It would seem to me that with the C8’s transmission, if you are advised to add transmission fluid (2 quarts or liters?) to track the Vette, and after doing so, you leave the additional fluid in the transmission, why did they not simply fill the transmission with the proper extra fluid before it left the factory? Is the lack of transmission fluid the source of the problem with a number of 2020-2023 transmissions? Your thoughts?

 

No. GM reps have been asked that many times. They always advise that if you are not tracking, you do not need to add the 2 liters. The issue is that in certain, difficult to reproduce maneuvers on the track while turning and going downhill, there can be brief period where the intake sucks air. But that never happens in street use, and almost never in track use (according to GM rep statements). The new transmission design does not require additional fluid and uses about the same fill as the earlier design - it just has some changes in the internal baffling that prevent that "rare" issue from happening, making it unnecessary to overfill the transmission. There is, in fact a reason not to add it even to the earlier designs - for sustained high speed driving.

It is discussed at this thread:
https://www.corvetteforum.com/forums...-engineer.html

Here is what appears in that thread:

"I drive my 2022 C8 Z51very fast in the open road races several times a year. This is the fourth Corvette I have used in these races. The C8 is by far the best above 150 mph. But my car has a problem that has shown up three times and cost me finishing well. No DTC codes ever show up. All the fluid and air temperatures are well within limits.

After 30+ miles of 150 mph driving on a warm day, the car slows down. Foot to the floor is 155, then 150, then 145. For the next 30 miles the maximum speed wanders around between 135 and 145 mph. No gear changing or mode changing makes any difference. It just won’t go fast. My dealer and service advisor are very good but they can’t find anything. This is a CND -Can Not Duplicate.



With Jeremy Welborn’s help (thank you very much), I managed to contact Corvette’s Chief Engineer, Josh Holder and Lane Rezek, Corvette Brand Quality Manager. Through several email conversations we tried to isolate the problem and eliminate false paths. I sent them PDR files and other diagnostic files from the road rallies. I use Cosworth’s PI Toolbox and already knew there was nothing in the normal OBD II at the ALDL that would help this problem. Josh and his staff concluded the same thing.

Before my last high-speed event in October, Josh had a field engineer meet me at the dealership. I left the car with Lee Williams for several hours. He installed a custom-made data recorder on the ALDL (OBD socket) and mounted it securely behind my seat. I had a button mounted by my right knee to push when the car started acting up.

Event day was cool and I was afraid that nothing would happen. I drive this car as fast as I can until I reach a finish solution, then I slow down as required. Well, the car decided to slow down first. I was in the middle of nowhere with no cell service or anything else. I had no idea if the recorder captured the slow down event data and whether it was able to upload it.

A few days later Josh emailed that the needed data had been received and his engineers were working on a solution. Today I got an email with the summary of his investigation and a simple solution. All this preamble to the solution is needed. There are going to be some people very surprised by his answer. But remember where this came from and his position. Here is his answer.

On October 27, 2023 Josh Holder wrote:

When you can schedule a visit to your dealer to return the data logger, please ask them to REMOVE 2L of oil from your transmission. The +2L fill was intended for “track” use as in “closed road course tracks” with sustained high G loading. You aren’t doing that in this event. Frankly most people would never see pressure drop outs even on a road course track at the factory fill level.

When the transmission has the extra 2L of oil, the sump level allows 6th gear (which you are in a lot) to spray oil directly onto a temperature sensor. When subjected to high load, like wide open throttle for long durations, the oil spray from 6th gear gets hot enough to cause the sensor to report to the transmission control module that it should begin to limit torque to protect itself. That is exactly what is happening to you. It’s a localized spray of hot oil. The sump average temperature is still quite low.

For the Z06 and all Corvettes built after July of 2023. The case and lube system have been revised so that the extra 2L of oil is no longer required for any situation.

That you very much for your willingness to help us out. We could not recreate the problem in our own vehicles because they were not track prepped for high-speed run (It does not meet our definition of track use), but I can completely understand why it would seem logical to add the 2L for your event. It just was not intended for that use case.

Drain 2L and you should be good to go."

 

Thank you for the quick response. I will not be tracking the C8 that would require the additional transmission fluid for the given conditions that are posted in your threads. Again, many thanks.