Keep 89 in there. A higher octane can cause detonation in the cylinders under high duress which can be a detriment to the performance. However, in a VTEC motor, the octane level that just prevents knock is what you want. This can vary depending on your elevation from sea level and moisture in the air. Using too low an octane level can cause the knock sensor to prevent VTEC from enaging. Using too high an octane can cause said detonation (premature firing of the cylinder due to too high of compression).