Can I Run half Tank E85 and 93oct fuel?

[Benji16]

2013 Audi A6 i know is not 100% flex fuel like the B8.5 A4 but will running half and half Cause any harm?

[Botbasher]

2013 Audi A6 i know is not 100% flex fuel like the B8.5 A4 but will running half and half Cause any harm?

I got a Octane Mix app and started mixing E15 (88 Octane) and E85 (100+ Octane) to avoid the rising prices ($95 vs $65 fill-up) and in prep for my upcoming E40 tuning.

It's easy to mix those up to something close to 93 Octane and does nothing to the car EXCEPT.... 1) Lowering your mileage, so you really need to do the costing to determine if you are actually saving any (30% less mpg) $$ and 2) that if you store your car for long periods, to fill up without a large E content before storage to avoid it's habit of sucking up water.

If you are doing it to straight increase octane, you tuner should be suggesting your mix ratio to partner with the tune.

My break even to mixing is about $4.50/gal... mpg loss vs cost per gallon... I'm seeing $5.50ish for 93 here, so I'm blending hard, usually 3/2 E15/E85 to get around 96 Octane. Car loves it and haven't seen any issues in several tanks!

Saying 1/2 tanks is fine on a perfectly empty tank (12g of 88 and 8 of E85 gives you 93), but if you fill up at 3/4... or 1/4 tank you can send it the wrong way quickly, you need to know what your putting in and giving your motor.

FYI - 15 gal of 88 Octane and 5 of E85 give you 91, so a general guide would be 3/1 for any fill.

Cheers,

KS

[Benji16]

So can i run lets say 6 gallons of E85 and the rest 93 pump gas? Will i be ok if i'm stock

[Botbasher]

So can i run lets say 6 gallons of E85 and the rest 93 pump gas? Will i be ok if i'm stock

I presume that you are trying to save $$ at the pump, correct??

You wouldn't mix with 93 at that point on a stock tune. Just run straight 93. The ECU isn't looking for anything above that and will not give you much additional under most conditions.

Get a Octane calc and start playing. Your motor won't really benefit from much more than 93, so you need to mix in a much lower grade to make this idea work.

For a FULL tank... 88 and E85(100) needs you to mix 12 and 8 gal to equal 93 octane.

For a FULL tank... 87 and E85 needs you to mix 11 and 9 to equal 93 octane.

For a FULL tank... 87 and E85 needs you to mix 14 and 6 to equal 91 octane.

The actual numbers on the last one are 13.85 & 6.15. I ALWAYS round towards the regular gas, not the E gas, since that's ALWAYS a much higher octane.

Get a calc and play with what you have local to you and see what you can do, but I would always suggest shooting for a 91-93 on a stock tune. The one I have is called "Octane Mix Calculator" and has yellow labels like the gas pumps do.

Good Luck!

KS

[2k16A6]

Not sure which engine you have, but if 3.0T I would not be arbitrarily adding E85 if you’re not running a tune for it and don’t have an upgraded hpfp.

[nosport]

I presume that you are trying to save $$ at the pump, correct??

You wouldn't mix with 93 at that point on a stock tune. Just run straight 93. The ECU isn't looking for anything above that and will not give you much additional under most conditions.

Get a Octane calc and start playing. Your motor won't really benefit from much more than 93, so you need to mix in a much lower grade to make this idea work.

For a FULL tank... 88 and E85(100) needs you to mix 12 and 8 gal to equal 93 octane.

For a FULL tank... 87 and E85 needs you to mix 11 and 9 to equal 93 octane.

For a FULL tank... 87 and E85 needs you to mix 14 and 6 to equal 91 octane.

The actual numbers on the last one are 13.85 & 6.15. I ALWAYS round towards the regular gas, not the E gas, since that's ALWAYS a much higher octane.

Get a calc and play with what you have local to you and see what you can do, but I would always suggest shooting for a 91-93 on a stock tune. The one I have is called "Octane Mix Calculator" and has yellow labels like the gas pumps do.

Good Luck!

KS

Its a joke?
Do you know what is stoichiometric?
Its not about the octane rating its all about ethanol content.
Stoichiometric afr for ethanol is different than for regular pump fuel. You cannot just add more ethanol to your fuel without a proper tune.

[Botbasher]

Its a joke?
Do you know what is stoichiometric?
Its not about the octane rating its all about ethanol content.
Stoichiometric afr for ethanol is different than for regular pump fuel. You cannot just add more ethanol to your fuel without a proper tune.

