Towing at high altitude

[techntrek]

I will be towing across the country this summer. Already confirmed the Diablo tune is only good for 91 or 93 octane, in another thread. At high altitude only 89 and in some places only 87 octane is available. I assume the only option at that point is to revert to the stock tune? Any advantage of using the 87 tune in that situation vs. the stock tune?

[jay p]

I live between Denver & Colorado Springs and buy 91 octane all the time. I have never seen true premium gas that is lower than 91 here. Some stations only sell reg (85) & mid grade (87). The reason for the lower octane is the air is thinner and you get less compression (unless you have a turbo or super charger).

Jay

[techntrek]

Yup, I understand the physics of it. Been all over the US several times and there have been times when I didn't see 91/93 for several fill ups. Even just one fill-up is going to force me to load the stock or 87 tune, just want to get some opinions on that.

[mikel]

id use the 87 tuneover the stock tune if stuck with low octane fuel for sure.

[techntrek]

If I'm able to get 89, wouldn't the stock tune do better?

[mikel]

No, if the stock tune was in any way better than our tune, we'd have no purpose

[techntrek]

lol

[techntrek]

After towing over 8500 miles from the east coast to the west coast, up the west coast, and back to the east coast, I have some observations. 2010 Suburban, 6000 pound camper. My primary goal is towing performance and I originally bought the tuner to adjust the fan temp settings (as discussed elsewhere).

I used the Diablo tune for the first 2000 miles, which included the Appalachian mountains up to the foothills of the Rockies, so lower mountains and plains. 1950 miles, averaged 7.3 mpg. Purchased the highest octane available at each stop.

I switched to the 87 tune the first time I could not buy 91 octane in the Rockies, and continued to use it for the next 6563 miles since half of that was in and out of the Rockies still, and I wanted to see how it compared mpg-wise to the Diablo on comparable terrain coming home. I still purchased the highest octane available so the only variable would be the tune and not octane, too. Over the last 2566 miles when I was out of the Rockies again (after Glacier National park) I achieved 7.2 mpg, so slightly less than using the Diablo tune going west. However, I had dragging rear brakes for some of that stretch before I got them fixed for the final 1000 miles so that puts a wrench in the works for a straight comparison. Also since the Rockies are only 60 miles wide at Glacier, if you include the flatter terrain we experienced going north to Seattle after Yosemite that puts the mpg at 7.6 over 4666 miles (including the dragging brakes and the mountain passes through Glacier). Over the entire time using the 87 tune I achieved 7.7 mpg over 6563 miles.

I did not notice any practical difference in available power at higher altitudes between the two tunes, since the limiting factor is the available air and no tune will fix that, only a turbo or supercharger. As I've noticed many times over the years even before owning the tuner, the lower the octane the less power available (duh). Since I definitely get an mpg improvement using the 87 tune during daily driving, and there appears to be a slight improvement in mpg using the 87 vs. Diablo tune with no real advantage to using the Diablo tune when extra power is really needed, I'll stick to the 87 tune with higher octane gas for towing, and 87 octane gas at home.

While I've kept good records of a prior long trip with the same camper last year, where I used the stock tune and a competitor's tow tune, I also only used 87 octane before I applied that tow tune so I can't compare the data.