Wondering what people's opinions are with the turbo engine as far as octane choice as it pertains to altitude (and outside temp) to some degree. I'm taking a road trip in my GTR going from Sea Level with temps in the upper 70's to high desert (3,500-4,500 ft) and low 90's then eventually to mountains (6500ft and low 80's). Not towing anything. I'll be driving in Washington, Oregon, and Nevada. I've read the HP increase on the higher octane fuel is really over 4000 rpm, but that's important when passing. Think I should just fill up with Costco Premium along the way and not give it a second thought. It's so much cheaper than elsewhere even if I don't use the extra power much between the Costco price and probably slightly better gas mileage on a 1,500 mile total road trip I'm not going to net much savings being a cheapo with the 87.

Also curious if the turbo will compensate for the higher altitude or if it will be more limited by the max spindle speed than by the max pressure?

 

I would use premium as you planned. If it was all sea level driving 87 would be fine but with the high elevation driving, I would at least use premium there. Not sure about the turbo question, but from what I understand, FI motors do compensate for altitude changes which is one of their benefits.

 

Actually the higher the elevation the lower the octane requirement. But I think that really mainly applies to normally aspirated engines. Because forced induction keeps the cylinder pressure more constant. But I could be wrong because it could be due to the oxygen content of the air not just the pressure. In the Denver Colorado area, regular is 85 octane due to the altitude. Still even if I lived there I wouldn't fuel up with lower than 87 on any car. And yes I also understand the turbo wastegate is controlled by the max pressure. So theoretically the wastegate will open later in the RPM range since the cylinder pressure will be less at higher altitudes so the turbo will be allowed to spin faster than it does at sea level before it gets to the max cylinder pressure. However I think most systems will also limit the turbo spindle speed so it can only compensate to a certain point. If the turbo can maintain the same cylinder pressure at altitude than it does at sea level, seems the math would indicate the octane requirement would still be 93 for max HP (unless the oxygen content plays a role.)

But why complicate things is my thinking. Just run premium all the time unless you are doing 90% commuting to/from work then that would be the only case I think the premium is a bit of a waste of money. But still, you have to think sometimes that extra power isn't just nice for passing but for freeway merging. I tend to think many of us (including myself) can get a little too analytical on the regular vs premium thing, at least from a cost standpoint. Now if you drive 30,000 miles plus a year and are on a tight budget, maybe it's a factor. But in retrospect, I think anyone worried about MPG should just be buying a hybrid or electric car.

 

Completely agree. Why buy a 250hp turbocharged car if you are worried about the cost of premium gas? Just buy the NA 2.5 and run regular...

 

one of the main reasons why they got rid of carburetors is because the manufacturer had to adjust it to different settings for each US state, because altitude required different settings. Just a few turns of a screw but it was a pain, if you drove from sea level to high altitude on same carb setting. Hence all car manufacturers moved to fuel injection etc. I think u should be ok if driving alone. If car loaded, give it the best octane to help it climb those hills.

 

last i remember premium in colorado was 91. lower octane in elevation

 

I for one prefer 6 cyl cars only, but now there are fewer 6's available in a decent price range. Most of the mid size suv's are 4 cyl, and totally underpowered for me at least, not to mention the load droning noise of the engine struggling to pull so much weight. Hence more power is required, so turbo is the only option, i had. The 2021.5 CX 5, Turbo is fantastic, and quite. It has the guts to climb hills and pull weight. I love it. Premium fuel or regular, both accented by the Mazda turbo. Again, i would prefer a 6 over 4 + turbo, to simplify repair costs. Who cares for a few cents. If one can afford $65k on Tesla, then they can afford a few pennies on premium. Still cheaper for ME at least.