What chemicals will affect the combustion efficiency of a fuel when added to the fuel prior to combustion ?

Some say acetone works. I have read between 5 and 10% increase, but I haven't tried it myself. I think the fomula is somewhere between 3 and 2 oz per 10 gallons of fuel. Acetone is the base ingreedient in a lot of paint thinners(can be bought at most hardware stores in 100%). Be sure to do your research though... some say it is hard on the seals in your engine.