Originally Posted by
Puggyberra
So you're to about where I am on that unfortunately. Limiting outlet pressure max to something like 26 psi will help in most cases, but the limp mode can still occur. You are correct that the throttle closing is what is causing the TIP to spike (since the TIP is before the throttle body), when throttle is fully open TIP and MAP will tend to be the same or very close. As for how you get the throttle to stay open, honestly there's not a great answer that I've found (not saying you can't do it, just that it will tend to cause other problems). If you change the throttle body surface area so the ECU thinks it needs it further open to allow enough air flow you create issues at part pedal where instead of the throttle opening saying 30% for moderate acceleration it thinks it needs to be open lets say 60% (obviously made up numbers). That would make part pedal acceleration jerky and unpleasant.
The issue seems to stem from the ECU iterating ahead in its expectations, that is (especially during a shift) the ecu expects it will require a certain amount of airflow in the near future, and so it wants to keep the turbo spooled and use the throttle body to control boost the engine actually 'sees'. This works phenomenally in the stock configuration, power feels smooth and linear despite fluctuations in boost that you can see when you log, and also allows the turbo to spool when the engine doesn't need power yet reducing turbo lag. Of course when you increase the boost as dramatically as we are the 'overhead' that the TIP sensor needs for this operation doesn't exist any longer. Reducing TIP actual could theoretically be done a few different ways, reducing or eliminating the torque reduction during shift would have a large impact on the TIP actual by not closing the throttle, of course it would probably cause your transmission to fail. Changing canister pressure FF could reduce airflow allowed into the turbo by influencing waste gate duty cycle, but the cells referenced during a shift are also used during some WOT situations where you don't want boost reduced. Changing the WGDC base table has the same issues.
Ideally what you want to change is the estimated MFRACT at the expected RPM/TQ requirement, indicating to the ecu that the current exhaust airflow is more than sufficient for the air flow the turbo will be required to make to hit the desired air load its expecting to needs, all resulting in the turbo spooling down somewhat. To my knowledge there is no way to adjust the estimated MFRACT, estimated air load, or estimated desired TIP/MAP, so this method is not possible. I've been told you can eliminate the throttle closing to limit torque by changing the fuel cut torque ratio and spark only torque ratio tables, though I've had no success with those tables, and they could potentially lead to fueling issues at WOT, so adjust them at your own risk.
Currently I have the outlet pressure of the turbo limited to 26 psi, and raised the torque reduction in the transmission while shifting ~15 ft/lbs so that torque isn't limited as much (while still trying to protect the transmission), as well as speeding up the automatic's 1->2 shift speed. This has eliminated most of the limp mode triggers, but on cold days with low traction it can still occasionally show up. If anyone else has ideas to chime in on this issue I'm also interested to see if there is a definitive fix besides upgrading the TIP/MAP sensors.