More power is great. Until it starts breaking stuff. We added more than 75 wheel horsepower to our C5 Corvette Z06 when we swapped out the original LS6 for a crate LS3. But what are the ramifications of 560-plus crank horsepower for a drivetrain designed around a 405-horsepower V8? Weak links were revealed, and we needed to address them.
Bulletproofing the Clutch
Stock C5 Corvette Z06 clutches should be filed under “barely adequate.” Our stock clutch was fine for street duty–back when it was doing such things–but even heavy autocross use with the stock engine started to cause issues. Heat would lead to slippage, which would lead to more heat, and the cycle would spiral until we just needed to let the thing cool back to ambient.
We initially replaced the stock clutch with a stronger, single-disc unit. While we were swapping engines, however, we decided to go all the way and upgrade to a twin-disc unit from Mantic Clutch USA coupled to an aluminum flywheel. This completely rebuildable clutch is rated for more than 1000 lb.-ft. of torque and features titanium drive blocks that further reduce rotating mass.
Our twin-disc setup also weighs 32 pounds, 6 pounds less than the single-disc unit it replaced and 23 pounds less than the stock setup. Retail is about $2750, but we actually spent more on that single-disc clutch.
Photography Credit: J.G. Pasterjak
Specs on paper are one thing, but the real test would be its operation and longevity in extreme circumstances. Operationally, we couldn’t be more delighted. The action of the clutch pedal feels very similar to–possibly even lighter than–the stock setup. It’s smooth and intuitive, and the bite point is easy to pick up for consistent launches. In short, we hardly think about it, which is some of the highest praise we can heap on a clutch.
As for its longevity, time will tell, but Mantic Clutch USA’s Geoff Gerko shared some best practices. Heat is its biggest enemy, he stresses: “Don’t slip the clutch any more than necessary. Get it rolling in the pits and get that clutch completely engaged as quick as you can when idling around. Push it on to scales, winch it onto the trailer, and make those upshifts and downshifts quickly and positively, and you’ll defer any unnecessary wear for a long time.”
He adds that lack of activity could lead to accelerated wear: “Don’t let it sit for months at a time without at least taking a drive to get some heat and friction into it to keep the mating surfaces happy.”
As long as the clutch feels good, Gerko concludes, there isn’t much need for regular inspection. “If you pull it apart for inspection, you actually risk contaminating the discs and friction surfaces,” he says, “so it’s best to just let it keep doing what it’s doing until it’s time to rebuild.”
Bulletproofing the Torque Tube
Instead of linking the engine to the rear-mounted transmission with a traditional driveshaft, the C5 does it with a torque tube. We had ours completely rebuilt when we performed our first clutch swap. The Vette Doctor in Melbourne, Florida, used OEM parts from front to rear, including new bearings, seals and driveshaft couplers.
We didn’t upgrade to aftermarket couplers, but our originals still looked good upon inspection. Perhaps those stock, thick rubber couplers, which damp driveline vibration both for passenger compartment NVH attenuation and mitigating component fatigue, are just fine for track use.
See those cracks in the torque tube’s drive coupler? (Don’t worry, it’s not ours.) It’s a common wear item in C5 Corvettes. Fortunately, factory-fresh ones seem to work well on track–you just need to regularly inspect them. Photography Credits: J.G. Pasterjak
An informal poll of C5 owners backs up our findings. The stock drive couplers–assuming they’re in good shape–seem to have plenty of excess capacity to handle additional power and torque. Plus, “upgraded” couplers–ones made of stiffer urethane or even solid materials like aluminum–still fail at about the same rate as stock pieces while introducing second-order complications in the form of additional vibrations.
So, are all aftermarket couplers junk? Probably not, but we aren’t worried about premature failure based on what we’ve seen and heard. Should we reach the point where we need to upgrade, we might move to OEM pieces from a Corvette ZR1–a popular upgrade for those who want some additional insurance but want to stay within the GM parts catalog.
Bulletproofing the Gears
Did we need to worry about the extra power tearing up our transmission? We checked in with GearFX, the firm that originally rebuilt and optimized our box.
GearFX’s Jeff Horton showed us some testing performed by REM Surface Engineering, the company whose technology GearFX uses to polish and prep the key surfaces of the transmission. Budget about $500 for similar treatment.
