Best octane fuel for fuel eceonomy? (5.3L only)

[swathdiver]

I wasn't sure if I should've replied to a post in your "Useless Information" thread, but I couldn't find it so I'll ask here:

I've been filling up (half tanks, actually, but from nearly empty) at a RaceTrac on my side of the river since it's the closest E85 to me. I was playing with my scanner Sunday evening and it showed my alcohol content to be 22%. I know E85 is often less than 85% alcohol, but this is excessive. I'm considering manually testing the fuel to compare. Is there anything I could check or do? I believe mine calculates the alcohol content based off of other sensor data rather than from a composition sensor. I got this reading while sitting in the driveway with it idling for about 30 minutes, so it was in closed loop. I fueled up Friday evening and the station is about 22 miles away, so it had plenty of time to adjust after the refueling.

You can always ask questions over there too, don't mind at all Chris. Well, that does seem a low but you were on regular fuel and nearly empty and then pumped in about 10 gallons or so of E85? Either the computer didn't finish making its adjustments or the Racetrac dumped 87 into that tank and diluted it.

Mine will often not settle down for a few days even after driving 18 miles home from the station up north. It keeps checking until the trims are where they should be, zero or near so while cruising or idling.

 

[iamdub]

I agree.

Ordered my daughter's birthday gift this morning and saw that test vial sitting in the cart, so I added those $15 worth of screen taps to the order.

One of these little test tubes is great but you will want to test the fuel as it leaves the tank. Would you tap some off the fuel rail at the test port?

I planned to stick the fuel feed into a Gatorade bottle and turn the key on for a second or so to get a few ounces to sample. I'll clean the vial, add water and bring it with me to the next refueling.

 

[iamdub]

You can always ask questions over there too, don't mind at all Chris.

I didn't wanna hijack this thread but I couldn't find your thread, not even when I clicked on "Threads started by swathdiver" in your profile.

Well, that does seem a low but you were on regular fuel and nearly empty and then pumped in about 10 gallons or so of E85? Either the computer didn't finish making its adjustments or the Racetrac dumped 87 into that tank and diluted it.

In chronological order:

Full tank of Chevron E10 93 for tuning that I ran down to about 1/4 tank

Filled up with RaceTrac E85 (so, 3/4 tank E85, 1/4 tank E10)

Ran that down to "Fuel Range Low" and added 13 gallons of E85 from another RaceTrac

Ran that down to "Fuel Range Low" and added another 13 gallons of E85 from the same Racetrac as the previous time.

So, virtually none of the E10 should remain and I should have a much stronger ratio than 22%.

Mine will often not settle down for a few days even after driving 18 miles home from the station up north. It keeps checking until the trims are where they should be, zero or near so while cruising or idling.

Ah. So there's a chance all is well and I just need to drive it more? Is there anything I should or shouldn't do while it's adjusting? I'll monitor fuel trims on my next drive. Maybe it was because I was idling for so long? I didn't drive very hard or fast after refueling this last time since I was trying to minimize disturbing the puddle of oil in my back floorboard any more than necessary.

 

[swathdiver]

I didn't wanna hijack this thread but I couldn't find your thread, not even when I clicked on "Threads started by swathdiver" in your profile.




In chronological order:

Full tank of Chevron E10 93 for tuning that I ran down to about 1/4 tank

Filled up with RaceTrac E85 (so, 3/4 tank E85, 1/4 tank E10)

Ran that down to "Fuel Range Low" and added 13 gallons of E85 from another RaceTrac

Ran that down to "Fuel Range Low" and added another 13 gallons of E85 from the same Racetrac as the previous time.

So, virtually none of the E10 should remain and I should have a much stronger ratio than 22%.




Ah. So there's a chance all is well and I just need to drive it more? Is there anything I should or shouldn't do while it's adjusting? I'll monitor fuel trims on my next drive. Maybe it was because I was idling for so long? I didn't drive very hard or fast after refueling this last time since I was trying to minimize disturbing the puddle of oil in my back floorboard any more than necessary.

Maybe, either that or your alcohol content needs to be reset. As for what to do while it's figuring things out, no, just drive. I've found that varied driving, like stop and go city driving, gets it figured out faster than a highway run where the motor is at a more constant state.

 

[cfmistry]

Well the results are in.

13.3mpg with 87-octane
14.0mpg with 93-octane

It gave me about an extra 15 miles per tank, which isn't enough to convince me its worth it (that small amount could be statistical error). I did think the trunk ran smoother on 93-octane, but that could've been in my head. I'll try 89-octane now but I assume it'll be the same result. I'm starting to wonder if a BBP tune is in my future... I think the increased MPG would pay for itself reallly fast.

 

[donjetman]

When gasoline prices dropped a few months ago, I switched from mid-grade unleaded too super unleaded. So far(6k miles) I've seen about a 1 mpg increase, from 16.5 to 17.5 mpg. Vehicle gets mostly hwy miles.

 

[bigdog9191999]

does the cost difference even out to the mpg difference? it may get better mpg, but how is the cost difference? which is actually cheaper to run?


and many guys say they are running higher fuel on tuned trucks and getting better mpg, which makes sense as regular fuel on stock tune should be the better mpg, but when changing parameters like spark advance this can change how it acts vs octane and mpg.

 

[swathdiver]

does the cost difference even out to the mpg difference? it may get better mpg, but how is the cost difference? which is actually cheaper to run?


and many guys say they are running higher fuel on tuned trucks and getting better mpg, which makes sense as regular fuel on stock tune should be the better mpg, but when changing parameters like spark advance this can change how it acts vs octane and mpg.

Gotta do the math, every locale is different because of the prices.

 

[GreyStone]

Gotta do the math, every locale is different because of the prices.

I wonder which octane is better for emissions/smog testing?

 

[gooffeyguy]

Are there areas that still actually put a sniffer up the tailpipe?

Around here, any OBD2 vehicle ('96+) just gets plugged in to the OBD port and the ECU readiness monitors are read. Vehicles older than OBD2 are emissions exempt, just a visual for required equipment (cats, smog pump, etc.)