Fuel won’t clean carbon off a better fuel will with the flame
Don’t understand what you are saying.
Direct injected engines suffer from carbon build up on the inlet valves due to not having the ability for fuel to wash them clean during the intake part of the combustion cycle.
Ok, i probably should have been explicit within my earlier post that it was inlet valves I was referencing but it’s implied as is the cause being blow-by and the PCV system bringing in oil mist into the intake which is that cause of the carbon build up on the intake valves.
It’s such inlet valve carbon build up that can cause engine misfire, stuttering and lack of power.
Any carbon build up within the combustion chamber itself isn't a huge cause of misfire, stuttering or lack of power and isn’t a differentiator between port injected and direct injected engines, so the “flame” reference is unclear.
Using 98 when 91 is ok from the manufacturers perspective (don’t know if it is) simply adds to running cost for no reason.
If indeed the engine issues are due to inlet valve carbon buildup, the solution is to mechanically clean* them and possibly to improve the PCV system by adding a catch can.
Basically there is zero way to clean the backs of the inlet valves by burning 98 so I don’t understand what you’re saying
* many methods to clean the inlet valve carbon build up; chemical introduced into the intake air stream as periodic preventative maintenance, walnut bead blasting, zip tie and cordless drill hokey pokey, or dismantling with old style mechanical clean and reassemble. Basically there is no easy solution (which is why engines with both port and direct injectors are the best of both worlds)…