91 w/o Ethanol, or 93 with 10%?
 

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

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

From our manual:

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?

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.

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.

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

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. :D

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.

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

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

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.

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.

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.

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. :burnrubber:

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. :)

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.

(Apologies for topic change) Sir, i am so sorry to report that a 25F difference in temperature results in only ~3% difference in ambient density, which would most likely be compensated for by the fact that hotter air flows easier (lower viscosity). Since ultimately the throttle plate controls air density into the engine, there are no benefits to colder air at part throttle.
Granted, i write without any knowledge of what your temperatures are compared to mine, but as a combustion engineer, the hysteria surrounding cold air and cold air intakes bugs me a little.

You're using mass. Ethanol has a higher specific mass than Gasoline due to the presence of Oxygen. Ethanol has closer to 66%(65.75%) of the energy per gallon(volume) of Gasoline. Meaning E10 will have 96.7% of the energy of a gallon of Gasoline. The presence of oxygen also makes it perform differently since it will absorb heat from the air and cool the intake and combustion chamber as it evaporates. Far more than Gasoline can. Which is why E85 even though it's 95 AKI octane will perform vastly different than 95 AKI octane Gasoline.

http://www.afdc.energy.gov/fuels/fue...ison_chart.pdf

Higher octane should get better MPG if lower octane retards the timing. The timing advance at partial load actually plays an important role in fuel economy. That's why some manufacturers use premium in their fuel efficiency kings. Peak power and peak efficiency is less important unless you're racing. Higher octane can improve both.

Keep in mind I'm in Ontario and my options are 91 pure gas or 94 E10. For the amount that I'm driving up in that rev range where it pulls timing (not that much) the better efficiency due to 94 doesn't make up for the better mileage of pure gas the rest of the time. I consistently see noticeably better mileage with 91 pure as opposed to 94 E10. But I don't like feeling the top drop off the way it does with 91 so that's why I'm switching back. You gotta pay to play and I like to play.

You should be able to feel a difference in the lower RPM response though.
How much more mileage and at what spreads?

@ Allch Chcar
Totally agree with you on E85.
E10 though, does not have the cooling benefit of E85, does not have high combustion speed of E85, does not have slightly higher specific gravity benefit of E85.
I used mass because fuel is sold by the gallon, but is used in our cars by mass, and all ecu calculations are mass based and not volume based.

We conducted few flex fuel studies in our research, and long story short, we matched results with the following paper,
http://www.sciencedirect.com/science...001005088#FIG3

Torque output does increase, but so does Brake specific fuel consumption, which affects MPG numbers the most. The torque numbers increased due to increase in octane number, but in our discussion we are dealing with commercially available fuels, which have equivalent octane numbers.

In conclusion, for the same octane number, E10 should have higher brake specific fuel consumption, i.e. Lower MPG. This was demonstrated by using 91 E10 and 91 Pure gas in a non flex fuel Chevy Malibu, at 70 mph on I85, 29 mpg on E10, 31.4 on E0.

The op should maybe try this same experiment, since i can't drive this car without a lead foot... :lol:

{edit}
Jut saw the posts posted while i was typing....glad to know fun took precedence over MPG.
Too bad the op does not have 93 pure gas, best of both worlds...

I don't know that I feel much of a difference in the lower RPM but on my next tank when I switch I will try to see if I notice a difference. It may take a little while for it to learn back though right?
As for mileage difference I probably see about a 2 - 2.5 mpg difference between the two. However it's hard to say for sure as I don't drive the exact same way each tank and some have more or less highway driving but from what I can see that seems to be about the difference.

What you're saying is correct. But at the same time, even though BSFC is 50% higher for Ethanol, MPG is "only" 30% less due to it having a higher specific mass than Gasoline. I've seem people confuse them before while looking at the different stoichiometric air to fuel ratios between E85 and Gasoline. I'm not an expert on the subject so I defer that I'm just trying to avoid confusion to anyone who might not be aware of the distinction.

What you saw for MPG seems to be abnormal though. I have heard of people losing far more MPG with E10 than is expected. But the studies I've seen have all shown a direct relation to BTU content. They just never followed it exactly. The improvement in octane peaks around E30-E40 but energy efficiency continued to improve the more Ethanol content there was. In fact power improves with more Ethanol content as well. Things like BSFC and fuel efficiency obviously decrease with Ethanol content. Pesky things that they are. ;)

It's a fun subject and I enjoy learning more about it.

It takes awhile, seems like at least 10-20 miles before the ECU makes any changes. According to what I've heard/read.

That's a big difference in MPG though. Some of it could be different situation and difference in driving style.

I wasn't that surprised by it because I saw about the same difference with my last car (2010 WRX) over a lot more consistent driving. (>90% highway)