87 vs 91 fuel mpg differance?

[Gman]

I can't remember the last time I had a vehicle that didn't require 91+ so it's nice to have options.

Isn't that the truth. I put 26 gallons in my truck, the full capacity of my previous vehicle that required premium (Mercedes Benz GL 450), for less money than a typical 21 gallon refill of that vehicle. The station that I use would cut me off at $75 with pay at the pump, and that didn't happen with the truck.

 

[Electrical]

Making power takes fuel. Getting a lot of power from a twin-turbo V6 is possible. More air in the charge also allows more fuel. You also won't get economy out of it under those conditions. The current Ecoboost uses both direct and port fuel injection. I'm not sure what you mean by "forced injection". I have heard of "forced induction", but not "forced injection".

If you're putting more fuel in, then you either have to somehow improve the amount of air in the charge to maintain the air:fuel, or you'll run rich.

Correct; forced induction.... not injection.

"Running rich" to me is 10-ish AFR. Very atypical to ever see this but it has its' place... talk to the Hellcat guys who play with their motors.

"Richer mixture" to me is 12 to 14 AFR... typical of any OEM map at wide-open-throttle... for the reason you stated (more air, more fuel, more power).

A complete burn can only happen at stoichiometric... 14.7 AFR.... therefore OEM's try to maintain stoich.

If a 14 AFR can accommodate wide-open-throttle without knock, OEM's would prefer that because, well, less fuel is being used and the burn is cleaner.

If knock or combustion temps are high, one of the levers that can be pulled is to richen the mixture... not to the point of 10-ish AFR, but enough to make the engine happy.


Bottom line is that engine mapping may very well be the most complex part of any vehicle these days. Seriously.

All I am saying is that octane affects mapping, which in turn affects economy, and that's why I believe it's within the realm of possibility to link octane to economy.

I'm also open to the possibility of God and space aliens

 

[Electrical]

Agreed and am continuing to test. In various threads, we've also discussed the possibility of build tolerances; some engines come out of the factory a bit 'hotter' than others and will respond to higher octane positively, while it's a complete waste for others. After my first few tankfuls were 93, I've now run a few tanks on 89 (same vendor and same stated ethanol percentage). So far, every tank of 89 has been 1-1.5 mpg lower than 93. It's hard to do a truly neutral test, given changing weather conditions, traffic patterns, and other variables. But I thought I'd benefit from reduced A/C demand over the past month, and I'm not seeing it.

That makes me think about Jesse James, Paul Teuttel, and other custom builders. These guys can build phenomenal one-off vehicles... but I wouldn't call them good builders. To be a good builder, you have to know how to handle tolerance stackups in volume production.

As you know, and I agree, it's a vexing design topic. I'm pretty impressed OEM's can be as consistent as they are.

Do you have different grades of ethanol-free in your area?

If you're testing with E10, I'm wondering if the varying concentration could affect results. Sometimes you might get 5% ethanol, other times you might get the full 10%, or anywhere in between.

 

[hessclark]

Since I picked up my truck last month, I have been getting between 13 and 15 MPG running 87 octane. I was at Costco the other day and decided to try 91 octane. Not only does my truck run a bit better, I am now averaging between 16 and 17 MPG. One interesting thing that I noted is that there is no longer a slight lag when switching back to eight cylinders from four-cylinder mode. When I was running 87 octane, it always seems like there was a slight bogging noise when I was in four-cylinder mode. This is completely gone away with my tank of 91 octane. One other interesting note is that since I filled up with 91, my ECO light no longer comes on on my dash. If I remember correctly from other posts, the eco-light only comes on when the computer thinks you are driving more efficiently. Since my average MPG has gone up, this must have thrown off the computer somehow and it no longer turns on the ECO light.

 

[Jako]

Since I picked up my truck last month, I have been getting between 13 and 15 MPG running 87 octane. I was at Costco the other day and decided to try 91 octane. Not only does my truck run a bit better, I am now averaging between 16 and 17 MPG. One interesting thing that I noted is that there is no longer a slight lag when switching back to eight cylinders from four-cylinder mode. When I was running 87 octane, it always seems like there was a slight bogging noise when I was in four-cylinder mode. This is completely gone away with my tank of 91 octane. One other interesting note is that since I filled up with 91, my ECO light no longer comes on on my dash. If I remember correctly from other posts, the eco-light only comes on when the computer thinks you are driving more efficiently. Since my average MPG has gone up, this must have thrown off the computer somehow and it no longer turns on the ECO light.

My Costco has only 87 and 93 octane. How many does your Costco have?

 

[Zinger]

My Costco has only 87 and 93 octane. How many does your Costco have?

Same as mine. They recently switched to 93 from 91. Southeast PA.

 

[50Front]

Ran 89 for the first 5000 miles and averaged about 14.5. The last three tanks I switched to 87 and there was no change. The truck seems to run the same on either.

 

[Gondul]

My Costco has only 87 and 93 octane. How many does your Costco have?

87 and 91 down this way....

 

[Gman]

My Costco has only 87 and 93 octane. How many does your Costco have?

85 and 91

 

[Ramit392]

Any others running on 87 and 89 both non ethanol? I cannot tell any difference with ECO light coming or off or any MPG difference between the two so far. Do not run any Ethanol in this area at all.

 

[moosem]

Before I knew 89 was recommended, I used 87 exclusively and averaged 16mpg. Since then I’ve been using either 89 or 93, depending where I fill up (BJ’s only has regular or super, so I use Super- otherwise, I use 89 at Shell) and am averaging right around 17. I drive pretty fast though.

 

[Daniel]

Before I knew 89 was recommended, I used 87 exclusively and averaged 16mpg. Since then I’ve been using either 89 or 93, depending where I fill up (BJ’s only has regular or super, so I use Super- otherwise, I use 89 at Shell) and am averaging right around 17. I drive pretty fast though.

Spending monet for not more milleage