Some people will never break their Corvette. For some owners, their Y-body is a piece of art – a collectable rarity that just happens to be able to get from one place to another under its own power. For others, an occasional romp on the throttle from a stoplight, or a freeway onramp taken briskly are enough to complete the “Corvette experience.”
We’re not like that. Be aware that if you toss us the keys to your car, there is a chance it will come back with a strange new noise or exotic smell emanating from beneath. We’re even harder on our own hardware; a perfect case in point is Project Y2k, our in-house C5. With 380 or so naturally aspirated horsepower to the tires, we quickly discovered the limits of the stock clutch when abused by drivers with more enthusiasm than skill.
Upgrading to a Centerforce DYAD twin disc solved that problem (keep an eye out for that article in the near future), but led to even more enthusiasm at the dragstrip, ending with a sudden clunk and no forward motion. When every gear is “neutral,” it’s time to call it a day and regroup, and that’s just what we did.
Just a Bad Break
The post-mortem examination revealed the ugly truth; the output shaft of the stock transmission had sheared right at the point where it enters the pinion gear in the differential, taking out the ears on the pinion support in the process. While the case was salvageable, nothing inside it was. To get back on the road, we turned to one of the most experienced names in late model Corvette performance: LG Motorsports.
They get too hot, and they start to melt the plastic bits inside of them. – Anthony Forney, LG Motorsports
Taking its name from the initials of founder-slash-owner-slash-road-racer Lou Gigliotti, Texas based LG Motorsports is to Corvettes (and recently, Camaros) what Wonka is to candy. If you’re building a hardcore late model Chevy track car, they have pretty much everything you need or want, from lug nuts to sequential clutchless transmissions.
Our driveline failure had a pretty obvious cause, but we asked LG’s Anthony Forney about the common weak links in late model Corvette transaxles. “The first issue with them is heat,” he explains. “They get too hot, and they start to melt the plastic bits inside of them. That makes the trans hard to shift, you see bearing failures, clutches in the diff start to go, and you can even lose the heat treat on the gears.”
“After that, on the T-56 transmissions you will see synchro failure, but more commonplace is the synchro keys bending or breaking,” Forney continued. “This either locks you into or out of a gear. The later 6060 boxes somewhat corrected that issue. On the road race cars we have to replace synchros every five to six races due to wear. The stock shifter fork pads are plastic and can deform or melt away giving you a sloppy feel. RPM Transmissions has a great set with billet keys, bronze pads and a number of upgrades to keep all of these alive much longer than stock.”
Speaking of RPM, they teamed up with us on our project as well. RPM’s Jeremy Jones knows the weaknesses of the Corvette transaxle, and may have hit our failure on the head. “Another weak link is the pinion carrier or pinion support,” he explains. “This piece houses the pinion bearings and is pressed onto the pinion gear and locked in place with a large nut that is torqued to 375 foot-pounds. Yes, you read that right, 375 foot-pounds. The whole carrier assemble bolts into the differential housing with eight bolts. The way the gears are cut and the way the differential rotates, the pinion gear naturally wants to be pushed out of the back of the differential housing. The stock pinion support is fairly thin, and in a high horsepower or heavy load application it cannot handle the force being applied to it.”
“When the force is too high, it will break the mounting lugs off the pinion carrier,” Jones continued. “When this happens the ring and pinion gears misalign and the gears will become damaged. Also, the transmission main shaft is splined into the pinion gear. So when the ring and pinion misalign, the main shaft can be fractured, causing a catastrophic failure of both the transmission and differential.”
Well, that sounds familiar, doesn’t it?
Jones adds that the C6 design addressed this failure, saying, “The pinion support was redesigned with thicker mounting lugs to prevent the part from failing in the way that the C5 piece did. The C6 pinion support is so strong we have seen it actually pull all eight of the mounting bolts out of the main case and the pinion support be completely intact with no damage at all. So on all of our Stage 3 and higher C5 differentials and all of our C6 differentials, we use the updated C6 pinion support.”
Left unrepaired, the broken pieces of the preload springs will be thrown around inside the rotator assembly destroy the inside of the LSD unit. – Jeremy Jones, RPM Transmission
While the stock T-56 transmission was salvageable, and would go to a local shop for a new mainshaft and overhaul, RPM would build us a fresh new differential using parts from LG, their own inventory, and the Eaton Trac-Aide limited slip differential we had already sourced.
