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Target AFR for different fuel blends using a gasoline calibrated WBO2 sensor

Understanding AFR

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Hello Andre, hope you are in good health. My question is, say I am running gasoline on my evo 8 (stock ecu) with water/meth injection sprayed at the intercooler pipe. I then calculate that the meth and gasoline content is roughy around say, 30% meth 70% gasoline. Using the ZT-2 WBO2 meter that's calibrated for the stoichiometric value of gasoline (14.7:1 AFR), what AFR do I target at WOT? Should I target the usual 10.8-11.2 (I prefer richer due to horrible fuel quality, around 88-90RON, potentially even worse) at 30 psi holding till 6000rpm and then tapering to 25-23psi at redline (stock 9 turbo) or do I target a richer mixture of 10.1:1 AFR based on this calculation:

(Maximum Power AFR of Gasoline x 70%) + (Maximum Power AFR of Methanol x 30%) divided by 100 = Target AFR

(12.5 AFR x 70%) + (4.5 AFR x 30%) divided by 100 = 10.1:1 AFR

My understanding could be completely off, so please correct me with the right information. Thanks.

Are you SURE about that RON value for your fuel - it's VERY low, roughly 83-85 using the US rating! I'd suggest buying a couple of cases of octane booster ASAP, and using it!

I may be missing your point, but you need to remember the "AFR" reading the gauge gives doesn't actually reflect reality - it is reading the LAMBDA and converting it to the AFR that corresponds to the lambda value. You could run pure ethanol, or methanol, and provided the lambda is 1.0, the gauge will still read 14.7 even if the actual is (don't recall exact values), say, 10:1 and 6:1, respectively.

Some gauges may have an option to show lambda, if so, switch to that - makes things MUCH easier if using different fuel blends.

If not, don't worry about the true AFR, just continue to target around the 11:1 range on the gauge, with the engine telling you exactly what it wants - might be richer, might be leaner.

Hey Gord, we have tested a fuel sample from one of the best fuel station out here. The Octane Rating was around 93 RON. Gasoline is mixed with other additives by the local stations to lower the cost. So my guess is, the octane varies from 90-93 RON from station to station. Sad reality. Anyways, to offset this, I am using water/meth injection as well as using Toluene as an octane booster. My fuel blend therefore, includes 70% gasoline, 20% methanol and 10% Toluene.

As far as I understood from the course, Lambda 1 for gasoline is 14.7:1 AFR. Lambda 1 for methanol is 6.4 AFR. With a gasoline calibrated WBO2 sensor, 14.7:1 AFR is considered Lambda 1. Now how do I determine the new stoich value for the blend of fuel I'm using? And what AFR do I target at WOT? Or do I ignore everything and tune the AFR just the way I would tune gasoline alone? We don't have access to a dyno, therefore there is no scope to test which AFR works best for my application.

To tune with Lambda, I guess I would have to configure the wideband with the new stoichiometric value of my particular fuel blend. I don't think its possible to configure that with the Zeitronix 2 wideband datalogger. So I am left with tuning fuel delivery with AFR.

OK, what you need is a chart that has the lambda to AFT conversion for the different fuels - such as this https://i0.wp.com/ls1tech.com/forums/attachments/forced-induction/349083d1334238324-e85-afr-procharged-car-11psi-lambda-sclae.jpg - use lambda for the tuning and convert to the AFR gauge value that represents. It may be easier picking up a gauge that gives a lambda display?

For example, if you wish to use a lambda of 0.80 as a starting point, that would be 11.75 AFR BY THE GAUGE, althought the true AFR will be different.

You may decide you don't want to go richer than 0.70 or leaner than 0.85, and those would correspond to approximately 10.25:1 and 12.5:1 on the gauge.

Don't worry about a specific 'target' - the engine will tell you what it wants as you adjust fuelling and timing for MBT - you may not be able to reach it if the fuel is that bad, just got to do what you can. Oh, and check exhaust temps don't exceed the turbo's specification.

I'd also suggest checking out the local distributors as you may find you can buy ethanol at a reasonable price and make up your own E85 (or whatever) blend - even pure ethanol, which 'should' remove the need for the methanol?

Sorry if we're at cross purposes.

Thanks for the Lamda conversion chart. Well, my gauge does have a Lambda reading. However, as mentioned earlier, it is calibrated for gasoline. I'm still a bit confused on this topic. Maybe, I wasn't able to explain my question well. Say, hypothetically I was running on 100% methanol in my fuel tank (not water/meth injection), what WOT AFR would I target on a gasoline calibrated WBO2 sensor? The usual 11.0 AFR?

