I admire that approach, Joe, because experiments properly performed are a chance to learn something that will be useful eventually.
Old racing fuel needs to be approached with care, emmo. First, some of it is methanol, and of course a petrol engine won't even run on it - it requires an air fuel ratio of 8 to 1 instead of 14 to 1. Second, a lot of it contains benzene/benzol (two names for the same stuff) which has a high octane but is carcinogenic. You used to be able to buy racing fuels of various octanes up to I think 115, with various levels of benzol, up to almost pure. Nasty stuff to leave laying around.
If an old engine runs better on higher octane fuel, either the ignition timing is over-advanced or it has so much carbon accumulation in the combustion chambers that its compression ratio has increased. Best solution is to begin by checking the ignition timing, then trying to burn out the carbon. Often the carbon build-up has been caused by worn rings causing blow-by and oil consumption - if that is the problem you probably won't be able to burn it out. However I had the problem quite a few years ago and it was caused by feather-footing when driving the car, believe it or not. I'd put a 215 cubic inch Buick in an MGB (the same little aluminium V8 that became the Rover 3500 V8 when Buick dropped it and sold the design and tooling to Rover). Probably unwisely, I'd chosen the Buick Powerpack version, with 4 barrel carburetor and 11 to 1 compression, then ported it considerably and fitted free-low exhaust headers. That engine in its original form had a really bad reputation for carbon accumulation, and since I was driving it to work every day in the Melbourne traffic, I just couldn't keep it from happening.
