requires careful suspension tuning and a host of specialized drivetrain parts. Of course, strength is a relative term, and what's strong enough for a 10- or 11-second street/strip car isn't as strong as the needs for low 9s and 8-second e.t.'s. As Massengale pointed out to us, there are options for every level of performance, from comparatively simple upgrades of the stock components to the complete-and, yes, expensive-replacement of essentially the entire drivetrain. He also told us the biggest killer of transmissions and rearends wasn't too high horsepower or torque channeled through weak parts, it was abuse and wheel-hop. "We've seen cars with only about 400 rear-wheel horsepower break the differential housing because of wheel-hop and we have customers who send twice that power through a basically stock manual transmission because they know how to launch correctly and shift precisely," says Massengale. "Once you've got the suspension dialed in and can launch without wheel-hop, you're half way there. But if you can't beat it or you keep missing shifts, the strongest axles in the world won't save you." The really quick Corvettes we've encountered, however, aren't running beefed up stock parts; and because we were interested in how they were making it happen, Massengale walked us through the setup his shop put together for Ohio customer Mark Carlyle. At the time we finished this story, his Atomic Orange, single-turbo '07 Z06 is the quickest IRS C6 Z06 on record, running 8.15 at 173 mph during the 2010 season. "Starting at the engine, Mark's car runs a new flexplate and bellhousing, along with a new driveshaft and driveshaft couplers," says Massengale. "The transmission is a Turbo 400 and the differential is a Quaife set in a Z06 housing. And, of course, the output shafts are stronger and even the CV shafts and wheel studs are stronger. It's a complete system from end to end, backed up with the right suspension." Starting at the front of the drivetrain, here's an outline of how and why the components are strengthened:  Typical of many power-adder...  Typical of many power-adder street/strip cars these days, Al Brodbeck's C5 Z06 uses a Vortech on a built LS7 to rip a best e.t. of 9.04 at 159.96 mph. The stock drivetrain is replaced with an RPM Transmissions-built 4L60E automatic and a C6 Z06 differential housing filled with a Quaife differential, as well as a number of other upgraded components.  This shattered diff housing...  This shattered diff housing isn't the result of too much power sent through the stock drivetrain. The damage was caused by violent wheel-hop, which stresses and cracks the housing. A stronger diff or transmission isn't the answer to this problem-suspension tuning is.  The twisted splines of these...  The twisted splines of these output shafts indicate parts not strong enough for an engine's torque and/or the car's traction capability. These shafts were supposedly high-performance aftermarket units, but still failed under high load. On even a moderate-level car, with say 550 rwhp or more, replacing the factory output shafts should be one of the first upgrades performed-as well as the transmission's main shaft. And the more power the car makes, the more important the shafts become.  Here is RPM Transmissions'...  Here is RPM Transmissions' SFI-approved flexplate on top of a stock Corvette flexplate. It is a single-piece, made of H60 steel that is thicker and stronger than the factory two-piece flexplate. On very quick cars, the stock flexplate just doesn't cut it-from a performance standpoint or a rules standpoint. Automatic cars running faster than 9.99 are required to run this, per NHRA rules.  To its credit, the factory,...  To its credit, the factory, two-piece flexplate is a relatively low-mass piece, designed for low inertia in Corvettes with the stock engines. It just wasn't designed for 1,000-horsepower drag cars. At that level, stress begins to pull at the fasteners holding the two pieces together, and cracks formed between open sections can turn the flexplate into a 6,500-rpm Chinese throwing star.  RPM Transmissions uses this...  RPM Transmissions uses this billet steel cone on the centersection of their flexplate, replacing the factory cast piece that simply isn't strong enough for high-power, high-load combinations. It's relatively small, but a very important part of the total drivetrain upgrades.  Another area in need of improvement...  Another area in need of improvement is the torque converter drive plate. Shown here is the factory stamped-steel part with RPM's upgrade. Like the flexplate, strength is more important in these applications than low weight-although it's not significantly heavier than the stock part.  At 1/4-inch thick, the RPM...  At 1/4-inch thick, the RPM Transmissions drive plate is 1/8-inch thicker than stock. This provides an exponential increase in strength, which stands up to 1,000-plus-hp power-adder engines.  An SFI-certified bellhousing...  An SFI-certified bellhousing is a must in fast cars, as a safety feature rather than one that delivers a measurable performance benefit. This one is made of spun steel. Factory Corvette bellhousings are made of aluminum, which weigh little, but shatter when hit by a wayward flexplate. SFI bellhousings are required on manual transmission cars faster than 11.50 and automatic cars faster than 9.99, per NHRA rules.
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