Quote:
Originally Posted by woodj
Your car is made to run regular gas. You may experience pre-detonation if you run higher octane fuel than is recommended. That is not a good thing. Someone can chime in if I'm mis-speaking.
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You are mis-speaking...it's just the reverse. You use high octane to lessen detonation, not increase it.
You don't need higher octane than is recommended. In fact, if you're looking for the best gas mileage, you're probably driving the car fairly easily, not matting the throttle, shifting at the recommended RPM, and keeping the engine load at an average, i.e. "road trip".
Increased engine load leads to detonation when the octane is inadequate. You shouldn't see any real increase in gas mileage if using the lowest octane available and driving steadily. That said, if you drive the car hard, and see detonation via monitoring, yep, your mileage will suffer. Higher octane, while more costly, will help reduce detonation and keep the power band up - leading to better gas mileage - ONLY if you drive the car hard.
The ECU has two different modes of timing vs. load events, and the first mode will be utilized when detonation is minimal - the second "lower timing" mode will be used if detonation is frequent. It stays in this mode if frequent detonation is monitored, and reverts back to the first mode if detonation is held at a minimum. FWIW, lower timing leads to less engine power, which indirectly affects gas mileage.
I have owned several V6's (well, turbo engines) and can tell you what detonation can do if it's severe. You don't want to go there. The turbo engines are a different story: Boost increases compression ratio pressure. That's really the detonation factor. This doesn't apply so much to normally aspirated engines. Octane rules change when you compress air.