What's the purpose of this mod?
As soon as I can actually type because this forum sucks so bad...
These shorten the intake runner distance, which effectively shifts your power-band upwards, resulting in a power gain because volumetric efficiency improves with RPM. In order to take full advantage of these you'd need to raise the rev ceiling to 15 or so.
These also result in less restriction on the intake path, though that result is minimal compared to the resonant tuning mentioned above.
Any air flow change should get a tune.
Not at all, in fact the ECU can adjust for a wide range of airflow perfectly fine for our uses. Now for the guy wanting every last horsepower, then there is some to be had. But really tuning doesn't adjust for changes in airflow, tuning adjusts fueling/timing for all airflow values. For example Bren gains their measly power numbers by leaning the motor out some, and advancing timing some. Both of these decrease safety margin for bad fuel and other problematic conditions, but that's something a lot of us can live with as BMW leaves quite the margin to cover their bases.
The engine is only going to inhale a certain volume of air through filter, air box, and into the engine. The cylinder is only so big.
I dont think it will do much without supporting parts.
You're absolutely incorrect, that's why we make 200hp out of a 1 liter while a Honda makes just 150hp out of the same size engine. This is because, quite simply, our volumetric efficiency is significantly better. VE is the amount of air introduced into an engine versus the volume of the engine. So 100% VE means we would injest 1 liter of air for every 2 engine revolutions. It is generally accepted that 100% VE is approximately 100hp/L. So the BMW S54, S65, and S85 all run at approximately 100% VE. This is quite remarkable considering the relatively large size of the engines, and that they need to be street tuned. But compared to smaller sportbike motors, it's crap. Now by this formula we pull in 2 liters of air, effectively running at 14.7psi of "boost", in NA form. This formula is obviously a wildly inaccurate approximation, I don't for a second believe we get in double our engine volume. But the point is, our motors are substantially higher than 100% VE.
However, you're right about this not doing much without supporting parts. This is because these really only add power if the rev ceiling is raised, and we need supporting parts to raise that because the factory parts are all tuned for 14,000.
Any fuel change would do the same...
Not quite, in fact without touching the electronics or the pressure regulator, you really can't change fueling at all. Bigger fuel pump, bigger injectors, bigger fuel line, none of it will do anything