Light spark knock- ping- that could be cured by less spark advance or a little higher octane fuel probably isn't harmful. It can also be caused by carbon build up. 200's and 250's in the late 70's were prone to carbon build up so severe it would cause piston to head contact. We never tore one down to fix it, we'd take the car out behind the shop, get the engine up to temp, and run it at 2500-3000 while slowly trickling water down the carb and burn the carbon out of it. If you try it on a vehicle with a cat be careful because it will overheat the cat fast. Heavy spark knock can cause engine damage, 302's and 351's in 80's-90's F150's and Bronco's would burn pistons from induction crossfire when the plug wires for cylinders 5 & 6 were run too close together. Removing the alcohol doesn't cure spark knock, increased octane does. Alcohol in the gas causes wasted money, foul exhaust odor from cold engines, small engine fuel system problems, and happy politicians. Other than that it amounts to nothing.