What's the root cause of not working particularly well?
Provided the RTA has top/bottom afc which you already know and maybe even own(ed) one, and altho the 3id coils in mind appear to fit while allowing ample af - or the id can be adjusted - what else beside getting them to ramp evenly? And quickly?
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The thing about the Fatality M25 RTA is that the bottom airflow is just far too restrictive to be able to properly use it like that. I never owned one because the 24mm Reload RTA (I own four of 'em) with 27/36 Ni80 aliens at 5 wraps 2.5mm ID dual coil at .11 ohms on a single battery mech has IME noticeably better flavor in direct comparison to it, and, this is despite the M25 does have pretty excellent flavor, also IME. Granted, the M25 has more build space than the (24mm) Reload so the latter RTA is limited to 2.5mm ID coils. But then, that's why I also own an Isolation RTA. With 30/36 Ni80 aliens at 6 wraps 3.5mm ID dual coil at .32 ohms in that one, on a stacked mech it too outperforms the M25... noticeably, again IME. Both the Reload and the Isolation vape more like an RDA in terms of the type of vape experience I get from them.
For the sake of reference, I'm an RDA guy with a passion for high end authentics and high powered
FLAVOR chasing DTL mech setups of various types and sizes─ranging from small build decks that fit inside a whole plethora of 20mm ID caps all the way up to the 52.8mm Colorado RDA. To me, personally, the flavor and all the other characteristics that make (or break) the vape experience are interlinked and can not be disconnected from each other in any way, shape, or form, so...
to me, this isn't just about applying some heat and some air. Rather, it's about fine tuning the parameters by keeping in mind that performance is limited by the weakest link. In part, it means that the relationship between coil specs, coil positioning, and all factors related to airflow also including the strength of my draw all need to be in perfect balance with each other.
As elitist as that may sound to some, it's all really nothing more than a, relatively strong, dedication toward a simple goal that is twofold: 1/ the benefits of variation and of being able to compare, and 2/ finding the one miracle kind of setup that vapes best for everyone else except me. ? (On a more serious note, I just think that flavor chasing is about so much more than just flavor, as the warmth, wetness, density, and texture of the vape are equally as paramount, and, both the speed of inhalation and the amount of restrictiveness are an inevitable part of that culinary equation.)