Convince Me to go e85

It's the ethanol content that matters, not the octane of the gas:
So E29 using 87 pump gas would be about 102 octane (assuming E85 having 150 octane), whereas the same mix on pump 91 would be 105 octane...not that big a difference. Adding another 3 gallons of corn, however, bumps octane up to 118.5. Bit more of a jump at ~13%.
 
Enki said:
It's the ethanol content that matters, not the octane of the gas:
So E29 using 87 pump gas would be about 102 octane (assuming E85 having 150 octane), whereas the same mix on pump 91 would be 105 octane...not that big a difference. Adding another 3 gallons of corn, however, bumps octane up to 118.5. Bit more of a jump at ~13%.
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Exactly. That's why I figured I could get away with 87 to save a few bucks, tried it, and it worked :D
 
Is it possible to "guesstimate" the ethanol content in your tank by looking at the long term fuel trim? Or maybe your AFRs?
 
Possible? Maybe. The problem is your trims will adjust and spread out over LTFT and STFT depending on how much learning has been done and the conditions of the air and whatnot; thus, you might go from being knock proof to not being knock proof and not know it (if your mix is thin enough already).
 
Finch204 said:
Is it possible to "guesstimate" the ethanol content in your tank by looking at the long term fuel trim? Or maybe your AFRs?
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TL;DR incoming wall of text - be forewarned, LOL

With AFR, no. Reason being, your ECU will use input from your primary O2 sensor to measure AFR and will adjust trims accordingly to hit whatever AFR target applies at any given moment, whether idling, @ WOT, etc. Seeing AFRs off does, however, often indicate boost/vacuum leaks because the system/logic in the ECU assumes everything is operating without mechanical issues, so your AFRs will be impacted by mechanical issues, like leaks (usually the worse the leak the bigger the impact).

LTFTs also aren't the best measure due to the fact that they're based on an average over an extended period of time. It's like the avg. speed or MPGs on your info center - the more miles you've driven, the longer they take to change because more and more data is being averaged the farther you drive. Now if you re-flash your map right when you fill up, LTFTs can be a good indicator when they start populating (takes a little while) because all of the averaged data is cleared.

STFTs are a better measure of instantaneously measuring how accurate your mix is, but even then it's a little tricky because they will also show figures as your car is adding or subtracting fuel to hit your targeted AFR. This is completely normal. So you'll see a lot of activity in STFTs, and if you're not used to the normal range they usually work within, they can be misleading.

The best method of ethanol equalization is using the same E85 pump every time and putting in a precise ethanol:regular in your tank every time you fill up. That's why I bust out the calculator to be precise. But this hinges on knowing for a fact you can fit x amount of gallons in your tank. For instance, if someone runs a 25% mix and assumes they can fit 12 gallons in, put in 3 gallons of E85, but can only fit 7 gallons of 93 (so 10 gallons total instead of the 12 they were shooting for), they just created a situation where they're running a good deal more ethanol than they intended. That's one of the reasons I always over-estimate how much fuel is remaining in my tank. This means that I have to refill more often because I rarely get the tank completely full; it also means my ethanol ratio stays on point.

Worst-case scenario, use your AP to monitor WOT AFR/KR to make sure you're still safe (not lean at WOT; no excessive KR, etc.), and if necessary, drive while staying out of a lot of boost until on fumes, go back to a full 93 tank, and start over, running it down until you can fit the proper amount of ethanol in and top up with 93.
 
The single best way to measure mix is with a flex fuel sensor and gauge.
Edit: The second best way is with one of those water based test kits, which are *reasonably* accurate.
 
Finch204 said:
Is it possible to "guesstimate" the ethanol content in your tank by looking at the long term fuel trim? Or maybe your AFRs?
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Not really honestly. E content in gas will vary as well as in the E85. And everyone's MAF cals are different.

Enki said:
Possible? Maybe. The problem is your trims will adjust and spread out over LTFT and STFT depending on how much learning has been done and the conditions of the air and whatnot; thus, you might go from being knock proof to not being knock proof and not know it (if your mix is thin enough already).
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This isn't the problem. No matter what is stored in LTFT, you always add it to STFT to get your total fueling correction. One of the first things I do in any log is add those two columns together and create a third column. As long as you have a consistent source of alcohol and gas, and a car that's free of leaks, if you're showing +10% at all MAF voltages (where previous tanks were sitting at around 0%), it's most likely that you've got too much alcohol relative to gas. And at any rate you don't want to go WOT and run the risk of leaning out in that state. I don't usually monitor my fuel trims, but when I run mixes, I always monitor my fuel trims for the first 10 miles or so to ensure I'm sitting roughly where I need to be.
 
I only do LTFT + (STFT/2) because STFT will shift with say, engine coolant temps, as well as like a hundred other variables.
 
