Boost threshold changes from winter to summer

I have a 2005 Legacy GT and I had it tuned at Topspeed in Atlanta last winter. Now that it is summer, the engine feels a little different. Full boost is a bit lower and comes 500 rpms later. Is this due only to the air temperature difference, or is something else going on?

The car has the BNR 18G turbo, AVO TMIC, AVO turbo inlet, post MAF, and throttle body hoses, 850 DW injectors, Walbro fuel pump, STi Up pipe, Cobb catted down pipe, Borla exhaust

The Winter datalog was taken in January at 35-40 F. The Summer one was taken in late June at 85 F. Winter boost hit 21.5, Summer boost is at 19.5

C.LogJan2015.csv

pullA6.24.15.csv

Full boost is a bit lower and comes 500 rpms later. Is this due only to the air temperature difference, or is something else going on?

Is there a boost leak or an issue with the car? I know hot air is less dense and a car makes less power in the summer. I saw another good post showing a mazdaspeed3 lost 30 horsepower on a dyno when comparing it from 2 pulls at 62 F and 83 F.

I have noticed that my peak boost during summer is lower indeed. If the temperature drops significantly (e.g. from 85 to 55-65F), my peak boost can jump another 1.5 psi. I believe the temperature drop makes the air more dense (more oxygen molecules per given volume). Whereas when it is warmer, you have less oxygen in the same given volume.

Regardless of temperature though, the car would 'swallow' the same given volume for a given load. So you can see by putting more oxygen in that same volume, more chances for increased boost.

In my opinion, I think it is safer to tune a car during winter than summer to avoid boost spike.

In any case though, always check for boost/vacuum leaks. If you keep an eye on a regular basis to your fuel trims (A,B,C,D) and they all are within +/-5%, then you should be OK. Outside of this threshold, then it's time to check for leaks.

This effect can be somewhat controlled via the WGDC IAT Compensation table. It's used to reduce WGDC at lower IAT, and I've seen some tunes increase WGDC in higher IAT. It took me about a year to get this table in my unlocked protune dialed in across the temperature range my car sees annually.

Originally my car would overboost by 1-1.25 psi in 75-80F temps and would overshoot by 2-2.25 psi in <60F temps. I also needed to specify more realistic boost targets at RPM below onset of peak boost (to reduce boost error at low rpm to keep turbodynamics from causing overboost), reduce WGDC Low to avoid overboost in the taller gears, increase WGDC Max to help achieve target boost in the lower gears (don't have per gear boost tuning in the 2005 ECU) and make the turbodynamics slightly less aggressive.

My 3rd & 4th gear WOT 2k-4k rpm boost curves are now typically within 0.5 to maybe 0.75 psi of each other across the 25F-85F IAT range.

There's a great thread that discusses boost tuning here http://legacygt.com/forums/showthread.php/having-issues-figuring-out-wgdc-171702.html