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Old 04-29-2014, 02:56 PM   #15
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Hi, newer vehicles are computer tuned to run whatever octane the manufacture recommends. Going up or down from the recommended octane can/will cause less power and mileage. In the old days we would advance the ignition timing and run the highest octane gas for more power and better mileage. [we did this when we planned to take our cars to Lion's Drag Strip] My Lincoln runs Premium (92) and on our Alaska trip some gas stations only had Diesel and 87 octane gas. In that case you have to burn what's available.
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Old 04-29-2014, 03:19 PM   #16
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Try to find out where your max hp and torque are developed and run near that rpm or above. As someone suggested, ask the dealer for a save max rpm, or do an internet search. Higher rpm Ina gasser is necessary for max performance. Only a diesel runs at lower rpm for max performance. Rev it up. Jim
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Old 04-29-2014, 04:18 PM   #17
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There are some circumstances where running higher octane will help. Yes the computer will take care of engine knock most of the time buy retarding the timing. This reduces peak HP. If I were running in the mountains, I would tend to run higher octane. Yes it is true you may not need it because the lower air pressure reduces compression which reduces knock. My Excursion will tend to ping a little with 87 octane at full throttle. It would run better on 89 Octane. 93 would be a waste of money.

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Old 04-29-2014, 05:07 PM   #18
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One thing that is bad regarding GM engine power management is that it will reduce engine timing whenever needed, but it will NOT advance it back immediately. The GM system "learns" patterns over time and adjusts several things to your habits, fuel quality, temperature, and driving conditions. What that means is say you usually run 93 octane for maximum power, then fill it up with 85 octane. At least on the SSR (6.0 LS2), timing will be drastically retarded for the less octane fuel. However, once you use all the low octane fuel and refill completely with 93, the computer will NOT immediately advance the timing back, so you are at a huge power disadvantage. At least GM on the SSR provides a fuse (number 21) to pull just for this purpose, to "reset" any de-tuning back to the original state (whether it be the factory GM tune or an installed performance tune). On the SSR, I've mounted a momentary switch on the dash so that when I'm out west and can't find good gas (sometimes 90 is all you can find...what's with this blasphemy??!?!), once I refill with at LEAST 91 (a Godsend to find out west, 93 is a pipe dream) then I reset the computer to get rid of any detuning the computer did for the poorer quality gas. The difference is amazingly noticeable. The SSR has a torque meter on the floor that will peg 400+ with adequate octane fuel; out west with 90 octane I'm lucky to hit 275. Once I reset the computer and fill up with adequate octane, the torque meter pegs right back at 400+. I reset the computer frequently just to let the engine adjust whenever I know I've purged out any questionable fuel in the system. Remember, the computer will only DETUNE and reduce timing for any problems (bad gas, altitude, excessive heat, etc) but it will NOT advance the timing back quickly...it takes several hundred miles to do that. That's why the computer must be reset.

I'm sure the Suburbans have a similar system...if GM didn't provide a fuse to pull to reset the computer, then it is imperative to reset the computer when out west by disconnecting the battery for a few minutes to allow the computer's power management system to optimize engine performance for the conditions you are immediately in without having to wait hundreds of miles for increased performance to return. Disconnecting the battery accomplishes the same thing that removing the fuse on the SSR does; it resets the computer and retunes the engine for the current conditions. Try it. This works.
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Old 04-30-2014, 12:12 AM   #19
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After reading this thread this morning before traveling through Idaho and Montana on many steep grades, I tried a tank of 89 and a tank of 85 octane in our 2012 Ram 5.7 pulling the our 25' Flying Cloud. Elevations ran about 4000 to 7000 feet.

We're not nearly as heavy as the o.p. but I could not notice any difference using the two different fuels. It pulled the grades with ease keeping the rpm in the 2500 to 3500 range and downshifting as needed. Surprising to me because I thought the engine would retard the timing enough with lower octane to be quite noticeable.

There should be a table somewhere on the internet showing the torque available throughout the rpm range for the o.p.'s engine. Probably over 3000 to get the most out of it.
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Old 04-30-2014, 03:20 AM   #20
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Tow vehicle fuel question

3,000 RPM is closer to cruise RPM than it is for pulling a really step grade for me.

Even 4,500 rpm or a little more won't hurt the 6.0.

I have pulled some pretty steep grades at 65+, I have only very briefly seen first gear once on an 8% grade south of Globe, AZ, and I never buy anything buy anything but regular gas.
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Old 04-30-2014, 07:04 AM   #21
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If the OP does a lot of mountain driving, another option would be to retrofit a small turbocharger on the gas engine. One would not be looking for lots of boost but the ability to maintain sea level power to say 10,000 feet which would cover most of the roads in North America.
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Old 04-30-2014, 07:58 AM   #22
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Thanks again for all the replies. Very helpful.
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Old 04-30-2014, 08:28 AM   #23
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By the way, I always select a gear to keep the engine above 2000 rpm when pulling the Airstream even on level, for us that is 4th (of 6). That keeps the transmission from down shifting on the usual rises in grade or strong headwind. I don't see that increasing fuel use at all because the engine pulls comfortably there and the transmission does not hunt.
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Old 04-30-2014, 10:23 AM   #24
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As a data point, my 2011 Flex with EcoBoost says this:

3.5L V6 EcoBoost engine

Your vehicle is designed to run on regular fuel with an octane rating of 87 or higher. For best overall performance, premium fuel with an octane rating of 91 or higher is recommended. The performance gained by using premium fuel will be most noticeable in hot weather or in severe duty applications such as towing a trailer.

So far, I've just run regular, to no apparent ill effect.
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Old 05-01-2014, 07:25 AM   #25
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I did order the Suburban with a 4.10 rear end. At the time I thought I would need the extra towing capacity, but we ended up buying a 7,300 gw AS rather than a significantly heavier brand X.

Regarding mileage, we get about 10 mph hauling and about 19 mph on open road without the trailer. Living on the east coast we expect to travel west only intermittently. But the mileage highlight (or lowlight) of our trip last summer was the 4 mph we experienced climbing the sixteen miles between Oray and Durango, CO. Ouch!

Thanks again for all the replies.
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Old 05-01-2014, 07:27 AM   #26
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Oh, and we found ourselves running in first gear for some of the Oray-Durango stretch.
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Old 05-01-2014, 07:54 AM   #27
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Might be good to reconsider the amount of gear in the truck and trailer you bring on those long trips, try to lighten the load significantly.
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Old 05-01-2014, 09:27 AM   #28
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Quote:
Originally Posted by FastEddieB View Post
As a data point, my 2011 Flex with EcoBoost says this:

3.5L V6 EcoBoost engine

Your vehicle is designed to run on regular fuel with an octane rating of 87 or higher. For best overall performance, premium fuel with an octane rating of 91 or higher is recommended. The performance gained by using premium fuel will be most noticeable in hot weather or in severe duty applications such as towing a trailer.

So far, I've just run regular, to no apparent ill effect.
The computer will back off engine timing so you don't have an ill effect.
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