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Default Octane ratings for different mixes - 11-20-2013, 04:29 AM

In order to give people a better understand of say E50 versus a 50% mix of E85 or E70...I did a chart to show the differences. However this chart makes 2 assumptions so don't take it as a concrete octane rating but as more of a comparison of different mixes and what they'll yield based on another mix.

The two major assumptions...

1. Pure ethanol has an octane rating of 113. I've read different things but used this number in the calculations.

2. The octane rating of the gasoline mixed with ethanol is 87 and this is the figure I used as well.

Here's the chart...if you see a mistake....highly possible as I started going cross eyed doing this...please point it out.
Name:  Octane chart.jpg
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Last edited by Steve @ BMS/Fuel-It!; 11-20-2013 at 05:40 AM..
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Charrigan Charrigan is offline
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Default 11-20-2013, 05:29 AM

Thanks for taking the time to do this.
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Default 11-20-2013, 03:20 PM

Quote:
Originally Posted by Charrigan
Thanks for taking the time to do this.
No problem...



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Default 11-20-2013, 04:28 PM

I wish you could get E100 so you could decide what kind of gas to mix with it


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Default 11-20-2013, 04:41 PM

Very cool!

+1 on bringing forward some excellent documentation!

Just curious...but what's the highest mix ratio guys are running with E85 and 91 octane? I've been using a 40% E85 mix with 60% 91 octane and that seems to run well with the JB4 Stage 2 + flex fuel but is anyone running higher mixes?

And if so, what's been the results?
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Default 11-20-2013, 05:21 PM

Quote:
Originally Posted by coolbho3k
I wish you could get E100 so you could decide what kind of gas to mix with it
Probably can at your local race shop...probably won't be cheap though.

Quote:
Originally Posted by boostedMPLS
Very cool!

+1 on bringing forward some excellent documentation!

Just curious...but what's the highest mix ratio guys are running with E85 and 91 octane? I've been using a 40% E85 mix with 60% 91 octane and that seems to run well with the JB4 Stage 2 + flex fuel but is anyone running higher mixes?

And if so, what's been the results?
No problem.

I run 100% E85 but I am not the norm and I run an N54.

You would need to supply logs, but from what I hear, there is no point in going above e50 with the N55. However I don't follow it much, so others should chime in on that.



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Default 11-20-2013, 06:18 PM

Thanks for posting this chart Steve, very useful!


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Default 11-20-2013, 07:27 PM

You can't just simply add those numbers and divide by two. You would have to test the exact octane rating, I think it's not possible at all to calculate it. For example, if you mix 50% 100 octane and 50% 90 octane you won't get 95 octane. It's more like 93 octane.

But still thank you for the sheet. At least we have a clue by how much our octane rate increases (+/- a few octane)

Edit:
We in Germany have about 5% Ethanol in our fuel. I don't know how you handle it in the US but you would have to add these 5% into your calculation


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Default 11-20-2013, 08:06 PM

^agreed very unscientific and I tried to express that in my post. It was to give people an idea as to the differences between mixes and the mixing strategies. Really..who gives a hoot what the octane...it's how the car responds to it...that's what matters.

Take it as you will..or completely discard it..makes no matter to me.



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Default 11-20-2013, 09:18 PM

Quote:
Originally Posted by 335iFFM
You can't just simply add those numbers and divide by two.
Yes you can, as a general rule of thumb unless, they react chemically or something, which shouldn't ever happen .

Gas stations here in my part of the US only carry 93 AKI and 87 AKI. To get 89, they use an exact 1/3 and 2/3 mixture of 93 and 87. This is also why 89 octane gas is only $0.10 more expensive than 87 here, while 93 octane gas is often $0.20 higher than 89 octane gas (because it's 100% premium gas rather than just 33%).

If they carry only 91 and 87 like in California, they use a 50% mix to get to 89.

Why would they premix it and have different SKUs and complicate things during shipping when they can do it at the gas station and only carry two types of gas, after all?

When I'm calculating ethanol content for my E85 fuel mix, I assume 10% ethanol because that's what is widely used in Michigan. The less ethanol in your gas, the more E85 you can put in to get the same ethanol concentration, the better your octane rating...


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Default 11-20-2013, 09:54 PM

Quote:
Originally Posted by coolbho3k
Yes you can, as a general rule of thumb unless, they react chemically or something, which shouldn't ever happen .

Gas stations here in my part of the US only carry 93 AKI and 87 AKI. To get 89, they use an exact 1/3 and 2/3 mixture of 93 and 87. This is also why 89 octane gas is only $0.10 more expensive than 87 here, while 93 octane gas is often $0.20 higher than 89 octane gas (because it's 100% premium gas rather than just 33%).

If they carry only 91 and 87 like in California, they use a 50% mix to get to 89.

Why would they premix it and have different SKUs and complicate things during shipping when they can do it at the gas station and only carry two types of gas, after all?

