i dont really know much about octane and all of that. i've been using 89 octane on my stock d15 ever since ive been driving it. thats what my dad has told me to use. my question is, from all that 89 octane usage, will it be bad switching to 87 octane? thanks.

 

No it wont hurt it at all. Now if you start raising compression, then yes you need more octane to an extent. You will be good.

 

well i say yea cus then your going to start building up carbon on your valves me i had a d15b and i used 91 octane and my motor had barly any cabon build up at all so i recommend 91 for safe and better gas mileage

 

actually you have carbon buildup from using too high octane.

lower octane burns faster, if your engine is not set up to run hi-oct then you will have unburnt fuel leaving the chamber
unburnt fuel=carbon
burnt fuel = CO2

as far as your octane, pay attention to your MPG and how your engine runs

if it gets worse, adjust your engine or your octane

there is no simple answer, depends on your engine/setup