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Old 11-08-2007, 11:02 PM   #11
Rafael
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Originally Posted by LAP TOP GAMER View Post
Yeah but is the efficiency gain enough to offset the cost of a premium fuel?:skep:
You win the prize. You'd have to get about 10 to 15% better mileage to warrant the extra cost of high octane fuel.

I've found the best way to get good mileage is to only drive downhill.
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Old 11-08-2007, 11:26 PM   #12
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Default Numbers for ya

Here's my math, which was based on numbers before my Volant intake or Edge Evolution:

1) 87 is $2.95, avg. fuel economy is 9.9 MPG
2) 93 is $3.25, avg. fuel economy is 11.1 MPG
3) 2.95 / 9.9 = 29.73 cents / mile
4) 3.25 / 11.1 = 29.28 cents / mile
5) Cost difference is negligible (in my favor), performance is better, and it's better for my engine.

It'd be interesting to put the PCM back to stock, and try this again to see if there really is still any difference, but since I'm not a paid researcher, I'll just keep things running they way they are and shoot for 13.5-14 MPG in mixed driving.

Bottom line for me: better performance for a hair less overall cost. What's not to like? Do the math for yourself - you might be surprised.
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Old 11-09-2007, 12:27 AM   #13
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Quote:
Bottom line for me: better performance for a hair less overall cost. What's not to like?
That's kind of my reasoning too. I live in Denver right now going to school. Driving up I-70 in to the ski country is really were I found the difference in performance.
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Old 11-09-2007, 02:02 AM   #14
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Quote:
Originally Posted by Getwired View Post
Um, maybe I remember chemistry incorrectly, but doesn't the higher octane rated gasoline autoignite/detonate slower, thereby creating a more steadily/slower burning fuel, increasing fuel economy? The lower the octane rating, the faster the gas burns. In a diesel engine, faster burning fuels is desirable (cetane rating), but in electric-spark ignition engines, slower is better for performance (compression is better) and fuel economy.

Reference: http://en.wikipedia.org/wiki/Octane_rating and over here: http://auto.howstuffworks.com/question90.htm

Check it out! Interesting stuff.

Sorry. I read over what I wrote and I did word it wrong. Higher octane fuel helps provide more power to higher compression engines than it would comparatively in lower compression engines because it combusts slower. Thx for pointing that out to me.
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Old 12-07-2007, 09:52 PM   #15
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Just be light on the accelerator
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Old 12-11-2007, 12:55 AM   #16
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Just be light on the accelerator
Ummm...REALLY hard to do trying to push a 2.5 ton truck around the city...:skep:
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Old 12-16-2007, 10:43 PM   #17
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now im beginning to realize why i got my tahoe for 10k under sticker.. just a little info to everyone.. if you put on an avg of 12k-15k a year for every dime gas increases its 10 bucks more a month out of your wallet....
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Old 01-23-2008, 05:01 PM   #18
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I'm always kinda surprised at the general misunderstanding about fuel octane.

Quote:
I know higher octane helps provide more power because it combusts easier.
First, higher octane fuel does NOT burn more easily. To be as simple as possible, it has a higher flash point. That is, it's more resistant to spontaneous combustion from heat and pressure. Then, once it's ignited, it burns more slowly, building combustion pressure more slowly. That's why you can increase ignition timing advance with higher octane fuel. That's also why, if you use high-octane fuel in a low-compression engine that doesn't need it you get more unburned fuel, resulting in black exhaust system deposits and possibly burned-up converters.

Quote:
Higher octane does increase fuel economy, its a not a myth.
Second (read all of this before you react), it IS a myth that higher octane fuel increases MPG, IF the vehicle is not equipped with an oxygen sensor and programming to use it.

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.

Optimum fuel economy and power, as they relate to ignition timing, is when the fuel is ignited at the best time to exert maximum down force on the piston during the power stroke, with minimum resistance to the piston when it's rising on the compression stroke.

If the ignition fires too soon, then combustion pressure can build too fast, and resist the rising piston. So too much ignition advance can actually reduce power and fuel economy. Go even further, and you get knock. Old school tuners used to believe that you advanced timing right up to the edge of getting knock. Computers have shown that this is not quite right. The best is to go right up to the edge of knock, then back off 2 or 3 degrees of advance.

If the ignition fires too late, then maximum force isn't developed in time to push the piston down.

If your Tahoe is E85-capable, you can advance the ignition timing even more, and get MPG with E85 approaching what you get with pure gas. BUT (and this is important), since the octane of E85 is up around 105, AND ethanol burns cooler than gasoline, it's VERY easy to over-advance the ignition timing with E85 and never trigger the knock sensor or hear a knock. Then you're into that situation where the fuel is being fired too soon, killing power and economy.

It's a balancing act. If your vehicle doesn't NEED higher octane and it doesn't have a knock sensor, then premium fuel is a waste of money, unless you can advance the ignition timing somehow. If it has a knock sensor and the right programming, and the compression to use it, then premium fuel will increase MPG, and might cost less per mile to operate. But if you over-tune the ignition timing, all the octane in the world won't get you more power and economy.

Hope this helps understanding.
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Old 01-28-2008, 03:49 PM   #19
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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...
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Old 01-28-2008, 04:11 PM   #20
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It means that the engine is designed for its maximum performance using 91 octane. If you use 87 or 89, then the computer compensates by retarding timing to prevent knock and/or pre-ignition which decreases performance.
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