5-Star leaves the knock sensors active, but they do adjust their settings per octane levels. You absolutely cannot use a lower octane fuel than what your current tune is rated for (i.e. you can't run 87 octane while using a 91 octane tune). However, you can run a higher octane than what you're tuned for (i.e. running 91 octane on an 87 tune), though you won't get the full benefit of 91 octane tuning. As for water-meth, whether the tuning will handle it will depend on how much you're spraying and when. If you have a low spray rate and are using water-meth as an octane/IAT stabilizer, then 5-Star's tuning shouldn't be effected, though your best bet would be to call us directly at 623-434-5277 or email us at [email protected] so we can get more specific about what you need.