GT-R TCM Clutch Pressure Tuning

Introduction

With the release of the latest version of RaceROM patches you can now increase the target clutch pressure past 16 bar, this aids in keep clutches engaged properly in big power cars and stopping slip. The key to getting your 20basr 1st gear clutch pressures is the RaceROM Clutch Pressure Max. This increases the factory limits to where you need them.

To up the clutch pressure use the latest version of patch and make sure the following maps have been adjusted:

Increasing maximum target clutch pressure

Use the Clutch Pressure - Maximum RaceROM map to override the OEM limit for target pressure (which is capped at 16.7 bar).

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You can raise this even on a stock transmission. early CBA cars appear to tolerate about 17.5 to 18 bar, which later transmissions or those with uprated seals can run a bit higher, and fully built transmissions have been known to go as high as 23 bar.

Do not set this unnecessarily high, more is not always better and this should only be set high enough to get the job done.

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Increasing maximum solenoid current

However it is not enough to have the target raised, as the rest of the control system will not deliver the high targets due to physical and software limits. One of these is the maximum control current for the clutch pressure control solenoids. This is normally set at 1.5 amps from the factory.

​​You also need to raise the current limits and they can also be used to limit the maximum acheiveable pressure (remember we only want as much pressure as required to do the job, and no more)

Usually current limit work out a bit like this, but you will need to test your specific setup as transmission components and builds can cause differing results.

Early CBA Transmissions.

1.5 amps stock about 16 bar
1.7 amps about 18 bar
1.8 amps about 19 bar

Later DBA transmissions:

1.5 amps stock about 17 bar
1.6 amp should give you about 18 bar
1.7 amps should give you about 19.3 bar

Clutch Pressure Duty Compensation

Finally in order to achieve the higher currents that the high torque targets require, and the increased current targets allow, you will need to trick the TCM into raising the current at high target pressure. We need to do this because the calibration of current required for a given clutch pressure has a hard coded input limit of 16.7 bar, and while the clutch is in a “hold” phase during steady state, it is all fully open loop control.

We need to change the Clutch A/B solenoid compensation maps which are not currently scaled but numbers are essentially milliamps.

Below we can see that where the fixed stock calibration would stop at 16.7 bar we add pressure target inputs up to 20 bar, and increase the current required.

Increased line pressure​​​.

Without sufficiently high line pressure the high clutch pressures may not materialise and in repeated fast shifting the fork movement may lag causing shifts to be delayed.

And set your line pressures (Min Tgt Line Press A & D and Target Line Pressure E)

Clutch Friction

Bad Shift behaviour with built transmissions and aftermarket clutches can often be improved by fine tuning the clutch Friction tables. These form the initial part of calculating the required pressure for a given torque transmission.

You can try reducing the friction coefficients for the low slip high pressure cells of the coefficient map to prevent increasing slip causing a reduction in pressure during extreme conditions. Normally this is correct when calculating just the right amount of pressure to give the required slip, but in maximum effort cars this can cause a runaway of slip, where increasing slip reduces pressure, leading to more slip.

Additionally at low pressure, and low slip values, an uprated clutch may experience higher friction, and lead to snatchy changes (if absolutely everything else, including the TCM torque calculations are well calibrated).

There are also shift limits, and schedules that may need adjustment to allow higher rev limits and improved shift points so these maps should be reviewed and adjust as per the requirements of the customer.

There is also a limit where the TCM is not able to see over 8192RPM from the ECM, when the RPM is above that it causes a large error in Clutch Speed vs engine speed. This results in a negative slip value effecting the estimated clutch slip and temperatures which can throw up a DTC's resulting in the TCM bailing out and releasing the clutch. This is a limit on the data the TCM receives from the ECM and cannot yet be worked around.

The DTC that appears is a P0746 and the thresholds for this need to be raised to accommodate the RPM above 8192 using these maps

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You can also disable P1725 to force the TCM into using the tach for engine speed instead of engine speed received from the ECM directly.