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That Guy
13,634 Posts
Discussion Starter #1 (Edited)
I can't even lie, I hate seeing conversations like that.

There is exactly one scenario where this generalized statement can hold to be true-the person that's telling you lives near you, travels the same general route to work passing the same fuel stations of convenience, and has done legitimate fuel-testing and can show you the data to support it. No one else can tell you what works best for you and your car-period. Not me, not your tuner (unless you've sent him logs with various fuels specifically for that information), and not any other random on the internet. The absolute only person that will ever know the best possible fuel for your car and conditions is going to be you, and it's going to take some work to figure that out. I see this general statement/conversation happen a lot, and in it's simplest form it is just not true. I've seen good logs from people that are not using a top-tier and poor ones from people that are.

There's nothing wrong with giving/listening to recommendations, just realize that you are the only one that can determine what is actually correct in your situation.

The first and foremost indicator is always going to be your OAR. If you cannot achieve -1 in that monitor, you absolutely must find better fuel or run a less aggressive map. Achieving -1 doesn't always mean the fuel is great, just that it is good enough to run the current map. I had never seen OAR budge off of -1 so I tried some crappy 87E10 and it did indeed change.

I don't have logs to show (as of this typing, anyways) because I'm already aware of the quality of my fuel and have been using literally the exact same station for years. I actually do myself a disservice in that because stations and their fuel source do change over time, and I should probably be testing another station or two yearly just to ensure that things haven't improved elsewhere or gotten worse where I currently go. At the very least, those that have seen my data know that the fuel I'm running is optimal for my custom tune (though technically, vice-versa). I will test a couple stations nearby and post charts if I can, but I'm not currently putting the mileage on that would make them appear relatively quickly. Either way, here's the best that I can do in verbally describing differences between good fuel and poor fuel. I stole this recent post as a visual aid in the meantime-

This is a good overall representation of a few different instances that good/poor fuel may show up, though we'll extrapolate it into a couple of scenario's. We're going to assume that you're either running an OTS tune rated for your mechanical setup or a completed custom tune. If you're only on a OTS map and are considering a custom tune, shoot for #1 prior to as you should ultimately receive better results on that fuel than either of the others-

1. (Not illustrated) You see similar positive corrections in all cylinders up to max that hold there until redline. Substantially better fuel than the map needs, power is left on the table, you might be throwing away money. This is the safest scenario, though you can consider trying a less-expensive station to drag your corrections down.

2. All of your cylinders are similar across a pull and through separate logs, but when graphed out they all look like cylinder 2 (as an example, though all cylinders individually have the same characteristic in the illustration). You're seeing increases in ignition correction up to a point, then they reduce, then they begin to climb again. Your fuel is good enough for the map, it just isn't so strong that it allows constant ignition increases. Basically, your fuel is better than what the map was designed for, but not substantially. So long as you're not seeing negatives, this is better in terms of power. The closer your lowest correction is to zero without a negative correction the closer you are to knock threshold, which is actually where you want to be for safety and longevity when coupled with cost.

3. You have one or more cylinders pulling negative corrections consistently, cylinders pair up, or you can't get repeatable results throughout separate logs (cylinders 2 and 3 individually or 2/3 compared to 1/4). You might have a bad tank of fuel or a suspect fuel source. This is where it's convenient to know if you previously had good fuel to begin with because if you have already tested it and nothing else has changed with your car then you almost definitively know it's fuel. If you're just now starting fuel testing then you should find a new station.

Problems have the tendency to compound or show similar traits, and this is why fuel testing is of importance. In any of the situations there are a multitude of legitimate problems that could be causing the issue-fuel just happens to be one of the easiest to rule-out when troubleshooting. This is part of how it is related to the other threads and why it's imperative to be able to view your entire procedure, from datalogging to troubleshooting, as a whole. Just because your graphs show traits that can be related to bad fuel doesn't necessarily mean it's bad fuel, but it's much simpler to ensure it's not the fuel first than it is to tear your whole car apart.

Ok, you've read a lot of words but we're almost done. You have an issue and want to rule out fuel, so now what? Assuming that you have logs to compare against, the simplest way is to go find some E85 and put in two or three gallons if it's available. What you're doing is boosting your octane to see what effect it has, and if your problems' cause is poor fuel then this should increase all of your corrections and make them repeatable. The other alternative is obviously to find a top-tier (or a different top-tier) fueling station and try their fuel for comparison. If your logs are consistent then it should be very clear as to what effect the fuel is having on your tune/engine.

Boosting your OAR to log- If your OAR is reading higher than -1 and you don't feel like it's moving fast enough for you, there is a technique to try and force the issue. First, you'll most likely want a relatively long hill to climb, preferably with minimal traffic because you're going to start at a low speed and may end up at something quicker, depending on the hill. You can do this on a flat surface if necessary, but it's more difficult without the added load applied by climbing.

1. Drive your car around until everything is up to temperature-OAR will not listen without it.

2. Starting at or near the base of the hill in fourth gear at around 2k rpm, build 5-10 psi of boost and hold it. You're not trying to make full boost, but the engine does need to be under a load for an extended period of time.

3. Continue to hold it. You may have to modulate the throttle, especially if you're on a flat road, but you want to stay in that boost range for as long as possible. I think the minimum is 7 seconds, but it's been a long time since I've had to do it. Just hold it as long as feasibly and safely possible.

4. Whenever you're unable to hold that boost range, let out and slow to your normal speed. It may take a few seconds, but you should see a change in the OAR. Repeat as necessary.

If you've done this procedure a few times and still have yet to have the OAR budge, you need better fuel.

That Guy
13,634 Posts
Discussion Starter #2
These logs aren't from the same car, but should give a general idea. If we assume the lowest correction in the chart in the original post to be zero and assume the other cylinders were all relatively similar, then that would be good fuel for the map.

1. This would be fuel that is 'too good' for the map-

2. This could be an example of poor fuel-

The reason that I say 'could be' is part of why I say that you have to take the entire situation into account. If my original logs had looked like 1 to start with and then I started getting negative corrections (obviously, sometime after having gotten fuel) and later took a log that looked like 2-with no other mechanical work having been done to the car-it would easily be a bad tank of fuel.

Otoh, if I had taken log 1 and then done a bunch of work to the car and changed maps (without having gotten fuel) before getting log 2, fuel is less-likely the cause. Still, it's the easiest to check and tends to be the first place that I'll start if it's suspect (and sometimes if it's not, just to rule it out).

Fwiw, the #2 graph is actually mine from my FoST and was caused by my RMM. Because I already knew that I had good fuel and was aware of RMM issues with the FoST I was able to go straight to it, but that community has shared more charts and faults so it was easier to run through the troubleshooting. That's the ultimate goal of the threads I've been creating-to help people become more comfortable with taking/viewing logs, more informed about what they're looking at, and hopefully ultimately leading to more legitimate data-sharing to help move the community forward.

Edit: Here's what car in #1 (my FiST) looks like with the same fuel but post pro-tune:
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