Though the factory LSD does a decent job for street use, it can’t compare to the new Eaton in terms of performance or reliability. So where do the stockers fail? Per Forney, “On the diffs, it depends on the year and power. For road racing, the clutch packs seem to go first. Once you start to make a bit more power in the car you will begin to twist or break the output shafts, then chip away the gears. Near the end of our runs in World Challenge we started to push the actual diff unit out of the side of the cases. This would crack the entire passenger side cover and main case. The C6 units made this stronger, and the C6 Z06 and ZR1 units are much larger in both the case and gear size, which have almost no issues to them, short of the clutch packs.”
Jones identifies other areas that need help, explaining, “The first major weak link in particular are the stock stub axles. They normally fail around 450-500 horsepower. Hard launches and wheel hop are the most common causes of failure. We replace them with both OEM C6 Z06 stub axles or custom made 300M steel alloy pieces.”
“The next common failure is the preload springs in the factory LSD unit,” Jones adds. “The stock preload springs are very thin and weak. The preload spring is a cone-shaped washer that puts pressure on the clutches when the differential halves are bolted together. This causes both of the wheels to spin together instead of the one tire spin you get from an open differential. Over the years these preload springs will fatigue and crack. When this happens the clutches no longer have any preload on them and the car will start spinning one tire more than the other. If this is left unrepaired, the broken pieces of the preload springs will be thrown around inside the rotator assembly destroy the inside of the LSD unit.”
Upgraded Driveshaft Couplers from DSS
One common area of driveline failure in C5 and C6 Corvettes is the “giubo” (pronounced “joo-bow”) couplers hidden inside the torque tube. Constructed of steel-reinforced rubber, much like a car tire, they crack and fail over time, also like a car tire.
While the driveline was out, we took the opportunity to replace them with molded poly upgraded versions from The Driveshaft Shop. While solid metal ones are available from LG and other sources, they aren’t really appropriate for anything less than a full-time racecar. The poly couplers offer increased durability while still preserving the critical function of absorbing harmonics and slight driveshaft misalignment.
Forney explains, “Heat is the friend and enemy of any mechanical device. Too cold and you do not get lubrication, and too hot and you start to wear because you lose protection for bearings and gears. Even a good synthetic oil will break down at temps over 285 degrees F if you run it long enough. Sure, they can take 300-320 degrees for a short period, but the amount of additives and protection agents in the oil starts to go away at a much more rapid pace, which is why it is so important to keep taps on temps and do oil changes during track days.”
A contributing factor is the underbody design of the Corvette, which makes things somewhat more complicated than a conventional driveline. On the Corvette with the transmission and differential being so close to the exhaust and tucked up under the body work, it does not do it any favors,” Forney admits.
Hot or Not?
So how much does the cooler setup help? Per Forney, “We have monitored both the the race cars and customer cars. Of course, temperature depends on the track, the outside temperature, and the horsepower of the car, but in most cases you can see upwards of a 45 degree drop on both the diff and trans. This should put them back in the safe range of the low 200’s regardless of the circumstances.”
The cooler system has a long track record (pun intended) – Forney says, “We did the kit before we even started to do the World Challenge cars. As you might remember, GM had this big clunky electric pump setup [for the factory-built race cars] that sounded like boat pumps. Between the hoses, the wiring, and the mounting of the pumps it just added too much weight to the car.”
“So going off of what we had done in the older TransAm series with mechanical pumps, we developed a self contained drive and pump setup that mounted to the diff,” Forney continues. “No more electrical wires, no more forgetting to turn the pump on; as soon as you start moving, it is pumping fluid. When the C5 came out and Lou got his first FRC we made the first kit for it, because we darn near burnt the diff and trans out of the car in the summer. When we did the World Challenge Corvettes it only made sense to run them in all of the cars. We’ve proved it works both on street cars and race cars with no issues. It’s quiet enough to drive every day and strong enough to hold up in real professional racing.”
No Such Thing as Overkill
Is a built diff and cooler setup like this strictly necessary for your Corvette? Well, that brings us back to where we started – for a certain proportion of owners, the answer is definitely no. But, as Forney puts it, “If all you did was drive back and forth on the street I don’t know if it would be my first choice. It wouldn’t be 100% needed. Now, if you are going to do some high performance driving events during the year, then yes, most certainly. Even a bone stock Corvette can get the over temp warnings on track with a good driver in it. Add more horsepower and it will only get worse from there.”
Since we will indeed be adding more horsepower to Project Y2k, we consider these upgrades from RPM and LG Motorsports some relatively cheap insurance against future problems. The differential is far stouter than the one that came with our car, and the cooler system adds both additional heat rejection and fluid volume to the differential and transmission. We’ll definitely continue to break things on our Corvette, but now we know these parts won’t be among them.