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."
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:
Flexplate: The stock flexplate is composed of two pieces of stamped steel bolted together. Under high load, it comes under stress at the fastening points. Also, the relatively thin steel plates can crack or tear. Failure of this part is messy and dangerous, so replacing it with a thicker, tougher single-piece flexplate is essential. RPM Transmissions matches the stronger flexplate with a billet steel cone that replaces the stock cast part. Flywheels (for stick cars) aren't nearly as fragile, but also a great upgrade and part of many quality clutch upgrade packages.
Bellhousing: RPM Transmission uses a spun steel bellhousing that is SFI-approved for maximum containment strength. For the quickest cars, the factory aluminum bellhousing is not only inadequate, it's not allowed.
Torque tube/driveshaft assembly: IRS or straight axle, a stronger driveshaft has long been a basic drivetrain upgrade. C5 and C6 Corvettes, however, incorporate a pair of rubber couplers with the driveshaft, inside the torque tube, which simply weren't designed for the rigors of 8-second drag racing. When put under extreme loads, the couplers tend to grow and rub themselves against the wall of the torque tube. Typically this chews up and destroys them. Replacing them with stronger polyurethane couplers is required.
Transmissions: Stick-shift trannies seem to be the hardest things to reconcile with IRS. Only a small handful of people have managed to go fast while rowing their own gears, even though there are plenty of modifications that can be made to enable the T56 and 6060 to handle gobs of power. Instead, specially built 4L60E, 4L80E, and even Turbo 400 transmissions seem to be the ways to go for drag racing. They're built to handle exceptional torque levels, while offering greater consistency. Mark Carlyle's Z06 runs a Turbo 400, while Massengale's personal Z06 runs 9s with a 4L60E. C5 and C6s were offered with 4L60E transmissions, so swapping one into a stick or 6L80E car is relatively easy. Going with a 4L80E or Turbo 400, however, requires surgery. Each is a few inches longer and taller than a 4L60E, so tunnel modifications are required, as well as a shortened torque tube and shorter driveshaft. It's a serious commitment to step up to one of these transmissions, but they're strong and dependable.
"The 4L60E is great for cars running from the 11s into the 9s, as I can personally attest," says Massengale. "If you're aiming for bottom 9s or the 8s, you'll probably want to go with the 4L80E or Turbo 400." And when it comes to deciding between the two, the Turbo 400 is ultimately more adaptable, with a greater range of gear ratios to select. It also offers a faster transbrake and there's no lock-up converter, which the 4L80E has in addition to a fourth (overdrive) gear. In short, it's a true racing transmission. So, what about a Powerglide? There is some debate in the community about whether the two-speed trans is a good option for the C5/C6. Massengale, however, favors the TH400 as he says the 'glide doesn't have the optimal gear ratio for the larger 3.42 ring gear of the Z06 or ZR1 differential. Without the ability to change gear ratios, while running the stronger diff, it would certainly be hard to optimize whatever combination you are running. Case in point, the two fastest IRS cars to date both ran RPM-built Turbo 400s.
Pfadt solid transmission mounts are typically matched with these upgraded transmissions to help minimize drivetrain flex, which translates to a reduced chance of wheel-hop. Of course, a torque converter must be matched with the vehicle's specific combination. Precision Industries or Coan Racing seem to be two very popular brands among the Corvette community.
Differential: RPM Transmissions upgrades stock differentials for cars mostly used on the street, using compressed carbon clutch packs in place of the stock bronze clutch packs, among other modifications. There are also a few aftermarket differentials that provide greater capacity than the stock diff, but when it comes to the quickest of the quick, the Quaife torque-biasing differential is the only option. Unlike other differentials, it does not incorporate clutch packs-it's strictly all gears inside, for maximum strength. Sets of floating helical gear pinions mesh to provide "normal" performance, but in the presence of wheel spin, torque bias is generated by the axial and radial thrusts of the pinions in their pockets. The friction forces that result and enables the diff to transmit a greater proportion of the torque to the appropriate wheel. According to Quaife, it's a progressive process and doesn't entirely lock the diff. It's not a cheap solution, but a bulletproof one.
Differential housing: C5 and C6 Corvettes have used a variety of differential housings, of different sizes and strengths. However, there are only two real choices for performance levels we're talking about in this story: C6 Z06 and ZR1, with the ZR1 offering the greatest strength. These housings are not only stronger than other Corvette models, they're large enough to swallow a greater diameter ring and pinion. The C5 housings are the smallest and weakest, and upgrading your C5 with the aforementioned Z06 or ZR1 housing means you'll also need to swap in the rear chassis cradle from a C6.
Output shafts, CV shafts, etc.: We're getting near the literal and figurative end of the drivetrain here, with the parts that convey torque most directly to the wheels. The output shafts from the differential need to be as strong as possible. Replacing the stock shafts goes without saying, but aftermarket shafts are not created equally. RPM Transmissions has had the best luck with shafts created in conjunction with The Drive Shaft Shop. They're stronger than 300M steel, with rolled splines that maximize their strength. After that, stronger CV shafts are needed and even stronger wheel studs. A pair of output shafts will run about $550, but the CV shafts are considerably more expensive. The Drive Shaft Shop's shafts run about $2,295 a pair. Again, not cheap, but that's the price of going fast.
There you have it-the basics of making an IRS-equipped Corvette deliver consistent and safe 8-second performance. And as Massengale mentioned at the beginning of our story, suspension factors must be matched with the stronger drivetrain parts, because they will all be wasted on the starting line if wheel-hop cannot be overcome. "It's amazing the numbers these Corvettes are running with an IRS," said Massengale. "You might be surprised by how little drivetrain breakage there is, but we've worked really hard over the years to find what works. If you can put the power to the ground without shaking the wheels off, the parts are available to make an IRS car run just about as quick as you want."
We'll be looking for 7s this season.