Hey, I am sure that like many people here, I would not mind to pay extra for high-octane fuel if I get better MPG.
Now, I remember reading that cars get better MPG if the ignition timing is advanced, but advancing the timing tends to cause the engine to knock, which is very bad. On the other hand, high-octane gas helps with engine knocks.
So, if the above is correct, if I put in high-octane gas, wouldn't I need to advance the timing to get some benefits? Do new cars have controllers smart enough to do that for me?
This is why I said some cars are designed to use high octane fuel. I don't want to write an engineering article here....so I'll try to keep it simple. What's truly important is the cylinder pressure within the combustion chamber at the time the spark plug fires. Advancing (making the spark fire earlier) cylinder pressure means you're exposing the spark to a condition where detonation is more likely. Detonation is bad in a gasoline engine...that's what you call "knock". Gasoline engines are supposed to have a "controlled burn", not an explosion. What I'm trying to say is that there are numerous factors that contribute to the in-cylinder condition that will cause knock...and timing is only one of them (others include valve timing, static compression ratio, humidity of combustion air, EGR function, and so on).
In most modern cars, you cannot advance timing...it is set by the computer. When you see these advertisements for high-performance computer chips you can plug in and make your car more powerful, that's often how they work...by adding more fuel, and advancing spark timing. I recommend against this unless you know what you're doing.
In some cases (again there are a myriad of factors), advancing spark timing can help fuel mileage, but this assumes you don't add more fuel and that you don't advance it so far that you enter the detonation zone. The only way this can be properly tested is on a chassis dynamometer using real time O2 sensor measurement...not a simple thing to do. I've had this done on my Mustang...the cost to get my engine modified to the >725 HP level was over $1,200 for the labor paid to the dyno shop and tuner who did the work.
Gasoline engines like "stoichiometric" A/F ratios of about 14.7:1. Tuners are able to adjust timing in the computer to get the A/F ratio down to a lower number (they call this "leaner", as you're using less air for a given amount of fuel, which burns hotter, resulting in more power). The A/F ratio on my Mustang during WOT (Wide Open Throttle) is about 11.3:1.
Can you buy chips that claim to increase fuel economy? yes
Do they work? Some do
Are they worth all the money you pay for them? Likely not
Are there risks to installing them? Yes, detonation
Hope that helps.