Re: Fuel mix for a 1966 Mustang HI PO 289
Octane requirement is solely based on compression ratio. What is your CR? Octane level required is to prevent knocking.
Also, if the heads have been reworked it is possible they have hardened exhaust seats already and you don't have to worry about burning up a valve. If you have aftermarket heads, not sure how it is in Oz, but in the states there is no worry at all.
Don't waste your money on octane boosters. They can only raise octane by less than 1 whole number. When they advertise "...will boost octane by 3 points..." they mean by .3 , not going from 94 octane to 97 octane, but from 94 octane to 94.3 octane. Just buy gas in the octane you need for your compression.