Thunderbird93, true but the .18's cannot rev as high as the .26's if it is dropped into the rs4. The reason is because the .18s and .26s are made to rev low but the .26 is tested using heavy loads while the .18s are tested with a lighter low. This is why when I dropped the .26 in my rs4, it revved higher than the AB MOD .18tz. There is barely any load on the engine causing it to rev freely.

You are right the .26 will not out-rev the .21. That is why I am planning on dropping an RB .21 in my rs4. I will update you guys as I get to it.