Get ready, here comes a short book!
There is apparently NOT a rule about what octane you have to run. I thought there was. I am running Amoco Ultimate 93 Octane pump gas with Lucas top end lube. I am going to add 4% Ethanol. This is all legal. Any fuel will be legal, as long as it meets the rule specs, which I posted a link to below.
Here's the deal. Octane in dirt road talk is a measure of how long it takes to burn that grade of gas. The lower the number, the less time it takes to burn, so you have a more volatile fuel that burns hotter and quicker than a higher octane fuel.
Increasing compression increases horsepower, but, compression, along with heat, can cause a volatile fuel to explode before the spark plug fires. This is called pre-ignition (you know it as dieseling) and it is bad. So, in order to counteract this problem, you use a higher octane, less volatile fuel. Even though the fuel has less BANG, the net effect of the increased compression is an improvement in performance.
So the trick is to select the right octane for the job. The first thing to consider is compression. We are just under 10 to 1 with these cars, and my handy dandy chart tells me that 89 octane is the fuel to use. BUT we have to also consider heat. A racing engine sees higher RPM and heat than this engine was originally designed to handle. It is also higher than what my chart is expecting. Running a less volatile (higher octane) fuel will help keep it cooler. Heat is the enemy!!! But remember that less volatile fuel takes longer to burn, so it requires that the spark fires a little bit sooner. So you have to advance ignition timing to compensate and give the fuel enough time to fully ignite.
There is not a huge difference between 89 and 93 octane, but it is plenty of difference to make up for the added issues we see. The Ethanol will simple help it burn more completely, and the Lucas takes the place of lead in gas and keeps your valve faces protected.
Below is the new link to the new 2007 rules. I copied and highlighted fuel related stuff.
http://probld.600racing.com/documents/2007%20INEX%20Rulebook%20-%20Legends.pdf