Do you have raw data you can share?
No big deal if you don't have it. My guess is most of these performance gains are related to differences in the sound of the motor when you removed the stock air box and not actual increases in performance. The mpg increase is interesting though. For those mpg increases to be real (not that you did not see it, but that conditions were close enough to be comparable), it could only be a number of things.
1) GM mapped the engine tune with higher airflow, then put on a more restrictive airbox. This seems unlikely given that fuel economy standards are at least partially based on an average across all vehicles of a certain class for a given manufacturer. Maybe they are just sandbagging? It isn't unheard of that a manufacturer just gets it wrong and simply putting on a new intake makes the car more powerful AND increases mpg. It is incredibly rare though, especially for any vehicle that uses fuel injection.
2) After installing the airbox, you were more conscious of how you were driving and you are comparing real world numbers to your newer numbers where your awareness to how you were driving influenced your driving habits. You did say you were using cruise control, but for the record cruise control is not generally better for gas mileage. You usually get far worse mileage with cruise control because mpg is not part of the cruise control algorithm.
3) By putting on a higher flow intake system you are operating outside of the parameters that the system is designed for and are now running very lean (too much air vs fuel). When your engine runs lean, your engine runs hotter, might get better mpg, might make more power, might have pre-detonation issues, might have reliability issues.
To get benefits out of an intake swap on any fuel injected car, you generally need to get the engine re-tuned to take advantage of the increase airflow of the intake. Even then, it is usually not a major bump in HP, Torque or MPG. If there is a major gain after a tune it is usually because you have sacrificed some other perf or efficiency metric for one that you care more about, for example, sacrificing MPG for HP is very common.
Last edited by coloradoninja; 05-21-2017 at 01:54 AM.