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It's very much on a case-by-case basis...
I run a hotter intake air temp for quicker warm-up, TC engagement, and whatever else it does. My studies have shown that air temps less than 100-110F decrease FE after warmup. Why? Not sure. It only has worked on the Integra and not the '99 TL or '06 TSX.
Now for the premium fuel argument. Again case-by-case. 3A-3B-3A tests (needed to run through 3 tanks of each to normalize the fuel content and vehicle's fuzzy logic) showed no appreciable gain in the "premium recommended" TSX. It has provided better in FE in rentals, such as the Volvo S40, 5AT non-turbo.
Back to Teggy, the timing is physically advanced to its limit; although somewhat regulated by electronic timing control, the overall index has advanced and (likely provides) more power (as detected by the "butt-dyno") and a tested improvement in FE. Check my 100+ fuel entries for some data.
So, to combat the effects of a hotter intake and advanced timing on detonation, I run premium, 91 Octane fuel. An extra $2.00 per tank for an extra 3+ MPG and engine protection seems reasonable. When it gets bitterly cold, I can back-off that octane rating since hot air is harder to retain.
RH77
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“If we knew what we were doing, it wouldn't be called research” ― Albert Einstein
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