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There are lots of things that can be changed, but the most common is managing the fuel map.
Basically, there is a giant table in the ECU of load in one dimension, RPM in another dimension, and things like temperature in other dimensions.
It takes air flow or some other RPM parameter as an input and outputs fuel. That controls the EFI to tell it what to do.
From the factory, the tables are designed with emissions controls in mind. Typically, they are not optimal for power – they are lean to reduce NOx.
If you modify airflow outside of the parameters the engine was designed to handle, and/or are going for power, then you can tune the fuel maps to be richer, maximizing power/response.
For example, I tuned my Indian Scout (RIP Nov 2022) after doing baffle-less slip-ons, and then again when I added an intake. From the factory, the bike was smooth, but slow to rev and very mild feeling.
After doing airflow mods, without the tune, the bike was very lean and the cylinders were imbalanced. It would surge idle, stall on throttle blip, lots of decel popping, and run hot.
The tune richened the low load/low throttle a lot, balanced cylinders, then richened the mid/top ends a little bit kept it running cooler, gave it enough fuel to handle throttle blip, smoothed idle, and dramatically increased its ability to respond to throttle input as it had some excess fuel to react to sudden air increase.
And, even more, it had enough fuel to respond to the new intake opening up on the top end, so while before the engine would fall flat at the top under WOT, with the intake full open and resonating, and fuel maps working right, it would keep pulling, even pulling harder with a torque bump kinda like you might have seen with old VTEC.
Both an audible and palpable difference on the intake side, and a big difference in behavior.
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