REM’s testing showed that its polishing process increased durability by 100%. Credit goes to reduced friction, fewer stress risers, and the associated benefits of the micropolished surfaces. Components that typically experienced a 90% failure rate after 200 million test cycles consistently needed as many as half a billion cycles to produce a similar failure rate after the REM polishing process.
GearFX ran our gears through the REM-polishing process. High-tech abrasives are used to polish metal components and reduce friction-induced heat and stress risers.
Still, you can’t REM every surface inside a transmission, and Horton says the biggest wear items will still be things the driver controls. “The area that REM does not affect or help is driver-induced synchro wear,” he notes. “The gear teeth and shafts will be stronger, but the synchro teeth are still a victim of the driver, even though we installed upgraded synchros. Since we REMed the shifting finger and guides, it will be a smoother shift, however I have been around long enough to know that some guys can wear out anything.”
Our mission going forward is to exercise some mechanical empathy. We need to learn how quickly we can shift without beating the synchros and causing premature wear. Shifting too slowly can be just as detrimental at higher loads, as it gives the engine more time to drop revs and induce more driveline shock once the shift is completed. Practice makes perfect.
Aside from those cautions, Horton says, maintenance should be exactly the same as any other track-used car. That means regular fluid checks and changes–we use synthetic GM Dexron VI ATF. Aside from that, don’t touch it unless something is going wrong.
Bulletproofing the Drivetrain Mounts
Even with the stock original engine, we’d begun experiencing a lot of wheel hop on launches–and even under rolling acceleration in lower gears.
We traced the issue to a failing differential mount–a single, center-mounted, flexible connection far more concerned with occupant comfort than power transmission. It did a lousy job of keeping that relationship consistent, allowing the differential to rotate in a plane perpendicular to the driveshaft. And since it was undamped, it turned into an oscillating spring once tension was applied.
Our solution came from aFe Power in the form of the brand’s Pfadt differential mount–about $500 retail. This billet aluminum mount features outriggers that more effectively transfer torque to the subframe while better supporting the differential through both leverage and stiffer urethane and metallic mounts.
Photography Credits: aFe Power (top, second from top), J.G. Pasterjak
Installation took only a few minutes, and adjustment of the outrigger pads took only a few more minutes. The benefits were instant and stunning: no more wheel hop, predictable torque delivery, zero worries.
We replaced our engine mounts with aFe Power units as well, and we recommend the same for anyone doing the diff mount upgrade. If you’re installing headers or a dry sump, you’ll need to upgrade those engine mounts from stock, as the OEM pieces won’t provide enough clearance.
Hard launches on grippy concrete with sticky tires produce a lot of driveline tension. Properly dealing with that tension increases both performance and reliability. The OEM diff mount–that soft thing at the top of the page–acts like a big, floppy, rubber pillow, but the aFe unit features stiffer construction thanks to more leverage and decreased flex. Installation is straightforward, but you might lose a little knuckle skin. It’s worth it, though. We also replaced our stock engine mounts–the black rubber things to the left–with stiffer ones from aFe Power. Photography Credit: Wayne Presley
Bulletproofing the rear end
Extra power can be brutal on the rear end, but GearFX had already future-proofed ours when it installed a REM-polished ring and pinion as well as a torque-biasing WaveTrac differential. Maintenance here should just involve regular fluid checks and changes. The polishing and blueprinting process considerably cuts down on metal filings suspended in the fluid, further reducing wear.
Wavetrac offers a drop-in solution for the C5 Corvette’s stock, clutch-based differential. While many gear-driven diffs can open fully when one wheel completely unloads, Wavetrac defeats this tendency with its proprietary wave ramp system that can lock together the driven wheels. Photography Credits: J.G. Pasterjak (differential), Courtesy Wavetrac (illustration)
As recommended by GearFX, we broke in our gears on some Driven Racing Oil rear-end break-in lube. After some miles and heat cycles, we switched over to Motul 75W-90 synthetic gear oil. This fluid doesn’t have a limited-slip additive, as the WaveTrac diff derives its limited-slip properties from gears, not clutches.
These diffs need proper lubrication; we’re running Motul Gear 300. Photography Credit: J.G. Pasterjak
The WaveTrac can require a change in driving style, however. For some advice here we reached out to our friend, former pro racer Jason Saini, who’s had experience with just about every type of diff.
