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Old 01-28-2008, 03:31 PM   #21
D'Hag
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Originally Posted by akim47 View Post
hmm... well the 6.2L denali "recommends" premium fuel, but does not require it... what does this mean? i've been rotating betweem 87 and 91... not sure if i notice any differences...
73Shark is correct. GMC built it to take advantage of higher octane. Now, whether there really IS any difference or not can be another story.

Your highly-tuned "seat of the pants" dyno may not feel any difference. But if you were to closely monitor fuel economy, you might get better mpg with 91 octane than with 87 octane. Or it might run 1 or 2 tenths quicker in the quarter mile, or go 1 or 2 miles per hour faster for top speed (if the speed limiter was disabled), with 91 octane. You won't "feel" that difference.

The key word here is "might." Suffice it to say, in a 5000-pound machine with a daily driver tune and nearly 17:1 weight/power ratio, you won't feel much difference in performance with only a 4 octane increase.

Even then, every engine coming out of the factory is a compromise. Construction, design, and tuning are set up so every one coming off the line will "run right." Exceptions are things like Ferrari, Lamborghini, and 8-litre Z06 Corvettes, which are all hand-built and custom tuned. But then, for what they cost, they better be!

What that means to you is, your engine may be at its best with 87 octane, while another guy with the exact same machine is forced to pay extra for 91 octane to get the same performance and mpg. Only way to tell for sure is at the drag strip, on a dyno, or at the gas pump.

If there truly is no difference in economy and mpg using 87 octane, then thank your lucky stars. You don't have to spend as much money for your Denali to be at its best.
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Old 01-28-2008, 03:50 PM   #22
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sounds great! 87 it is =)

i was worried remembering back to my mom's old '85 monte carlo SS... she ran 87 all the time and the thing developed bad knocks...

do those over the counter detergents work to prevent deposit build up over time?
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Old 01-28-2008, 03:54 PM   #23
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im confused, why is premium different in different areas, here its 92, you head south and its 93.

up north its 91 or 90, when i was in denver it was 85 for regular.

why do they vary the octane rating, ALL FROM SHELL.
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Old 01-28-2008, 09:04 PM   #24
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Quote:
Originally Posted by akim47 View Post
sounds great! 87 it is =)

i was worried remembering back to my mom's old '85 monte carlo SS... she ran 87 all the time and the thing developed bad knocks...

do those over the counter detergents work to prevent deposit build up over time?
I doubt your Mom's SS req'd anything over 87 octane. If it had a lot of miles, the knock could've been due to carbon buildup in the combustion chamber which effectively raises the compression ratio leading to pinging or knocking.
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Old 01-29-2008, 09:27 AM   #25
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Quote:
Originally Posted by akim47 View Post
sounds great! 87 it is =)

i was worried remembering back to my mom's old '85 monte carlo SS... she ran 87 all the time and the thing developed bad knocks...

do those over the counter detergents work to prevent deposit build up over time?
If those "bad knocks" were from carbon deposit build-up in the combustion chambers, it would be from the brand of gas she was using, not the octane. (If they were from old age or poor maintenance over time, then it could have been piston slap or loose bearings, and not related to fuel at all.)

Many brands only add the minimum amount of cleaning additives that they can get by with to meet federal standards. A certain amount of cleaning additives is required by law to help cars meet emissions standards. Trouble is, the federal standards aren't enough.

Certain big-name brands (Shell, BP, Conoco, etc.) put in enough to make a difference. In my experience, Chevron is the best about this. These brands' fuel will actually clean out deposits and keep them from forming, preventing that knock problem.

Over the counter detergents (the good ones, like Techron or Sea Foam) only clean out deposits that are there. They will not prevent build-up over time unless they are used regularly.

Personally, I run name-brand fuels only (those that are on the "Top Tier" list or BP). My preference is for those that run their own trucks. (See "the story" below.) Also, I run a bottle of Techron through the system once every spring.

Yes... all gasoline in an area comes from the same distribution depot. But that does not mean they're all the same. All those different tanks at the depot are different "recipes" of gasoline. And when the tanker is filled, the particular brand's additive package is mixed in at that point. And yes, the tanker will go to different brand stations with the same load. But the tanker has at least four different tanks in it. Gas is not all the same!

"The Story"--
I used to work at a convenience store in Kansas (where they have oil wells). One of my jobs was to check in the gasoline deliveries. One morning, the truck came in 3 hours early. He said he had to make a "salt run" later, so he was running early. (Oil wells also produce large quantities of salt water, which is separated from the oil at the well and stored in tanks. The salt water is later dumped back into "empty" oil wells.)
I said, "Ah... so you have to go back and get a different trailer, then."
He said, "Nope, I'll just use this one."

That's when I started buying gas from companies that run their own trucks. Texaco, Phillips, etc., don't run anything but gasoline, diesel, or ethanol in their trucks!

