>But adding high-octane gas to a normal engine doesn't make it go any more boom, in fact, it goes less boom because it doesn't burn as much when it booms.
It’s probably not your fault. It always annoyed me when old (20-30 years ago) Shell gas commercials would imply that high-octane (read: much higher profit margin) gasoline would cause your engine to make more power. Now, they didn’t actually say that outright, because it’s not true.[0] But if you put Shell gasoline in your car, you were told that you’d “feel the power your car is capable of”, or some weasely bullshit. The word “power” was used a lot in that commercial, just never next to the word “more”.
I was a professional mechanic around that time, and I had coworkers that thought that not only would high-octane gas give more power, would also swear it increased fuel mileage. Neither is true. High-octane fuel has less energy per gallon than lower octane fuel, it can’t give better mileage. (Now, “mechanic” != “engineer”, but c’mon, guys.)
[0] Yes, Captain Pedantic, I’m aware of knock sensors and computerized ignition timing. It was a quarter century ago.
> I’m aware of knock sensors and computerized ignition timing. It was a quarter century ago.
This stuff started to show up in the late 1970's-- the 1981 280ZX Turbo my dad had was equipped with a knock sensor and would retard timing and add fuel.
And just to highlight and explicitly explained the weird edge case that GP mentioned...
On many modern cars, high octane gas gives you more boom. They are designed for high octane gas, but the engine management computer is able to detect low octane gas and prevent damage from predetonation-- giving up some power.
This is probably not true of modern naturally aspirated cars. Maybe they could lean out the mixture a little to get a bit more efficiency on higher octane, but they largely can't leverage the higher octane fuel as well as a car with an electronically controlled turbocharger system.
>They are designed for high octane gas
The vast majority of modern engines are designed for 87 octane, and not downrated to 87 octane, because that would give you poorer performance than a clean sheet 87 octane design, and single digit percentages matter here.
> This is probably not true of modern naturally aspirated cars.
There's a whole lot of cars that say "Use premium fuel only" with compression ratios of >10.5:1, but rely upon anti-knock sensors to retard timing (and adjust mixture) to avoid detonation with 87 octane fuel.
And that's exactly where i was wrong :-)