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We hit speed limiters in 6th gear on the Mustang MD1100. Dyno is good at 220mph but seems the run lasts forever lol
We tune in 5th and are very happy with ending tuning results
6th gear is the most desired for the A8 because the input/output shafts are "locked" together and there is no power flowing through any meshed gears meaning no HP losses to gears in the transmission. However, all chassis dynamometers have speed limitations with most models not being able to go over 200 MPH...the top of 6th with an A8 is over 210 MPH. You'll need to find a chassis dyno with the correct speed range to dyno in 6th or you'll have to use 5th.
There's nothing special about 6th gear being "locked", it's just 1:1. Either the converter is locked up (which I doubt it does at WOT, but I do not know) or the converter is slipping. But at high RPM the centrifugal force inside the converter is so high that it doesn't slip much.
If my last assertion is true, you should be able to run in an underdrive like 4th and keep the wheel speed down. A -quality- dyno operator should be able to tell you if that's practical or not, it's more of a guess. Looks like a few above are doing it though!
There's nothing special about 6th gear being "locked", it's just 1:1. !
In the 8L90, there are 4 planetary gear sets. In 6th gear, the power flow from input shaft to output shaft all goes through planetary gear set 1. The input shaft, in this case, is locked to both the ring gear and the sun gear of planetary 1. This means that the planetary carrier of gear set 1 is locked to the input shaft. Since this planetary carrier is also connected to the output shaft, the input shaft and output shaft are locked together.
The important thing here is that in 6th gear, there are no gear teeth in the planetary gear set slipping against each other while transmitting full input shaft torque. This reduces parasitic losses.
Contrast this to a conventional 1:1, in which two 1:1 gears incur gear tooth slip, and load-dependent parasitic loss, as they rotate against one another.
10-18-2015, 10:55 PM
Dyno Z06 A8