Last edited by D'Hag; 01-29-2008 at 09:50 AM.
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Old 01-29-2008, 09:42 AM   #26
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Originally Posted by Rollin Thunder View Post
im confused, why is premium different in different areas, here its 92, you head south and its 93.
up north its 91 or 90, when i was in denver it was 85 for regular.
why do they vary the octane rating, ALL FROM SHELL.
The octane rating varies with the climate and altitude of the area where the fuel is being sold. That's because an engine's octane requirement changes with conditions.

High air temperature increases the combustion chamber temperatures.
High altitude reduces the amount of air/fuel mixture entering the engine (lower compression).
High humidity raises the fuel/air octane rating (like water injection).

So, in Colorado (my home state), premium is 91 and regular is 85. High altitude and cool air calculates out to those octanes needed.

Head south-- low altitude and warm air needs higher octane.

They don't figure in humidity too much, because that varies from day to day, sometimes widely.

Ever notice how strong and smooth your engine runs on a damp 60-degree day?

Of course, they COULD sell 93 octane everywhere. But remember-- It's all about the money. Notice the gas isn't any cheaper just because it's in an area with lower octane.

Last edited by D'Hag; 01-29-2008 at 09:52 AM.
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Old 01-29-2008, 01:09 PM   #27
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I should note that i do not have a tahoe yet...

I originally opened this thread and expected to see how to gain the most out of the cyl switching feature of the tahoe/yukon. Most of the things listed are common sense but people just need to re-read them at times to be reminded of the "oh, i should remove the 300lb's worth of crap in my trunk?" scenerios.

Anyway, so what are the "methods" to really take advantage of the cyl switching? On the few i test drove, i really had to be light on the gas to even see it engage. I heard their may be an update to make it less sensitive? I imagine you just need to stay light on the gas on the highway to really see any benefit from it.

Anyone have experience to backup data on this?
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Old 01-29-2008, 01:14 PM   #28
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Quote:
Originally Posted by D'Hag View Post
The octane rating varies with the climate and altitude of the area where the fuel is being sold. That's because an engine's octane requirement changes with conditions.

High air temperature increases the combustion chamber temperatures.
High altitude reduces the amount of air/fuel mixture entering the engine (lower compression).
High humidity raises the fuel/air octane rating (like water injection).

So, in Colorado (my home state), premium is 91 and regular is 85. High altitude and cool air calculates out to those octanes needed.

Head south-- low altitude and warm air needs higher octane.

They don't figure in humidity too much, because that varies from day to day, sometimes widely.

Ever notice how strong and smooth your engine runs on a damp 60-degree day?

Of course, they COULD sell 93 octane everywhere. But remember-- It's all about the money. Notice the gas isn't any cheaper just because it's in an area with lower octane.

makes sense, ist 59 and 88% humidity today and man did she run good this morning.
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Old 01-29-2008, 01:25 PM   #29
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Quote:
Originally Posted by D'Hag View Post
The octane rating varies with the climate and altitude of the area where the fuel is being sold. That's because an engine's octane requirement changes with conditions.

High air temperature increases the combustion chamber temperatures.
High altitude reduces the amount of air/fuel mixture entering the engine (lower compression).
High humidity raises the fuel/air octane rating (like water injection).

So, in Colorado (my home state), premium is 91 and regular is 85. High altitude and cool air calculates out to those octanes needed.

Head south-- low altitude and warm air needs higher octane.

They don't figure in humidity too much, because that varies from day to day, sometimes widely.

Ever notice how strong and smooth your engine runs on a damp 60-degree day?

Of course, they COULD sell 93 octane everywhere. But remember-- It's all about the money. Notice the gas isn't any cheaper just because it's in an area with lower octane.
Thank you for posting that, that has been a mystery to me for years :smilielol: . I always wondered why the octane in Crested Butte, CO, where my parent's second home is, was lower than it was at home. I could never figure out why regular would be 85 there and 87 here. That makes a lot of sense.
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Old 02-01-2008, 10:55 AM   #30
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Quote:
Originally Posted by D'Hag View Post
Our Tahoes have knock sensors, and the ECU advances the ignition timing according to fuel octane. Basically, the ECU advances the timing periodically to test the knock sensor and to check the octane. Then it settles in on a timing advance that matches the fuel and operating conditions. THIS is what increases MPG with higher octane fuel.
I have a theory that if the ECU actually will advance the timing on its own to check octane, the best way to take advantage of Premium without a chip or tune is to fill up with Premium, reset the ECU and continue to run Premium while the vehicle "learns" to run advanced timing to take advantage of the Premium. Of course if this actually works, the first tank or two of Regular would run like crap unless you reset the ECU or ran it long enough for the ECU to retard the timing.
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