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Old 11-14-2012, 11:07 AM   #3
IA Performance
Former Vendor
Member#: 25194
Join Date: Sep 2002
Chapter/Region: RMIC
Location: Boulder, CO
2008 Forester XT 5mt
Dark Gray


Vacuum at idle should be close to 16-20 inHg (inches of mercury). Keep in mind, boost cannot be a negative value, hence inHg. 8 psi = 16 inHG.

Boost Pressure Oscillation
This is often a result of over boosting, where the car over shoots the ECU's goal of PSI for that given Throttle Position and RPM. Example using arbitrary numbers:
- ECU only wants to see 8 PSI at 40% throttle and 3500 RPM
- Car makes 10 PSI at 40% and 3500 RPM
- Resulting, ECU will decrease the waste gate authority, and often decreases too much, making a oscillating pressure curve (fluctuation)

Car is Not Achieving Target Boost
From the data log, assuming in 3rd gear, the car is not achieving target boost. You should run the HWG map or shorten the wastegate arm by 1.5-2 turns. The larger TMIC may be contributing to this depending on the pressure loss it has compared to the stock TMIC.

Taper In Boost
During a full throttle pull the boost pressure will gradually decrease. This is not in the tuning, it is a limitation of the stock waste gate. No matter what a tuner tells the ECU to do, it will always decrease in the higher RPMs as the higher exhaust velocity/volumn forces the waste gate door open.

From Cobb's website:
Stage2 ACN91 v310
o Fuel Requirements: 91 octane fuel found in Arizona, California, and Nevada or better
o Intake Requirements: Stock airbox or COBB Tuning SF Intake
o Exhaust Requirements: Upgraded turbo-back exhaust
o Boost Targets: ~16.0psi peak boost pressure tapering down to ~12.0psi by the 7250RPM redline, +/- 1.0 psi

All Stage1 and Stage2 performance maps include a “HWG” and “LWG”:
HWG: Higher wastegate duty cycles (HWG) for use when normal wastegate calibration produces lower than targeted boost (i.e. under boost).
LWG: Lower wastegate duty cycles (LWG) for use when normal wastegate calibration produces higher than targeted boost (i.e. over boost).

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