GT-R RaceROM Failsafes

Introduction

Adding an element of safety to tuning, a range of thresholds for important engine variables that if not in a desired range will trigger a torque limit to cut power.

The limp mode will self heal after a default 60 seconds, alternatively to clear the limp mode torque limit, the ignition can be turned off for a few seconds and then the engine restarted.

Operation

Open

Live Data Parameters

Failsafe Flags - A single integer value that displays the trigger to go into limp mode

Failsafe Stored Flags - The last thing that caused the failsafe to become active

Table of flags below

They can be combined so a value of 3 would mean both knock retard and low oil pressure were active

Note: These values are displayed as a sum e.g. 3 = 1 + 2 = BOTL Active & Throttle Integral Active  

Reading and Clearing (ECU Connect)

To reset the stored Failsafes ECU connect is required, you can also read the failsafes current and stored as well as check the MIL source

To reset the the stored failsafes open ECU Connect and connect to the car then go to the “Malfunction Indicators” Menu, Press the “Failsafe, Stored“ Menu Option and then Press the “X” button

Open

 

 

 

AFR Correction Max

This sets the maximum allowable Fuel Trim Short Term Bank #1/#2 values, that if reached for AFR Correction Timeout while over the LM Min Load and LM Min RPM will trigger the limp mode limit. Note the default value for this is less than the maximum possible fuel trim of 120% so if enabled, poor tuning or inadequate hardware will be quickly found.

AFR Correction Timeout

Fuel Trim Short Term Bank #1/#2 will need to exceed the AFR Correction Max thresholds for longer than this delay for the limp mode torque limit to be applied.

AFR Lean Max

This sets the maximum allowable AFR Bank1/2 values, that if reached for AFR Lean Timeout while over the LM Min Load and LM Min RPM will trigger the limp mode limit. After ignition-on, AFR Bank1/2 is monitored and ignored until both sensors are observed outside of the range 14.6 to 14.8 to mitigate AFR limp mode trips on sensor warmup.

AFR Lean Timeout

Delay timer  in seconds for the AFR Lean Max safety trip.

Coolant Pressure Max

High coolant pressure can be encountered on high power maximum effort engines as an early sign of head gasket failure due to combustion gases compromising the gasket seal. This limp mode trigger allows an additional sensor configured using coolant pressure options to be used to trigger the torque limit, typically a 0-10bar sensor similar to those used for fuel pressure measurement plumbed into the top radiator hose is suitable. Due to the losses around the coolant system the pressure will vary greatly at different locations, you should first familiarise yourself with normal pressure levels before relying on this.

Coolant Pressure Timeout

Delay timer in seconds for the Coolant Pressure Max safety trip, this is set to 0 Seconds by default to ensure immediate drop in pressure if dangerously high coolant pressure is encountered.

Fuel Pressure Relative Min

As detailed in the section on fuelling, relative fuel pressure should be consistent with boost, so a single value can be used to determine if the fuel pump is coping with the current demand. Be aware that far too many moderate and high power GT-Rs do not have fuel pumps capable of maintaining the set pressure. If you are adding a fuel pressure sensor for the first time you may find that this threshold causes the limp mode to be set, but ignorance is not bliss!

Fuel Pressure Timeout

Delay threshold for the Fuel Pressure Relative Min limp mode to be set. On maximum effort cars you may need to lower this threshold to around 0.2 seconds to avoid lean AFRs while experiencing fuel supply issues (although the lean trip and fuel trim trips will offer additional safety).

Knock Maximum

A threshold of the maximum level of knock correction that is permitted before limp mode is triggered. The values in the map equate to retard, so a limit of 5 will be triggered when Knock Correction reaches -5 degrees. (should this be swapped so values in map match the values on the log for knock correction?)

Knock Timeout

Delay threshold for Knock Maximum limp mode.

LM Enable (Mode1 – Mode4)

Each limp mode test can be independently enabled per Mapswitch mode.

LM Max LatG

The limp mode torque limit will not be imposed until the lateral G has dropped below this absolute threshold. However even while the Lateral G exceeds this limit, the all limp mode tests will continue and if triggered will latch on, only the torque limit itself will be delayed. The default value of 1 will impose the torque limit once the lateral G is within the range of -1 to +1.

Failsafe Min Load

All the tests except oil pressure will only be carried out while engine load is greater than this value.

Failsafe Min RPM

All tests except oil pressure will only be carried out while the engine RPM is greater than this value.

Failsafe Reset time

Once a safety trip has been triggered, the car will run with the reduced torque limit, but the failsafe condition can “heal” after this time expires. The default value of 60s allows the engine to cool, regain sensible fuel trims or whatever.

Failsafe Torque Limit

Once a safety trip has been triggered and the ECU puts the car into limp mode a torque limit will be applied using this value.

Oil Pressure Minimum

When using a RaceROM equipped ROM in the TCM the ECU can receive oil pressure information and use this for checking for issues. To trip the oil pressure limp mode, the measured pressure needs to drop below Oil Pressure Minimum for at least Oil Pressure Timeout.

LM Min Load and LM Min Load are ignored when testing oil pressure for limp mode purposes, instead the engine RPM must be above 600RPM to avoid triggering the limp mode when the engine isn’t running or on startup.

Oil Temp Max

A maximum safe oil temperature above which limp mode will be entered. Primarily useful for cars used on circuit or in situations with sustained high power.

Oil Temperature Timeout

Oil Temperature will need to exceed Oil Temp Max for this time in order for limp mode to be triggered. Given the slow rate at which oil temperate changes, this can be extended in order to ignore brief peaks that might be encountered.