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2015+ WRX Cold Weather Boost info.

Every year around the colder months we get emails in asking why boost numbers are lower than expected. Wanted to be able to provide useful information for anyone who wishes to know more about how any it does this.

This article is going to be wrote with the assumption the car we are speaking about has the proper parts is not experiencing boost creep ( Another topic)  This also holds true for other years and models how ever we are using the 2015+ WRX with the FA20 here. 

This is the stock tune.   What you see is the top axis is the intake temperature(-40 to 230) at the maf (Unless changed in tune) Below that is the compensation applied to MAP boost target and boost cut.  Take notice as the ECU sees cooler temps it is now targeting less boost. Reason for this? To keep it simple, air quality. The air is more dense, and there for takes less air to produce the same HP. 
Why is this important? 

  • TQ control. If it made the same boost it would now be making more tq then the tune was intended to. 

  • Fuel Limitations.  Depending on power and fuel used you could be riding a fine line between enough fuel and a dangerous spot. The ecu can request more fuel. If the system cannot support this. A lean condition will be present.

  • Exceeding the tune.  Without a table like this in colder weather the car would achieve airflow and boost numbers that the tune and sensors may not be ready for.

  • Engine Safety.  Allows the tune to maintain safe TQ levels to prevent excessive TQ. Which leads to connecting rod failure.

  • Hot Temps. You can also view here that in hotter temps it again starts to target less boost. ( When custom tuning this table really helps with intercooler heat soak control)

Top Axis here again is Temperature at the MAF (-40 to 230F) Below this is compensation to waste-gate duty cycle. Soon as the temperature starts to drop notice the large amount of waste-gate duty cycle that gets compensated.  This table works with the above table to help the turbo hit the new targets and control boost with the more dense air.

Both are simple tables. So while you may be seeing less boost on the gauge if tuned properly you are running within the limits the tune/tuner set and should be making close to the same TQ regardless.