Saini says gear-driven diffs like our WaveTrac are extremely forgiving and smooth. “The amount of lock is determined by the amount of slip that happens,” he explains. “So as slip increases, the worm gears counteract that and transfer torque to the wheel with more traction in a very linear fashion. This happens both in acceleration and deceleration, so a gear-style diff tends to be very predictable and intuitive.”
But there are a few complications, he continues. “The system does require some resistance to function. So if a wheel becomes completely unloaded, you can get anything from inside wheelspin to unpredictable behavior based on how the manufacturer of the diff set it up.
“There’s also no real adjustability like there is with a clutch or ramp diff, where you can alter the lockup and unlock through different clutch plates, springs or ramps. But the cool thing there is the way the gear diff is constantly reacting to traction conditions in real time. If the manufacturer did their homework with gear angles, you shouldn’t need to make any adjustments.”
Saini’s bottom line on torque-biasing diffs: “Just enjoy them, because they’re easy to drive with and predictable. If you’re getting into a situation where you’re completely unloading a wheel and experiencing unpredictable behavior, start looking at roll stiffness with springs and sway bars and do what you can to keep that inside wheel loaded enough to keep the diff functioning.”
Bulletproofing the Hubs
Hubs are overlooked workhorses. They only, you know, hold the wheels to the car. In nearly three years of track and autocross use, we never needed a hub upgrade or replacement.
But then we threw nearly 100 more horsepower at the ground and mounted some sticky tires, and suddenly we were going through hubs like they were bottomless chips and salsa at Chili’s. The rear hubs seemed to bear the brunt of the increased load, regularly displaying telltale slop when rocked in a vertical plane, but we also snapped a flange completely off a front hub.
We first tried to “save money” by skipping the full-race hubs and sticking with OE-style parts–to the tune of $130 to $260 each depending on brand, supplier and time of year. After all, we reasoned, we usually only run a few laps at a time, and the stock-type parts cost so much less.
“Well, certainly we can get by with these ‘premium’ versions of the off-the-shelf hubs,” was our next rationale. “They only cost a few dollars more, but [insert fallacious reasoning here].”
Nope, but louder.
Moral of the story: Buy the good hubs. We should have bitten the bullet up front, but instead we got nickel-and-dimed by inferior products that lasted as few as 20 autocross runs and about a dozen track laps.
Stock-grade Corvette hubs aren’t up to the task of hard track driving. The solution is SKF X-Tracker hubs, identifiable by their black oxide finish. They may seem pricey, but you get what you pay for. Check out our YouTube page for the step-by-step. Photography Credits: J.G. Pasterjak
The “good hubs” in this painful lesson are SKF X-Trackers, and we sourced ours from Sam Strano at Stranoparts. With a street price around $400 each as of press time (prices have been fluctuating a lot lately because of supply chain issues), the X-Trackers are “buy once, cry once” items. In all our calling around, we never found a Corvette owner whose X-Trackers failed due to anything short of a crash. That’s a heck of a lot worse than 20 autocross runs and a few hot laps.
Accoding to SKF, the X-Trackers, which are available for a variety of applications and are even used as OEM pieces on some factory hotrods, increase durability via optimized design and materials to resist deflection between the fixed and rotating portions of the hub. Even minute deflections in the hub assembly lead to heat spikes due to localized areas of increased friction, and heat plus stress is as sure a killer as a diet of doughnuts and PCP.
That deflection can also lead to second-order effects like increased brake pad knockback, especially when fixed-position calipers are also being used. Tiny physical deflections at the hub face are magnified at the outer diameter of the rotor and can work the pistons back into the calipers, resulting in a low pedal. Yeah, we had some of that, too.
Fortunately, installing hubs on a C5 is straightforward–see our YouTube channel for a complete step-by-step. Long story short, you’ll need to remove the brake caliper, lower ball joint and tie rod to disconnect the upright so it can swing out far enough to reach the bolts securing the hub assembly to the upright. In the rear, you’ll also need to remove the axle nut and slide the axle out of the hub. We got to where we could do all four in less than 90 minutes. We wish we’d never gotten the chance to get that good at it.