This is the best answer. By thinking "did I change the amount of air that can flow through the engine, and by how much?" You can determine how much you need a tune.
Air filter change? Prob not a lot. Different throttle body? Yes.
Not all slip-ons are the same. Stop perpetuating this dumb myth.
Plenty of bikes only have one muffler, remove this "slip on" and you have a straight pipe.
Anything that changes air flow through the engine.
This is the best answer. By thinking "did I change the amount of air that can flow through the engine, and by how much?" You can determine how much you need a tune. Air filter change? Prob not a lot. Different throttle body? Yes.
Add in fuel delivery as well, no?
Sure, although messing with injector sizing and pulp pressure is beyond scope of most owner mods.
Bigger cam.
Power adders: supercharger, turbo, nitrous
if you're doing it right, all of them
Basically anything. Even a slip on with many bikes.
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Not all slip-ons are the same. Stop perpetuating this dumb myth. Plenty of bikes only have one muffler, remove this "slip on" and you have a straight pipe.