When I'm calculating ethanol content for my E85 fuel mix, I assume 10% ethanol because that's what is widely used in Michigan. The less ethanol in your gas, the more E85 you can put in to get the same ethanol concentration, the better your octane rating...
I don't think you can calculate it that way with Ethanol and gas mixes. Octane is a measure of resistance to knock. When you're mixing two different fuels together where one knocks at a lower threshold, there is no guarantee that the combined knock resistance is a linear blend of the two. In fact it is very likely non linear and will assume a knock resistance closer to the lower of the two values. I like to think of it as a "contaminant" in the blend. Conceivably just the mere presence of a little substance at a lower KR could bring the octane down many points.

For our motors though, unless you're on a kill tune it doesn't really matter anyway. I ran 50% E85 mixed with 50% 87 and supported 17.5psi with no timing corrections. Didn't bother trying lower mixes but know I can do 40% E85 to 91 and be fine too.


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Default 11-20-2013, 11:23 PM

There's definitely a molecular bond that takes place and my thought is "if you bring X amount of octane to the party...you still have X amount of octane when you get there whether it is mix with Y or not". Dr. Euro some of this in a post and we discussed it briefly.

I don't feel as though I'm educated enough on this subject (yet) to argue it...so do I defer...

Here is the information he quoted. Here is the thread.

"Octane is a chain of eight carbon atoms each carrying a pair of hydrogen atoms and an additional hydrogen atom to cap each end of the chain (C8H18). Iso onctane (race gas) has three of the end clusters of the chain moved toward the center, making it a more stable molecule, requiring more energy to break. "



"Fuels with a higher octane number have a stronger concentration of these more robust molecules versus the more easily broken longer chains. For this reason, they require more energy to break these bonds, and have more resistance to knock. The energy required to break these bonds comes in the form of heat during combustion. Heat transfer from one molecule to the next requires a small amount of time. Even though this is a small amount of time, more total transfer means more time for it to happen. Adding more heat to separate their ions means more time is required. This means that the burn rate is slightly slower for "high octane" fuels. This becomes an important factor when choosing spark advance timing."



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Default 11-21-2013, 12:05 AM

Quote:
Originally Posted by coolbho3k
I wish you could get E100 so you could decide what kind of gas to mix with it
Ive found a supplier here in RSA that sells a hyper ethanol. E99.
But my car doesnt like it...


fuel injection is cool, but i'd rather be blown!
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Default 11-21-2013, 12:32 AM

Quote:
Originally Posted by bmsportzn135i
Ive found a supplier here in RSA that sells a hyper ethanol. E99.
But my car doesnt like it...
Let's see the logs and see if we can figure out why your car doesn't like it. It could be hardware or tune related.



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Default 11-21-2013, 01:07 AM

I have sent Terry some logs before, but i think its got to do with my LPFP.
Which i dont really wanna change if i dont have too.
When i fill up again i'll send though a log.
I used to mix 20L BP98 race fuel (yes thats our race fuel this end of the world) 10L E99 and 1 can NF race fuel concentrate.
Ive now changed that to 5L E99, 1 can Nf and if the tank is low enough, 40L BP98.


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Default 03-08-2014, 12:08 AM

Old thread but I somehow didn't see it and I love E85... lol

I calculate it a little differently, kind of based on how how the interceptft.com site does it. I just focus on the Ethanol, rather than octane. I know my E85 fuel pump is E77 through testers. So at my last fill up I was on straight 93 since I had been stock for a few weeks. I have verified E10 93. So my last fill up went like this here:

10.000 gallons of E77
3.462 gallons of E10
2.638 gallons of E10 (still in tank) assuming tank is 16.1

So I add up the ethanol per gallon and divide by 16.1. So 770 + 61.0 = 831 / 16.1 = E51.61

This is how the http://www.intercepteft.com/calc.html site is based.

At my next fill up I will use 51.6 ethanol for the remaining in the tank. I can usually do the calculations sitting in my car at the pump. I just use the hash marks on the gauge rather than the iDrive trip thing. Take each hash and they represent 0.8 gallon. Figure out the remaining in tank and decide what I need to put in to achieve the ethanol content I desire. I usually put 8.5-10 gallons on every fill fill up with the 93 being 3-4 gallons.

Just the way I do it


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Last edited by musc; 03-08-2014 at 12:14 AM..
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Default 01-12-2020, 03:38 PM

I know this thread is old, but the OP chart is really helpful. Any chance you could post the source spreadsheet or update for E25 and E20?
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Default 01-14-2020, 09:24 AM

I don't understand the difference between true E30 (yellow lines) and mix E30 (white lines), can someone explain?

EDIT: Disregard, I figured it out. Blends of "True E30" take a stronger mix of E85, for example, True E30 created from E70 is a ~43% mix. Reccomend this table be re-arranged in order of octane to better understand the stronger blends that are needed to create "True" E values.

Last edited by Rustl3r; 01-14-2020 at 09:47 AM..
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