@Finch204 The good news is that fuel trims are a thing. And they are more than capable enough to address even moderate variations in your mix. Addressing variations is exactly why they exist, primarily due to fuel variations and weather changing, but the same applies to ethanol content. Heck, they adjust all the time across the span of the year due to the oxygen/volume content of ambient conditions even on 93- or 91-only cars, such as when it's cold in the winter and hot in the summer.

The short of it is that with my method, I've never had a single issue. Not once. And I've been running ethanol mixes continuously for going on 4 years now.
 
I don't use a calculator, I just never top off. Put in the corn first, and then try to get the right amount of pump gas in after. If you can't make it fit, it's better to be on the heavy side of corn than the thin side.
 
Enki said:
I only do LTFT + (STFT/2) because STFT will shift with say, engine coolant temps, as well as like a hundred other variables.
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I haven't found that my STFT's vary much with coolant temps, but I've found the ECU in my car (after a reflash) will just grab pretty much any old number from the STFT while I'm driving and throw it in LTFT and just leave it there seemingly indefinitely unless I have a day where my STFT's are exceptionally stable, which is pretty rare since I have a 3.5" intake with a shitty air straightener. With your approach, your corrections will be biased if your LTFT deviates significantly from your STFT's, which may or may not be good. I guess it just boils down to the fact that you trust your LTFT reading a lot more than I do. And might reflash your ECU less than I do.

I prefer to just average a lot of data to make fueling correction decisions in tunes.
 
Thanks a lot for all the replies guys. I'm just about to try out Stratified's E85 Flash Tune, so I'm trying to learn more about keeping the mixes in check.

Looks like Enki and Redline are suggesting filling up with precise amounts of E85 and gas, which I'm inclined to follow. On the other hand, the instructions on Stratified's E85 Flash Tune page says to drive till the tank is near empty, then fill up with 4 gallons of E85 and top off with gas. I'm going to ask Stratified to find out how safe their tune is in the event that the ethanol content changes.

Those flex fuel sensors and gauges look pretty cool. How come not a lot of people have them installed?
 
VTMongoose said:
I haven't found that my STFT's vary much with coolant temps, but I've found the ECU in my car (after a reflash) will just grab pretty much any old number from the STFT while I'm driving and throw it in LTFT and just leave it there seemingly indefinitely unless I have a day where my STFT's are exceptionally stable, which is pretty rare since I have a 3.5" intake with a shitty air straightener. With your approach, your corrections will be biased if your LTFT deviates significantly from your STFT's, which may or may not be good. I guess it just boils down to the fact that you trust your LTFT reading a lot more than I do. And might reflash your ECU less than I do.

I prefer to just average a lot of data to make fueling correction decisions in tunes.
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TL;DR We have modern, high-tech cars and are very blessed to have wideband primary O2 sensors from the factory. In particular, Pu ECU logic includes O/L (i.e., WOT) fuel trims. All of these features, taken as a whole, make Pu's very easy to address (or even avert) O/L MAF calibration. This makes going ethanol blend (and even WMI) on them comparatively much easier than on a Won (depending on your nozzle size/mix, of course).

Just my .02: Your experience with STFTs/LTFTs is completely normal. I'm sure you know that LTFTs are nothing more than averaged STFTs over time. I the direction of this conversation has made the matter more convoluted than necessary (which I'm sure is hilarious to hear coming from me, haha). LTFTs take easily 30-40 miles to equalize, and that's assuming you're doing driving in all of the breakpoint ranges that are setup in your tune. For instance, if it's mostly cruising miles on the highway, you may have next to zero aggregated information for an accurate idle breakpoint range's LTFT. I personally like to go at least 100 miles before putting a lot of confidence in what my LTFTs end up being. With that amount of driving (mixed driving, mind you, not just highway cruising), I can be reasonably sure that each LTFT breakpoint has had a lot of time to sample data and derive increasingly accurate figures.

I, too, have a 3.5" intake (an HTP) and I run no straightener. Zero issues. I'm a pragmatist regarding tuning. Fortunately, as Pu owners, not only do we have superior piston design, VC design, and ECU hardware (faster sample rate) and tuning logic (for the most part, excluding when we don't have enough FP on stock HPFP internals), but we also have O/L fuel trims. Practical wisdom is that these can vary from +/- ~10% and be totally fine. Wons, not so much. I barely needed my MAF curve touched when until I went CM7 full 100% methanol due to these O/L trims (and only then was the adjustment minor - zero issues on 50/50). It was when I went CM10 that I finally found myself outside of the +/- 10% range and needed my calibration adjusted/MAF scaled down a good deal.
 
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I wish we could see that shit. I thought we were working towards that with Cobb, but things seem to be going backwards with them.
 
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