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      11-09-2018, 03:57 PM   #1
voteman
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How to bring up E85 mixture without draining fuel tank

Hello, I made a mistake when filling up my fuel tank, it was at 1/2 full with 91 so all I had to do was fill the other 1/2 with E85 to bring the mixture to E50 which is what I want. But like a dingus went 1/4 91 and 1/4 E85 which only brought it to ~E30 and now my fuel tank is full. I would like to avoid draining fuel tank with siphon through rear seats.

** So my question is, how do I do the math to find out how many gallons I need to use with it full (14 gallon cap. incl. reserve) (just by driving it) to then only fill it back up with E85 to bring it to an E50 mixture?

I only need to get the mixture right from the start and will then make sure to never make that refilling mistake again. (yes I realize the E85 at pumps can vary in actual ethanol content but nevertheless)
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      11-14-2018, 05:05 AM   #2
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Quote:
Originally Posted by voteman View Post
Hello, I made a mistake when filling up my fuel tank, it was at 1/2 full with 91 so all I had to do was fill the other 1/2 with E85 to bring the mixture to E50 which is what I want. But like a dingus went 1/4 91 and 1/4 E85 which only brought it to ~E30 and now my fuel tank is full. I would like to avoid draining fuel tank with siphon through rear seats.

** So my question is, how do I do the math to find out how many gallons I need to use with it full (14 gallon cap. incl. reserve) (just by driving it) to then only fill it back up with E85 to bring it to an E50 mixture?

I only need to get the mixture right from the start and will then make sure to never make that refilling mistake again. (yes I realize the E85 at pumps can vary in actual ethanol content but nevertheless)
6 gallons of E85 (with an actual ethanol content of 85%) will get you to just over E52.

You need to be accurate with your ethanol content values. Do you have an ethanol content analyser?

If you had exactly half a tank of 91 (with 10% ethanol) and you added exactly another quarter of a tank of 91 and exactly a quarter of a tank of E85 (with an actual 85% ethanol content), then you should have E28.75, which is why I'm guessing you said ~E30. Like I said, you need to be accurate.

If all this is correct so far, then you need to use 6 gallons to ensure you're not under E50 (depending on your tune, you'll most likely be better off to be a bit over E50 than a bit under if you don't have flex fuel maps).

Here's the formula:

Fe = Ft(Et-Ec)/(E-Ec)

where:
Fe = the amount of ethanol fuel required to fill
Ft = the total amount of fuel, i.e. fuel tank capacity
Et = the target ethanol content
Ec = the current ethanol content
E = the ethanol content of the ethanol fuel

The formula is unit agnostic, i.e. equally relevant for gallons or litres.

For example,

Fe = 14(52.857-28.75)/(85-28.75) = 6.0
Fe = 14(50-30)/(85-30) = 5.1

Using 5.1 gallons of E85 based on rounded values with the actual current ethanol content of E28.75:

Et = 5.1/14(85-28.75)+28.75 = 49.2

As you can see, inaccurate data will skew the answer, resulting in less E85 being added and a lower actual ethanol content, E3.5 lower in this example, which could be detrimental to your tune. Maybe, by chance, not so bad in this case but it could be worse if you aimed for E50 but ended up at E45 or lower due to rounding or estimating.

You could always just use half your tank and fill up 7 gallons of E85 - then you'd definitely be over E50 (almost E57) and then put E50 in after that. Depending on how low you run your tank between fills, you'll stabilise at E50 after a few fills.
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      11-14-2018, 09:48 AM   #3
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All the more reason I don't want to bother with E85...
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      11-14-2018, 12:22 PM   #4
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Use an online calculator and combine with logging to see if the octane is sufficient. Easier.

Also. 50/50 is not 50%
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