rtlm

It does, but not in a way that lets you buy mid-grade instead of premium. The lower oxygen content at altitude means less early detonation (pinging), but here in Denver the "premium" gas is 91-octane (instead of 93 at sea-level) for exactly the same reason.

dougnash

It's not precisely the oxygen content in the air/fuel that causes the BTDC detonation. It's the extreme heat from the pressure. And I think you may be a little off the mark on the regional octane system. Here in California (with more cars at sea level than any other state) we only get 91 octane. That's it. Each state decides what kinds of fuel it will allow, and what those parameters will be. And quite often, it's more a reflection of EPA rules, than anything else.

If the rules were more flexible, I would be able to fill up with 95 in Death Valley, and then only use 87 up in Mammoth Mountain. But, it's not, and the fuel grades are identical from below sea level, up to 12,000ft.

Plasticar

You don't NEED premium, but as many have said, it allows the engine to run the best performance tune available in the ECU. If it senses the early detonation that can occur with lower grade gas in turbo engines (mainly at full boost), it will retard the timing and other changes to eliminate it. This reduces the available power, and may also affect the mileage.

Pardon me, but I find it a little silly to spend $45k on a vehicle and try to save $3.00 on a fill-up.

OttoNY

My sentiments exactly!

georgehf

Just curious - this has nothing to do with what I'll put in my Q5 - what grade of fuel is the best deal? i.e. if there is a $0.50 spread between low / mid / high grade gas, which on average is the best bang for the buck?

uberwgn

Long term? It's going to be the grade recommended by the engine mfr

123Audi4me

I did find this in a Car and Driver article about octane requirements. "Hot temperatures and exceptionally low humidity can increase an engine's octane requirements. High altitude reduces the demand for octane." I'm almost always driving at an altitude of 5000'. I wonder if that might allow the use of mid-grade fuel.

dougnash

For a naturally aspirated engine, I might imagine so. But, and not that I'm more experienced than C&D's research staff, but the advantage that the forced-induction car has over the NA one, at altitude, is the ability to constantly adjust the level of compression. Where a NA car will rapidly lose compression (and therefore HP) as the altitude climbs, the much lower cylinder pressure would arguably allow for a lower-octane fuel.

But the turbo-charged engine would keep the bypass valve closed more, and more - thus, boosting pressure and heat. It would (up to a point) be able to mitigate the high altitude, by approximating a lower altitude with denser air. Get it? Therefore, you're going to want an appropriately detonation-resistant fuel, to go with that.

Don't take this info from me, nor a commercial car rag. Just do some extra reading in engineering and racing books / web sources, and it'll explain a great deal.

123Audi4me

Makes sense. Darn, with premium gas and diesel about the same price now, I hope I don't regret not waiting for the 3.0TDI Q5. When I made my decision, mid-grade gas was about $.40/gallon cheaper than diesel, making the 3.0TDI hard to justify, especially with the likely higher price for the TDI engine.

DennisM

Thanks for the info!