Sadly the 2019 STI probably has one of the better OEM tunes performance-wise compared to previous years. Although I've heard mixed reviews about the safety of the OEM tune..I just want to do a COBB Stage 1 tune to 1) Get rid of the annoying stumble, and 2) Protect my car from the (obviously) sub-par factory tune - that's it - nothing crazy, not even bolt-on parts. Sad, sad days.
The video below talks about the OEM tune being more aggressive on the 18 STI. I believe this held true for the '18 Type RA & '19 STI as well. They mention the OEM tune leaning out in higher RPMs (above 5,400rpm) and pulling timing. Cobb's map runs a bit richer to avoid this.
2018 STI Baseline and Cobb Stage 1 Data on the Dyno - YouTube
I also found the information below from Cobb regarding the OEM tune's aggressive timing:
The consensus seems to be that the OEM tune may encounter some knock and pull timing on 91 octane. Both sources claim that the OEM tune is still safe as the knock correction system is functioning as designed. The Cobb stage 1 tune claims to rely on the knock correction system a bit less while also increasing mid-range power.COBB Tuning – Subaru WRX STI Type RA Accessport and Parts
The Type RA is loaded with a much more aggressive tune than we are used to seeing from Subaru. In fact, on ACN91 octane fuel the car was seeing enough knock to shut off the boost control system completely. Using higher quality fuel (93 Octane) we did see much more consistent power out of the Type RA however it is still leaning fairly hard on the knock detection system – with hits of 4 degrees seen relatively consistently. With that said, our internal calibration team was still able to find a bit of power, specifically in the mid-range, which can be realized in our Stage 1 Off The Shelf Maps. By using a different tuning strategy, we were also able to mitigate the concerning levels of knock. This makes for noticeable gains and peace of mind, especially for those using ACN91 fuel.
The factory calibration essentially starts with higher base timing values and the knock system rarely allows it to fully take advantage of dynamic advance. Our calibrations will start at a slightly lower base value with a higher allowance of dynamic advance. It’s not necessarily disastrous for longevity as the knock correction system is indeed doing its job. Our approach is to not lean on that system as hard as the factory calibration does.
How lucky I am to live in California where 91 is the only option unless I go the flex fuel route..