They suggest the higher octane to get the best performance from the car. The knock sensor will retard timing and will prevent the car from predetonation. Add the 91-93 and you get all the ponies. Add the 89 and timing is cut as to knock that is detected. Its all about compression. They are right at the edge of whats acceptable with todays high test. More compression more HP... but you need fuel with retardents to keep the heat of compression from igniting before the spark plug fires. So use the 89 but when you really need the go pedal use 91-93.. All the cars I own I have to use high test. I even have a car with 8 to 1 compression, but its turboed and the same principal applies.