[kismetcapitan]

For some reason I can't do the math in my head - if standard gasoline is about 14.7:1 stoichiometric, and toluene is 14:1, then with a high blend of toluene (I've recently decided to massively increase my toluene fueling up to 70% from 25%), what happens to the AFR as displayed by my wideband?

On pure pump gas, my tune is set for 11.0:1 at full boost (sometimes dips to 10.9~10.8). The more toluene I add, will that go up or down (read like 11.3:1, or read like 10.5:1). This is with zero change in fueling or ignition btw.

In hot weather, the car does love the tol. Where pump gas loses knock resistance when heated, toluene vaporizes better (I have to run 25% in colder weather or it won't light off). The engine note mellows, and the revs become smooth as cream when running very high levels of toluene. I guess I could bump the ignition up for the higher toluene? Or just let it run with that much more knock protection; I'll have to reduce the toluene load when the weather cools. I might start tweaking when my modified twin turbo pipe arrives

 

[Marky-25]

The AFR is calculated on mass, so it depends on the specific mass of the toluene relative to the specific mass of the petrol. It's not a question you can answer based on stoichiometric ratios alone. They only tell you the mass of air needed relative to the mass of the fuel.

Some figures from wiki
Gasoline - Wikipedia, the free encyclopedia
Gasoline 740kg/m^3
Mass, Weight, Density or Specific Gravity of Liquids
Toluene 864kg/m^3 (average of 2 temps.)

Hence toluene is 1.1676 times as dense as gasoline

At 25% your fuel absolute weight indexed against pure gasoline is

(0.25 * 1.1676) + (0.75 * 1) = 1.042

At 70%

(0.7 * 1.1676) + (0.3 * 1) = 1.117

AFR = Mass of Air/Mass of Fuel

Mass of fuel has increased by 1.117/1.042 = 1.072

Hence AFR decreases from 11.0 to 11.0/1.072 = 10.25 if everything is unchanged

However using your given stoichiometric values and applying a safety factor (SF)

[(0.25 * 14) + (0.75 * 14.7)] * SF = 11 Gives SF = 0.757

[(0.7 * 14) + (0.3 * 14.7)] * 0.757 = 10.76

So in short if you apply the same volumetric flow rate of fuel at 70% C6H6CH2 as at 25% AFR will drop from 11.0 to 10.25. Ideally you need to reduce the volumetric flow rate by ~5% to give an AFR of 10.75 based on stoichiometrics and margin for error.

But of course with more toluene, less margin for error is needed so you may get away with a higher AFR.

 

[kismetcapitan]

so actually, interestingly, I have to lean out my fueling not because of stoich, but because of fuel density? I do feel a slight bit down on power; part of it is that toluene burns slower and timing has to be advanced to compensate at high RPMs, the other part is that I figured I was running too rich, which always puts a wet blanket on power no matter what fuel you're running more or less.

 

[Marky-25]

You can get problems running too much toluene. As you say it burns slower, so much so that it can still be burning when it exits the cylinders and take down exhaust sensors.

 

[Marky-25]

Just a word of advice though. You have to know how your car is measuring AFR. If it's measuring AFR by measuring fuel flow and then calculating fuel mass based on the density of gasoline, the 'measured' AFR will be unchanged from where it is now, however the actually AFR will have reduced by 7.2% as indicated. However, the end conclusion that you need to reduce the fuelling by about 5% is still accurate but this may not give the measurement of 10.76 if you see what I mean because all the computer sees is reduced fuel flow and not fuel density.