5.3 Ecotec3 L83 - E85 vs 87 octane

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I have been doing an experiment with my 2014 Silverado with the 5.3 Ecotec3 L83 that is flex fuel approved. Full disclosure - a large portion of my yearly income comes from farming. I have not generally been a fan of E85 even though I benefit financially from it.

Factory 87 octane ratings are 355 HP & 383 LB-FT.
Factory E85 ratings are 376 HP (or 380 depending on source) & 416 LB-FT.
87 octane is rated at 16 city & 22 hwy
E85 is rated slightly less but I can't find the official GM numbers

Simply switching to E85 is worth 21 HP and 33 LB-FT according to GM.

There has been much internet debate on this motor and fuel choice since it was released by GM. People like to argue that 11.0:1 compression needs a higher octane fuel but then others argue that direct injection mitigates the need for a higher octane.

My experiment is far from scientific and far from ideal conditions - I admit it. These are just my observations and I hope I can help at least one person that was having the same thoughts and questions I was having.

On 87 octane my truck DIC shows an average of 17.7 MPG over a tank of fuel. This includes my daily commute and a mix of city & highway. The DIC will show a range of 430-440 miles for a full tank of 87 octane. The truck drives fine, has plenty of power but does seem to lack grunt from a dead stop with a steep throttle input. Initially I attributed this to torque management and PCM/ECM nannies. On a rolling start, steep throttle input results in loads of acceleration.

On E85 my truck DIC shows an average of 19.5 MPG over a tank of fuel. This includes my daily commute and a mix of city & highway. The DIC will show a range of 450+ miles for a full tank of E85. The truck drives great, has gobs of power and does not have the off idle lack of power that 87 did. Clearly GM has added timing in the E85 tables - no surprise here. Also, the truck goes into V4 earlier, more frequently and stays there longer on E85. This could be where I am seeing the increase in MPG from. On a rolling start, steep throttle input results in insane amounts of acceleration for a 5,300 pound crew cab truck.

I just filled up with E85 again for $1.99 a gallon when 87 octane was $2.39. Not much of a savings if you experience the typical 25-33% MPG reduction. However, in my case I am getting more power & more MPG for less $$$ per mile. Clearly this engine favors a high(er) octane. Will it run fine on 87 octane? Absolutely. Will it run better on E85. In my experience so far, the answer is a big yes.

Now, this brings us to the next part. Can the truck recognize 89 or 93 octane and give similar power & MPG results that E85 is providing? Multiple people that have run gasoline octane higher than 87 report better idle quality, more power and more MPG. Is it a quirk? Does the PCM/ECM add timing until it hits KR and then backs off? Is it doing this constantly? Does it do it at all? Does it only switch to the 'high' table for E85? Something noteworthy is that GM even rates E85 MPG lower than 87 octane on the window sticker when the truck was new.

Multiple websites/forums report that GM has two timing tables, basically a high & low table, with the high being for E85 and low for 87. Can running 89 or 93 trigger the high timing table? Multiple people have reported similar gains to mine by running 89/91/93 octane versus 87. It seems that 87 is adequate and will work just fine but a higher octane fuel, whether it is 89/91/93 or E85 is producing better results. Luckily for me, E85 is readily available and currently $0.70 - $0.80 cheaper than 91/93.

The following questions/assumptions are currently unanswered.

Was my truck experiencing low(er) MPG due to additional timing being pulled from the 'low' timing table?
Did GM rate the truck on 87 knowing the average consumer wouldn't pay up for mid-grade or premium that the truck actually needs?
Does the PCM/ECM add timing until it hits KR and then backs off?
-Is it doing this constantly?
-Does it do it at all?
Does it only switch to the 'high' timing table for E85 and not 89/91/93 octane gasoline?

Stay tuned, I will continue testing.
 
Originally Posted By: A_Harman
Have you ever tried running a mixture of 50% 87 and 50% E85?


Not yet and here is why...

My gas station has an ethanol pump with E10, E15, E30, E50 & E85. The E85 tag shows that the fuel could be 51-85% ethanol but the E50 tag shows it is 50% ethanol, it doesn't give an operational range for E50. The E85 was $1.99 this morning and E50 was $2.09.

This same station is always the lowest priced E85 station in the area. Generally they are near $1.69-$1.79 when gas is $2.39 as it is today. I suspect they are still receiving shipments of winter blend E85 that has more gasoline and less ethanol, resulting in higher prices on E85. So, I just select the E85 for now as it is the cheapest and most likely is not 85% ethanol right now and actually closer to E50 than E85.

I plan on testing the E50 blend this summer, in the hopes that ethanol blend prices will drop with warmer weather and higher ethanol concentrations.
 
Yes, I've seen a few of those gas stations in IL where they have a selection of different ethanol blends.
In my town in Michigan, one station has E85 priced at $3.24/gal! This is even more expensive than 93, which is $2.95. 87 is $2.35.
 
I would run E85 in anything that can take it. Less dependence on oil and burning it is supposedly better for the car and the environment(?) also. Can't see any reason to dislike it.
 
I like E85 or any blend. But for me it is all about the cost per mile to operate. I have good working numbers on what various blends gives me for mpg, so I can just see what the prevailing prices are and the spreads between fuels and pick accordingly the fuel that gives me the lowest cost per mile to use.
 
Our results here are similar with the service van I drive. E85 results in a noticeable increase in power. This is a 9000 pound 3500 long wheelbase Chevy Express van with the 6.0 flex fuel engine.

A really noticeable difference, plus a huge driveability bonus in near perfect shifting due to less timing being pulled at the shift. Love it.

But since 87 octane E10 gives us the best dollars per hour costs that's what we usually use. Our local stations that carry E15 and up really don't give you a commensurate discount for the alcohol content...
 
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