The pinion (spider) gears also balance the power load between the side gears while allowing unequal axle speeds when the vehicle is turning.  Notice in the above diagram how the spider gears will spin in opposite directions when one wheel spins faster than the other.

By the way, the parking gear and prawl are just that - when you shift the transmission into "Park", the parking prawl is forced into the parking gear, locking your transmission up.

The only failure mechanism for the differential / final drive assembly we've seen is this:

• As demonstrated in the explanation above, whenever one wheel turns faster than the other the pinion gears will spin on the pinion shaft.  This occurs on curves as well as on the drag strip when doing a burnout.  The differential was not designed for the pinion gears to spin at high speed for sustained operation, and eventually the pinion gears will gouge into the pinion shaft due to lack of sufficient lubrication, eventually causing the pinion gear to seize tightly onto the shaft!  When this happens, the roll-pin which holds the shaft in place will snap, allowing the shaft and pinion gear to spin as a unit.  (Of course, the other pinion gear is also spinning on the shaft in the opposite direction.) 
• With the roll pin broken there is nothing holding the shaft in place, so it will slide freely in and out of the carrier, hitting the transmission housing and gouging into the aluminum case.  This of course, throws chucks of aluminum all into the transmission and converter, destroying everything.
• This failure phenomenon will only occur on vehicles where one wheel is spinning much faster than the other for a sustained length of time, meaning it has the highest probability of occurring on vehicles stuck in mud, ice & snow, or on vehicle which are subjected to heavy sustained "burnouts" (yes - the one doing the burnouts on our home pages indeed qualifies!).

Potential Solutions for this problem

There are only a few potential solutions for this problem:

• Replace our "open" differential with a limited slip one.  Remember, if both wheels turn at equal speeds in the same direction, the pinion gears do not spin at all!  If the pinion gears don't spin, naturally, broken roll pins and free-floating pinion shafts are not going to happen.  This of course would be an ideal solution except for a few issues:  potential cost, and a limited slip differential is not available for the 4T65e transmissions.

• Yes, we realize there are others hard at work as we speak working on just this issue, but we've had prior experience in 1998 working with a major limited slip differential company (who currently manufactures and supplies limited slip differentials of the same design to GM for their current hi-performance vehicles) and were not successful in keeping the unit together.  The conclusion at the time was that due to small physical size of these 4T65e differentials, the components were just not strong enough to withstand more than 300 HP given the weight of our vehicles.  The units would have probably been fine behind the factory 240 HP level, or with a higher power but lighter car,  but obviously, that would not help us.  We are not publishing photos of the failed unit out of respect for the manufacturer but those of you that attended the second annual Kansas City Grand Prix Gathering may remember seeing it.  The design and concept of the unit we tested was very similar to the unit people are pursuing at this time.

• Welding the pinion shaft to the housing was tried (we were actually not a part of this endeavor), but because the housing is cast-iron and the pinion is steel, the weld penetration was not very deep and therefore the weld was not very strong - hence it broke.  We considered trying a stronger weld, but decided against this approach since the pinion gear would continue to gouge into the shaft and eventually cause a failure anyway.

• Our current approach is to machine the pinion gear for roller bearings to prevent the gear from seizing to the shaft and snapping the roll-pin.  This can be seen in some of the above photos and the ones below.

On the left is a close-up of the modified 4T65 pinion gear with the roller bearings.  Yes - we know the photo is fuzzy, but you can still see the bearings.

Click on thumbnail to expand

Conclusions

We've seen in severely abused 4T65e transmissions almost no problems with the differential and final drive assembly except for the problem noted.  We all agree that a limited slip differential is the ultimate solution, but whether or not one can be made to last in the cramped confines of the 4T65e behind our power levels with our vehicle weight is the question.  

It's too early to come up with any conclusions about our potential fix at this time.  The modified differential with the roller bearings have just been put into the customer's car for testing (4/01), and naturally, it'll take time and lots of burnouts to determine if this fix is acceptable!  We will be working through him to develop this idea and produce a marketable package.  For more information, visit his website at www.c-ya-racing.com!

Our advice at this time is to avoid doing extensive burnouts.  This failure mechanism is unavoidable and WILL happen to you eventually.  If you intend to do burnouts and want to try a modified differential with the understanding that this product is still in the experimental stages and there are no promises or guarantees, contact us.

To purchase the THRASHER / C-Ya Racing LSD Differential, click here.

NewsFlash!!!  Update on differential 4/16/01

For all of you having the same problems and breaking pinion shaft roll pins and anxiously waiting to try this bearing fix, we have really bad news!  Apparently, machining material from the pinion gear weakened it too much, and they broke.  This was after 5 passes at the Grand Prix session at Norwalk, OH this Easter weekend.  So, it's back to the drawing board - stay tuned for updates!

Mike Dye participated in this Easter Weekends Grand Prix session at Norwalk, OH with his Thrasher-prepped car to test the ported blower, torque converter, differential, Thrasher built 4T65e trans, 2.6" (yes, that's not a misprint) pulley with intercooler (16psi boost with no knock retard)!

Conclusions?  Tranny worked great - shifted HARD, the differential did not fare so good.  The stall speed of the converter seems right, but we're not certain about it's efficiency, because it looks like the engine's torque curve has shifted too much toward the lower RPM range, meaning he's got massive amounts of lower RPM torque, maybe too much, but his high RPM horsepower seems to be suffering; either that or the torque converter is losing efficiency.  Remember, smaller blower pulleys always give more torque at the lower RPM ranges, but tend to sacrifice the high RPM horsepower.  The car was not achieving the high MPH that our test Regal GS has (of course Mike's car still has the stock cam and heads with no port work), and his 1/4 mile performance could be suffering because instead of merely barking the tires on the shifts, he's lighting the tires up in smoke on the shifts, losing valuable time and speed.  This isn't the fault of the hard-shifting tranny, but perhaps it's due to the torque converter stalling right into the beginning of his peak torque band, which has been increased and moved to a lower RPM range.

In a nutshell, more R&D will have to be done to figure out the right combination.  Here's some movies of Mike's runs (warning - these files are extremely large! 1.2 and 1.4 MB)  These are .mpg files and Windows Media Player should automatically run them.

Click to enlarge