Cobb Custom Features: GT-R Auto Shift Point Tuning Guide

Auto shift point tuning guide


4/25/2016




Auto Shift Point Tuning Guide

The TCM (Transmission control module) for the GTR contains a complete instruction set for transmission function. These instructions include great number of tables that influence the up and downshift points for the transmission when run in "auto" mode. The full function and interactions of these tables is not fully understood. Therefore, these tables are labeled as BETA. This guide is meant as a starting point for tuning. Your tuning application may be unique and require testing to verify. However, because these tables are not fully defined, there is an opportunity for talented calibrators to create new and valuable intellectual property. 

The most common use of autoshift is for high HP cars where a computer instigated shift routine is more reliable than a driver. Under these circumstances a higher than "stock" shift point is desired. Changing two sets of tables will permit higher than stock auto shifting in both "race" and "normal" transmission modes. The most simple way to do this is to increase the shift points at high throttle for tables "Auto Upshift Group A - xx Gear" in addition to "Auto Upshift Group B - xx Gear – 1, 2 and 3". Shift points are indicated in mph and must be translated to RPM for each gear.


For example:



This is the Upshift table for gear 2. When translated from mph to RPM it looks like this:


Throttle

0

9

11

18

25

38

50

75

88

100

MPH

14

14

16

22

29

39

47

57

60

63

RPM

1521

1521

1739

2391

3151

4238

5107

6194

6520

6846


The scalar to change speed to RPM in second gear is: 0.009203

So if you desire to increase the Upshift point to 7200 RPM the new indicated MPH in the table is:

 7200*0.009203 = 66.2 mph or 66 mph in the table. 


A completed table with the 2 to 3 upshift changed to 7200 would look like this:



The approximate scalars for each gear are:


Gear

Scalar

1

0.005253

2

0.009203

3

0.013725

4

0.017548

5

0.022085

6

0.027706


In order to change shift points in auto mode we advise that users change Upshift points in a coordinated manner in both "Auto Upshift Group A - xx Gear" as well as all "Auto Upshift Group B - xx Gear – 1, 2 and 3" tables. A sample TCM (KJ10B) set up to Upshift at 7200 rpm in Auto, Race and Normal Modes can be found here.

For example = to upshift at 7200, 7500, or 7800 in each gear the max mph shift points are:


Gear

Scalar

7200 rpm

7500 rpm

7800 rpm

1 to 2

0.005253

38

39

41

2 to 3

0.009203

66

69

72

3 to 4

0.013725

99

103

107

4 to 5

0.017548

126

132

137

5 to 6

0.022085

159

159

159


With the current tuning approach the maximum 5 to 6 gear change is limited to 158 mph.