Aussie Mike,
I did some research on the exact same topic a couple weeks back. Here is what I learned using 91 octane (that's the best they will sell you at the pump in So Cal...pretty sad, huh?).
Toluene (also known as methyl benzine) is rated at 114 octane. The average Toluene concentration in pump gas is about 30% (so some of these articles say).
Add 1 gallon of Toluene to 16 gallons of 91 pump gas and you end up with 92.4 octane and a Toluene concentration of 34%.
Add 1.5 gallons of Toluene to 15.5 gallons of 91 pump gas and you end up with 93.0 octane and a Toluene concentration of 36%.
Add 2 gallons of Toluene to 15 gallons of 91 pump gas and you end up with 93.7 octane and a Toluene concentration of 38%
(even at 38%, you are still considered at a (relatively) safe level).
A local paint store will sell Toluene in the $6/gallon range. So I'd have to spend $12 + 7.75% sales tax ($12.93) for the additive. NOS makes an excellent "cure in a can" for about $10 + tax. Even mixing AV GAS (aviation fuel) has got to be cheaper than $6/gallon Toluene. I'm sure I could do better buying in bulk...I haven't checked it out yet though.
I just might leave the chemistry to real chemists.
Home Made Octane Booster
Linear Mode