Yeah... and I even know what Lambda is too!!

I've been doing it for a while... running high E values (~E20) on non-E tuned cars... without issue. Even ran it on my Megasquirt-ed '84 A1 Jetta GLi without any mods or tunes (although I did allow a larger INJ Duty window) without harm when E85 was new and at only 3 pumps in the whole of Los Angeles!!

My Lambdas never strayed out of the safe zone (but they did tip toe up to the line sometimes on higher mixes) and I would never expect any performance from it (OP didn't state but easily presumed he isn't looking for HP!).

Currently my STOCK S6 is running 3:1 ratios of E15:E85 and I have yet to see a DTC (couple hundred miles and some soft, short pulls), so don't think it can't be done. It's pushing the Lambdas on the stock set-up about as far as I am willing to go for long term, but it's doing just fine!

Mileage sucks but it's still cheaper than 93 right now!!

Cheers,

KS

[himself]

Here's my $0.02. Probably worth less. Don't rely on internet pundits.

You can definitely add E85 to your stock tune. In the US, your car is already set to accept 10% ethanol (US pump gas). There was a recent study done by University of Nebraska-Lincoln that showed modern cars can accept up to E30 [30% Ethanol]. https://www.greencarcongress.com/202...10809-unl.html. The article mentions that most car manufacturers set a 20-25% limit on Ethanol. I have no idea what Audi does, though.

Assuming you accept the study and want to run more ethanol, you have to remember that E85 is NOT 85% ethanol. It's a range from E51-E85 [51% to 85% ethanol]. With gas prices through the roof, some folks are now reporting almost 100% ethanol. doh!

In order to get an accurate ethanol reading, you need to measure the ethanol content at the pump or get an ethanol sensor. But, if you just want to stay under 30%, you can do that using some math and a little guessing. Folks that can actually MATH will comment/adjust these numbers, but if you fill up @20 gallons, getting to around 20-25% ethanol would be ~16:4 [16 gal gas and 4 gal E85]. That is, if the 4 Gallons of E85 are actually 85%E, this would be a 25% mix of Gas/E. If the 4 gallons are 50%E, this would be 18% overall E content. So you are between E18-E25. [and, even at 100%E, you would be at E28 mixed.]

At a 15 Gal fill up, 12:3 gas/E85 nets about the same ranges [18%-25%E].

At a 16 Gal fill up, 12.75:3.25 is the same E18-E25 range.

As a kind of loose general guideline, each 1 gallon of E changes the percentage around 2%-5% overall. So, you can scale up/down to get to the E% you want. For example, if you want ~15%E, you can put in 1-2 gallons when filing up. It won't be a perfect measure, but you'll be in the "range." Your fuel trims should figure it out.

I think any adding more than 5 gallons is going to be out of range, so 4 is probably the most you want on a full tank considering the max 30% target the article mentioned. Adding 2 gallons might be the sweet spot for most people. Doing a 1/2 and 1/2 fill up at 20 gal would be a E30-E50 (30-50% Ethanol), which your stock tune probably wouldn't like.

Lastly, when you add E, you get octane boost, but mileage decrease. That's another tradeoff that might be relevant to your situation. The article says that a 2.5% price difference is enough to offset the mileage loss. [E30 is .64MPG worse than E15]

-td

NB: my car usually takes ~16Gal when I fill it up. When I use E85, I put in 3-4 Gal. This gets me around E20-E25. Car seems to like it.

[Botbasher]

...Folks that can actually MATH...

Love it... and don't forget.. Folks that can think critically.

Sadly abstract thinking is a fading skill. No one is willing to test the boundaries and find new limits.

Cheers,

KS

[Alabama]

There was a recent study done by University of Nebraska-Lincoln that showed modern cars can accept up to E30 [30% Ethanol]. https://www.greencarcongress.com/202...10809-unl.html. The article mentions that most car manufacturers set a 20-25% limit on Ethanol.

I have trouble understanding the data tables in the original paper, but almost all the cars used were Dodge; I assume naturally aspirated. I wonder if the results transfer to turbo cars.

[paul-g]

if your stock 89octane is acceptable, but i'd drive it easy not jackass around. like if you drive mostly highway where your just cruise 89 is not going to hurt anything, computer can pull timing if need be

[DetRebel]

Mixing 87 and E85 to save money sounds dicey but chances make champions, good luck 🤷🏾*♂️

Don’t forget to account for the 10% ethanol in pump gas! That can really throw off the calculation. I’m going to shoot for an e20 mix most of the summer on my IE tune.


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