Self Tuning EFI

[fabr]

I pirated this from a flex fuel site. It basically explains how auto tuners work. Some of you are using Bazazz and such,I'm using the FAST EFI . All work on the same principle. The info below was a reply to a different question but truly applies to how the auto tuners work. I found it pretty interesting and enlightening.


I'll try to keep it somewhat brief how this all works. You already know that E85 has 30% less energy content than gasoline, thus it takes 30% more fuel to extract the same power. What you may not realize is how the computer actually runs in the background, behind everything.

You probably know that 14.7:1 AFR is stoich for gasoline, stoich means that there is exactly enough air for that volume of gasoline. In lambda terminology, stoich is a value of 1. In contrast, since E85 has less energy and takes more to run, it has a stoich value of 9.75:1 AFR. If you read that out loud, 9 parts air to one part E85, that's a lot more gas than 14 parts air to 1 part gasoline, by a factor of 5 parts air so to speak.

Now, oxygen sensors cannot calculate AFR values because they don't know how much gas or how much air you are actually adding, they also don't know if the gas has 10% ethanol, or 0% ethanol, or 85% ethanol which all run different volumes for a stoich reading. All they can do is sniff the exhaust and tell you if the mixture has enough air for how much gas you added, regardless of their energy content. If it's rich, the value goes below one, if it's lean the value goes above 1.

So here's the trick. On a full E85 tune, if you are looking at a AFR log and it says stoich around 14.7:1, it's lying to you. It's actually at 9.75:1 by volume of fuel injected, but the gauges are all mathematically calibrated to gasoline's energy content. If you switch to lambda, however, 1 is 1, regardless of fuel type. There is no backend calculation required since it's a RAW output so to speak. I hope that part makes sense, because this concept is the biggest one.

So how to tune for flexfuel on a modern engine WITHOUT an ethanol sensor, that's easy. The car you drive will target specific lambda values (AFR values that you input for gasoline in Cobb get converted to lambda in the background, which are universal across all fuel types). The car reads the lambda of say 1.1 or higher, meaning lean, when you mix in E85 to gasoline. So, the car will increase fuel trims to bring it back to stoich. Even at WOT, the car is having to increase fuel trims to keep lambda targets in check (it's very fast at this). Now stock, the computer see's it's taking way more fuel to hit the correcet lambda (rich/lean) reading, and throws a check engine light. With a true flexfuel car, it makes a note that it's taking X units more gas to hit target readings, meaning the energy content has lowered in the fuel. The computer interprets this as E85 being used, and can calculate the effective % ethanol based on injected fuel and engine load to meet O2 targets. It can do this very precisely because we have a returnless fuel system that is more precise than you can imagine. It can then adjust timing up and other values appropriately.

Stock, your car will throw a check engine light because it doesn't understand why the fuel trims go up so high to hit target lambda readings (it's adjusted for gasoline). In a JB4 car, it basically lies that the fuel pressure is going down, causing the computer to increase injector duty cycle, and keeps lambda readings in check. Since the stock advance curve is very agressive, that takes care of itself. The JB4 reads the better timing as fuel that is supportive of current boost levels, and adjusts boost targets up. You will never run lean with lambda tuning with any fuel type assuming the fuel system can keep up, which is why you log the pressures.

This is my last smart post today. I made some generalizations, and you guys can feel free to correct stuff, but that's the idea. If you want to monitor ethanol that's fine, it's always nice to know exactly what you have. Especially on track days, but day to day it just really doesn't matter. This is all also why you can have a tune for E30, and just run E50. The car will target the same lambda values, it will just require some additional fuel to hit them. That is all taken care of for you, your car runs in closed loop ALL the time (not many do this). All you have to do is watch the pump pressures and you're golden.

[dsrace]

very good info......... i do run the bazzaz and i am not 100% positive but i don't believe bazzaz can control timing. that would be nice and could be bad at the same time. i only say that seeing how the smallest and largest leak effect my wide band o2 sensor that it is basing the fuel  adjustments on. i haven't looked nor asked if there is a wide band just for e 85 but according to that reading it really wouldn't be needed once caliberated in that manor. i do like the idea of the flex fuel sensor however and enemy has had nothing bad to say about it yet.

[dsrace]

fabr is your system allowed to adjust timing?

[fabr]

Yes,it will be controlling timing. I set the parameters and it does the rest. Using a crank trigger to control initial 30 degree starting point. It can adjust timing ,I think I remember correctly,+- 20 degrees from there. Nothing wrong with a flex fuel capable efi sustem at all. I kjust will not run this motor wit anything other than dead consistent e85,hence the e98 blanded with 87 no lead pump gas. Everything I have researched supports this mix makes the best overall power along with cool weather easy starting. Pretty interesting stuff to be learned about e85.