Damien-63, technically it would be helpful to know the rod length of your engine and the compression ratio. Even so it will probably be an experiment you will have to try. Pragmatically you could start with mid-range 89 octane and see if it knocks even when you adjust the timing, but it may be likely that you will have to use 93 octane. If you have the short-rod version of the 383 you will probably need 93 octane. Tell us more about the details of the engine as to C.R., flat-top or dished pistons, squish volume and rod length. Others know more about this than I do, I am just giving you a first guess response, but I considered this when I decided between a 350 and a 383 and I chose a 9.0 C.R. 350 to try to be able to use 87 octane but still I may have to use 89; I won't know until I get my wiring in and start the engine. At this point the price of gas will soon be over $3/gallon so if you have to spend $3.20 for 93 octane maybe the difference in price is now less as a percentage of the overall price?

Don Shillady
Retired Scientist/teen rodder