The donor XDF's identify a map that limits fuelling when overboost occurs, which I've named "Overboost Fuel Cut".
That naming seems to have created a bit of confusion as I've had a few people email asking if they should increase this map to match the torque limiter.
Where the confusion arises comes from common belief that the kangaroo hopping or jerking that occurs when the wastegate fails to operate is a fuel cut caused by an overboost condition. What is actually occurring is quite different.
Back to the ADC
I've posted previously about the signal flow from MAP sensor through the ECU Hardware to the Analog Digital Conversion module.
This is roughly:
- Sensor converts pressure to voltage
- Resistor divider drops voltage by approx 10%
- ADC converts voltage to a value between 0-1023
- The value is checked against ADC Max and Min
If the value is above ADC Max, a logged high fault is set, and if below ADC Min a logged low fault is set, and the sensor default value is used.
And this is what occurs when the wastegate fails to open. The pressure quickly increases to a value higher than ADC Max, and the sensor default is set.
Limit checking happens at the first stage of processing after AD conversion, and well before any fuelling calculations. While this might appear to be a fuel cut that is simply a side effect of the dramatic drop in calculated airmass.
The real overboost fuel cut
Overboost Fuel Cut Map is used specifically when the current boost exceeds the boost limit value.
The current boost value is calculated as the difference between Manifold Absolute Pressure (MAP) and Ambient Absolute Pressure (AAP). This is obviously influenced by the pressure drop across the air filter and airbox.
Say for example the reading are 240kpa MAP and 100kpa AAP, in this case boost is 140kpa.
However if there is a 3kpa drop caused by the airfilter, that becomes 240kpa MAP and 97kpa AAP which gives 143kpa boost.
The stock ADC Max limit for the MAP is the equivalent of about 242kpa and stock Boost Limit is set at 142kpa. Due to the drop in AAP at high boost you'll generally reach the Boost Limit at least 2-3kpa before the ADC Max limit.
Once boost exceeds the Boost Limit, the ECU limits the amount of fuel injected using the Overboost Fuel Cut map. This limiting remains in place until the current boost drops below the level set by the Boost Limit Recovery parameter.
A few thoughts
My take on this map is that it is used by the ECU to help control boost levels after overboost is detected, and prior to the MAP value exceeding the maximum limit. With stock values this is a small window of perhaps 3kpa // 0.4psi. It is basically a last ditch effort by the ECU to control overboost, and I personally don't' see any legitimate reason to mod this map.
The best approach is to set boost limit and boost limit recovery values at the maximum value you expect to run.
If this is higher than 150kpa/1.5bar boost you will need to replace the MAP sensor with a wider range unit.
Then adjust the ADC Max value to suit - some thing like 250kpa if you have set the boost limit to 150kpa.
Working in this way you will only hit the Overboost Fuel Cut when the boost reading goes above your maximum expected value.