Ethanol is super expensive here. So the best alternative is Toluene which I have found to work.

If I understand the question, yes, the lambda value would correspond to the same fuel to air ratio richness.

Gasoline/petrol gauge AFR of 11.0:1 is ~lambda 0.75 which would correspond to a methanol AFR of ~4.8:1, or ethanol AFR of ~6.75:1.

Conversely, if you were looking to run a base methanol AFR of 4.0:1, that would be a lambda of ~0.63 which would show as ~9.25:1 on the gauge.

I hope that makes things a little clearer.

[edit] You think you have the option of lambda? Just use that - there may be a small re-learning curve but it is easily the best solution if you are using different fuel blends - the lambda value will always be right.

Gord, I'm still not clear about the concept. Let me ask a different question. Say I run my Evo 8 on gasoline alone. I find out that the car responds well to 11.5:1 AFR at peak boost pressure till redline. Then I go on an add methanol injection to my car, which changes the fuel blend. So the car now runs on 80% gasoline and 20% methanol. Do I still target 11.5 (AFR) or richen the AFR mixture?

Keep in mind, I'm using a wideband which is calibrated for gasoline. And I'm tuning with AFR and not Lambda. Avoid answering the question in terms of Lambda. That would clear out my confusion.

Yes, if the engine ran best with a gauge AFR of 11.5:1, then it should run best with the gauge showing ~11.5:1 regardless of the fuel

I suggest you try hard to understand the lambda relationships with the guage and the fuels' AFRs - they ARE important, and will actually reduce confusion!

Whatever you are mixing stay with AFR for gasoline number on your gauge like you did before. If you were getting your best power at 10.8:1 AFR (which represents lambda 0.75-.074) keep using it after adding water-meth since it is best lambda value for best power of your engine and regardless of fuel type your gauge will be always showing AFR 10.8:1 to represent lambda 0.75 since it reads residual oxygen.

So Gord, even if we used different fuel blends with different stoichiometric values, we would target the same AFR as gasoline alone. So what’s the point of understanding the fact that methanol, ethanol or other fuels have lower stoich values and are supposed to run richer if they don’t affect the gauge AFR?

Thanks Shota. It seems my Evo 8 likes to run at around 10.8:1 AFR (0.73 Lambda). Maybe it has to do with the high boost pressures I’m running coupled with very low octane rating. Guess I’ll have to sacrifice fuel economy.

Different fuel does affect your gauge AFR reading because of different stoichiometry. If you run gasoline at 14.7 (lambda 1) the gauge will be showing 14.7. If you'll switch to e85 (for example) without adding more fuel ( using the same settings for fuel delivery as for gasoline) your AFR gauge reading will become very lean, not showing 14.7 but 20+. In order to bring it back to 14.7 reading you will need to start adding more e85. Once you add as much e85 as enough to make it 9.8:1 your gauge reading will come back to showing 14.7 again, representing lambda 1 for e85. If you'll keep adding more e85, your gauge AFR reading will be showing richer AFR, depending on how much e85 you will add. Basically wbo sensor is measuring lambda only. That is why it is easier to remove AFR dash and replace it with lambda one to avoid confusion. But if you get used to making quick conversion from gasoline gauge AFR reading to lambda, then it is not a problem at all to figure out if you need to add or remove fuel.

My 4g64 also loves 10.9 AFR - makes best power with that.

Good explanation Shota. That clears up a few of my questions. Thanks.

Well said. I think the biggest mistake made by new tuners is understanding an O2 sensor narrow or wideband is an oxygen sensor NOT a fueling % meter. It does not know what content the fuel is just how much oxygen is or is not passing the sensor. That sensor's element at the right temperature produces a voltage. Think of it like a MAF sensor in the exhaust measuring oxygen and the "density" of air measured is % of O2. Consider a high voltage reading of an O2 sensor as having MORE Oxygen than fuel (lean) and a lower voltage (rich) or less Oxygen. The perfect ratio of air to fuel changes and is based on fuel type and make up but Lambda of 1.00 equals that perfect ratio regardless of fuel type or blend. That is why tuning a target in Lambda is better than targeting AFR which is fuel dependant.

I hope this helps.

Probably not - he certainly didn't pay any attention to what I wrote, several times...