no offense, but most of you guys don't know the first thing about octane. the higher the octane, the better the fuel, the more power, period. the higher the octane, the more resistance to detonation, and the more you can advance the timing meaning more power. so, in EVERY scenario, keeping the tune the same but putting in higher octane can do NOTHING but increase horsepower.

i tested it recently on my last car, my supercharged rsx-s, with hondata k-pro. on the SAME tune, 87 octane caused knocking, but 93 resulted in the engine running smooth. at least 20whp increase on the dyno (don't remember the exact numbers). if i upped the gas to 100 octane (keeping the tune the same), a few more whp. if i then TUNED for the increased octane, another 20whp, easy.

yes, tuning for the higher octane results in MUCH higher hp numbers, BUT in our mustang gt the computer advances the timing as much as it can (from 87 to 93 octane) based upon the gas we put in the car. when i first put in 89 octane from 87, no difference, but after a tank or two, there was a noticeable gain in rwhp as the computer adjusted. this car was designed to be smart, so that if an idiot put in 85 octane, no serious damage would result as the engine would pull timing. at the same time, it was designed to adjust for higher octane, to increase hp and performance.

i'd guesstimate the following:

93 octane + 93 octane tune = 25whp more than stock
93 octane + stock tune = 5-10whp more than stock (after running a full tank or two for the computer to adjust)
91 octane + 91 octane tune = 15whp more than stock
91 octane + stock tune = 5whp more than stock (after running a full tank or two)
89 octane + 89 octane tune = 5-10whp more than stock
89 octane + stock tune = 3whp more than stock
87 octane + 87 octane tune = 3-5whp more than stock
87 octane + stock tune = stock hp