91 w/o Ethanol, or 93 with 10%?

[Superhatch]

I've been trying to find a solid answer to this question, and found this on a Corvette forum:

http://forums.corvetteforum.com/c6-t...anol-free.html

Here is another way to look at it. The 91 octane and the 93 octane E10 are the exact same 91 octane fuel. By adding 10% ethanol, which has an octane rating of 113, to the 91 octane, it brought the octane rating up to 93. ((91 x 9) + 113)) / 10 = 93.2

So basically by buying the 93 E10 you are paying around 10 - 12 cents per gallon more for 90% of a lower grade fuel that has been cut with 10% of a product that will cut your fuel mileage by 2 -3 miles per gallon.

The vettes in question in that thread were all tuned for 91 from the factory as mentioned in the thread.

From our manual:

Fuel types
Unleaded gasoline (93 AKI [Research Octane Number 98] or higher)
If unleaded gasoline with an octane rating of 93 AKI (98 RON) is not available,
unleaded gasoline with an octane rating of 91 AKI (95 RON) may be
used with no detriment to engine durability or driveability.

So my question is really in regards to what octane 93/10 actually is. I've also read some threads where they talk about using a flex fuel system with an octane reader and seeing large swings in the actual octane of E85. I've read "E85 fuel is said to contain 85 percent ethanol and 15 percent gasoline, but the fuel mixture can vary anywhere from a low of 70 percent all the way up to 85 percent ethanol, depending on the place of refinement and the season of purchase." If you apply this to the 10% in 93/10 could we actually be running an RON of 91 even when we think it is 93?

[A5D5TRYR]

I don't know that 10% ethanol gas would vary by as much as that but I would still be cautious if you're tuning it to be right at the safe limit for what 93 or 94 octane can do. Also, whenever I've looked at the pumps it sometimes says "Up to 10% Ethanol" which says to me that it can vary. I don't know that it would ever get as low as 91 though.

[Superhatch]

Originally Posted by A5D5TRYR

Also, whenever I've looked at the pumps it sometimes says "Up to 10% Ethanol"

Ah, that's a good point. Our company does some work with Ethanol plants, I should ask about this next time one of our guys goes there.

[Someguyk]

well we only have 90 ethanol free gas here..and that is the highest besides the 100 octane at the airport.

[reeves]

Originally Posted by Superhatch

I've read "E85 fuel is said to contain 85 percent ethanol and 15 percent gasoline, but the fuel mixture can vary anywhere from a low of 70 percent all the way up to 85 percent ethanol, depending on the place of refinement and the season of purchase." If you apply this to the 10% in 93/10 could we actually be running an RON of 91 even when we think it is 93?

I don't remember exactly but I think the label on most pumps will say "Minimum octane rating 93" if it is indeed 93 octane.. regardless of what the actual mixture is.

[Superhatch]

Originally Posted by reeves

I don't remember exactly but I think the label on most pumps will say "Minimum octane rating 93" if it is indeed 93 octane.. regardless of what the actual mixture is.

Hmm...so if the octane rating of the ethanol is low, then they would likely use more gas to make up the difference? I guess I'm curious if this happens or not. One would assume they test the batches of gas for the octane rating prior to shipping. If so, nothing to worry about.

[Allch Chcar]

If it was 91 w/o Ethanol it would be true due to spec blending. The octane posted is the minimum. But if it's 91 octane w/ 10% ethanol it's actually 90% 89 octane mixed with 10% Ethanol. While 93 octane w/ Ethanol is 90% 91 octane Gasoline with 10% Ethanol which makes it 93 octane.

In general assume it has Ethanol unless otherwise noted. Also "pure" gasoline will be significantly more expensive than Gasohol. I haven't read the actual law on how much Ethanol is considered to be Gasohol but I believe it's about 5% for the Federal requirement. It could vary between states though. Some states require signage if Ethanol is present and some allow Ethanol free gasoline to be sold statewide.

Originally Posted by Someguyk

well we only have 90 ethanol free gas here..and that is the highest besides the 100 octane at the airport.

Nope, and you probably won't since it's Alaska. But they use Ethanol in Canada.

[jpit]

I think the Govt just approved 15% ethanol for cars 2005 and newer.

[StormTrooper]

in idaho and montana it's about 10-15 cents more for ethonal free gas and i've never seen it over 91....we used to have 93 with ethonal but now it's all 91. kinda blows since at elevation i'm down on power anyway.

[arghx7]

If it says 93 octane, it's 93. Normal pump fuel is sold by the octane rating, not by the ethanol %. It might be 10% ethanol or it might vary a little bit, but basically it's gasoline cut with ethanol in some blend to meet the spec.

And yes, when buying nominal E85, the blend varies throughout the year. It's harder to start the car in winter with high ethanol concentrations, especially in cold climates.

Also, the vehicle itself is certified on 97RON which is about 92 octane AKI. That's the spec for CARB Phase II fuel and EPA Indolene fuel. Now that doesn't mean the engine wasn't designed with various octane levels in mind.

[A5D5TRYR]

I'm going to switch back to 94 with 10% ethanol. With it getting warmer out it feels like I'm starting to lose a bit of power up in the rev range and given what I've read on these forums I wouldn't be surprised at all if it's starting to pull a bit of timing. I'd rather get mildly worse mileage but not be pulling any timing.

[200hp/tonne]

Ethanol has energy content of ~27 kj/g while gasoline is closer to ~45 kj/g.
So any E10 will have ~43 kj/g energy content. Day to day driving, where you stay below 70% throttle all the time, you will never be in the knock limited region of the spark timing map, so the 91 w/o ethanol will give you ~5 % better gas mileage. If you go WOT a lot of the time on 91 oct, then you will run into the knock region of the map (low speed high throttle, high speed high throttle) and the ECU will pull timing from MBT (max brake torque) timing and you will lose some torque. Here, 93 oct will be most beneficial. I try to keep no alcohol 93 oct in my tank all the time provided the price difference is not more than 10% compared to 93 E10.

[200hp/tonne]

Originally Posted by StormTrooper

in idaho and montana it's about 10-15 cents more for ethonal free gas and i've never seen it over 91....we used to have 93 with ethonal but now it's all 91. kinda blows since at elevation i'm down on power anyway.

At elevation you make less power because ambient air pressure is low, which means less mass of air (and consequently fuel) in cylinder at WOT. Less pressure also means less pressure at end of compression stroke and much lesser peak pressure inside cylinder during combustion. All these lead to a much lesser tendency to knock, so lower octane fuels are sold at higher elevations. I hope I was helpful.

[StormTrooper]

Yeah i've heard that before and had a pretty good idea of why......doesn't mean i couldn't get more power from a tune with better gas though.

My only consolation is that it's usually pretty cold which helps with air density and power.
kinda sucks to know that every turbo car or truck has an advantage regardless of rated power.