Volvo is saying the XC90 T6 should run on 91 Octane but isn't that at sea level? I've always heard you can drop the octane by one rating (i.e. Premium to Mid) for high elevation due to thinner air.
Since Octane rating is the point of combustion where at sea level 87 octane will detonate sooner than 91. At 7000 feet, 87 will detonate inside the combustion chamber at a a slower point due to the thinner air.

In both cases, newer vehicles will self adjust to the thinner air but the XC90 twin turbo may still run better at 91. Also, premium is usually not that much more in cost.

The question is: will 87 octane at 7000 feet damage the engine? Are you wasting your money running 91 at 7000 feet? Will there be a noticeable difference?

 

well considering the T in T6 says you have a boosted engine your combustion is running at manifold pressure not ambient pressure. The octane drop rule would apply to a naturally aspirated car but not a turbo.

So yes if you run too low an octane, you risk pinking (aka pinging) - the knock sensors will tell the ECU to dial down the boost and retard the spark so you get some protection but much less HP and possibly an increase in fuel consumption (since the ECU has moved the engine off the optimal fuel/air/spark setting). Also considering that if you are driving at 7000 feet you are likely in the mountains where you'd be going up steep inclines regularly which will increase the stress on the engine - all adding up to a solid reason to run the available 91 and avoid the 87.

 

Agree and will continue running 91.
Thanks for the response...

 

I have not been at high altitude for over a decade, but my recollection from years ago is that the gas you bought at higher altitude had lower octane rating at the pump compared to lower elevation gas stations. But I was at 10k feet, not 7k when I observed that.