Timely that someone should bring this thread back to life. My father has recently bought a car with a high compression engine (2005 Toyota Mark X). I have the same car so had done a little research when I got mine, and the general consensus online was that it should be run on 98, but could likely get away with 95 (but with reduced power as computer would adjust timing to prevent knock). Everyone seemed to be in agreement that you shouldn't run it on 91.
I explained this to my father when he got the car, but just discovered last night that he has since filled it up with 91 "because the dealer never mentioned needing 95 or 98". I, of course, had words with him about why he would just ignore my advice!
But my question is, just how much can the anti-knock sensors compensate for? Surely it can only adjust the timing so much? Could he be damaging his engine?
At this point, with half a tank of 91 left, should he top up with 98? Will this bring the overall octane up as the fuel mixes together? Or is he better to wait until the tank is near empty and fill it completely with 98?
If the manual says use 98, use 98. If the manual says use 91, use 91. If you need to switch then I'd probably let it get pretty low first.
Yeah, I'd made it very clear to him he should be using 98 - I don't know what possessed him to put 91 in it.