Q: I recently rebuilt a 383 for my 1968 Charger. It’s not a race car, but a typical street cruiser and occasional quarter-mile warrior.
My question is about fuel. I’m not certain how much fuel additive or 100 octane aviation gas is enough, too much, or even necessary at all. Some people say it’s pointless to use more than 20 percent aviation gas (the rest is 92 octane) because it burns so fast. Others say the higher octane gas adds power. To make things more confusing, the old cars like mine ran on regular gas, not 92 no-lead. Nobody has convinced me that any of those cans of octane booster do any good.
A: To help explain the use of fuel and fuel additives, let us give you a lesson in gasoline and octane ratings. Old muscle cars ran on leaded gas because they didn’t have hardened valve seats; the lead acted as a lubricant for the valve and seat contact area. Cars from 1974 and up have hardened seats that allow the use of unleaded gas. When it comes to power, there is no difference between leaded gas of a given octane and no-lead of the same octane.
Fuel octane is related to the fuel’s ability to burn. A slower burning fuel is better than one that burns fast; the faster fuel burns, the more detonation problems you will have. Fuels with higher octane ratings burn slower, which helps cylinder pressure to build up steadily instead of all at once. That reduces the chances of detonation in higher horsepower engines. Feed the same engine low octane gas, and it will detonate and ping.
You said you were adding 100 octane aviation gas to your car. While the octane is good, aviation gas burns very hot and could eventually damage a car engine. It is also a very dry fuel; spill some on the ground once and see how fast it evaporates.
Those octane boosters really do work; 104+ is one of the better ones. If you still think you need extra octane in your Charger, try using VP Racing gas in a 70/30 ratio with your 92 octane pump gas. Or, you could use 104+ mixed according to the directions. Make sure your ignition timing is set to take advantage of the extra octane.