True, a carb and distributor are key for running good, however when it comes to determining what octane fuel an engine must have, it is all based on Dynamic Compression Ratio (DCR). It is actually the cylinder pressure that is a result of the DCR that causes the issue. As any fluid is compressed (in this case the air/fuel misxture), it heats up. The more it compresses (higher cylinder pressure) the hotter the fluid gets. When this temp reaches the flash point of the fluid, combustion occurs. The higher the cylinder pressure the higher octane you need since higher octane is fuel with a higher flash point. Here is a web site that explains in detail and also has a DCR calculator that you can download.

http://www.empirenet.com/pkelley2/DynamicCR.html