h3mpking

I noticed while making half mile pulls this weekend that right around 130 MPH the RPMs will take a slight dip and right back up. I took it to the dealership for service and they are looking in to it. They have escalated the case to the GM engineers. Anyone else ever see this? YouTube link attached.

Ahmed_otb

I've had a similar issue wit my 2014 shelby gt500, where in 4th gear there is a little prolonged dip, took it to the dealer and they replaced the throttle body and it got fixed.

hopefully its a simple thing


have you noticed any thing with the drivability of the car around town, like stop and drive type of driving?

Mr. Gizmo

Maybe it's loosing traction for a moment due to the road surface. Did you have traction control on or off when this happened.

h3mpking

I ran with traction control on and off. Didn't seem to matter.

h3mpking

I hope it is as simple as that! Around town driving seems normal. I haven't noticed anything.

NSC5

From your video the speed seems to be increasing normally through that RPM drop. It looks like you have the A8 transmission and perhaps this is the point where the converter clutch locked again after the shift resulted in this slight temporary RPM drop?

dar02081961

This is likely the torque converter locking up after the 4th to 5th shift.

If you notice when you shift to 5th the rpm drops to around 5k then as it starts to build rpm it drops back to that exact same rpm of the 4th to 5th shift.

Locking a torque converter gives about a 200-400 rpm drop in most transmissions I have encountered.

I have seen some transmissions hold the rpm at a particular rpm on the hp curve (allowing greater slippage of the torque converter) by not locking the converter right away so as to not lose momentum by dipping to low on the HP curve.

Once it is going fast enough to lock the converter and not go to low on the hp curve the computer will allow the converter to "lock". When this happens you will see the 200-400 rpm drop I mentioned.

This locking of the torque converter reduces slippage in converter automatics and thus reduces some of the drive line loss associated with hydraulically coupled (torque converter) automatics.

It may or may not do this at other shift points depending on the transmission modules programing and weather or not it would aid in overall acceleration.

It is a transmission engineers way of keeping the engine "on the boil".
Look at it as another gear ratio and another shift that only last for about 300 rpm.

Just my guess though having not driven an A8.
Semper Fi

h3mpking

What you guys explained sounds very logical and I did think about that. Appreciate the insight. I will post the update when I hear something.

NSC5

Also take a look at Hennessey's 0-185 run posted on YouTube. It is more difficult to see with the video of the instrument panel versus your PDR view but it looks to me like there is a little tach wiggle at about the same point as your video:

0NORCAL-SS

It's the converter lock up. Hp tuners will also confirm this

four0nefive

I'm gonna take a guess and say that shouldn't happen...

0NORCAL-SS

Yes it does. Even 4l80e and 4l80e will se sane drop when converter locks up

h3mpking

Yes, there is definitely some movement there.

Snorman

I've never seen that in my car. You can look at my track videos, and the car is ran up to ~140-150 mph a few times each lap and reviewing the video, there is no rpm drop like that after the shift.
S.

h3mpking

I don't know if it is more mental than anything, but I feel the car slow down during that point as well. You can feel the slight drop.

dar02081961

Was the OP in track mode? I know you where. I am speculating mode selection could affect the lock up. I say this because I know the mode selection affects torque management in the PCM, which in turn affects the converter lock up points. If you and the OP were at different levels of PTM or in a different driver mode this could be a reason for the disparity. Hard to really know.

I do know the transmission algorithms (hardness and shift times) vary based on several inputs and the mode selected. So it wouldn't surprise me if the torque converter lock up varied as well.

h3mpking

I noticed this in both Track and Touring modes. Interestingly enough, my top MPH's were ran in Touring mode. Felt like the smooth shifting was a benefit.

dar02081961

Yea I just looked at it about 10 more times but with the audio turned way up this time.
It actually sounds like the traction control is cutting the throttle. Even though we don't see the throttle percentage drop in the PDR.

Are you always doing this on the same surface/course?
Wondering if it changes based on the road surface? If its traction the issue would change based on the surface (available traction).

If you could data log you could look at the throttle and timing to see if they reduce at this point. If they do you would know its traction control.

And contrary to what many think you may not catch the traction control light and the message at high speeds if the traction event is short and quick.

davepl

It's impressive to me that the clutch in the lockup converter is used at WOT, since you could be pushing 650ftlbs. I guess they designed the clutch to handle it, obviously, but not something you should expect your Dad's 700R4 lockup converter to handle!

NSC5

GM released an updated calibration for the 8L90 truck/SUV applications to deal with converter clutch "shudder" sometime around late May or June. Passenger car applications weren't included in this program but I wonder if there is a similar running change that was applied to C7 applications during later production. It is possible the OP has a different calibration than yours Snorman.