I am not going to get into MAF transfer functions or other tuning functions.This is just about fuel.
First lets define a few terms:
AFR: Air Fuel Ratio
Stoich Rating: In simple terms is the AFR to have chemically complete combustion that is neither rich or lean. For Example most pump gasoline is about 14.64 to 1. Which means 14.7 parts of air to 1 part of fuel.
Lambda: Is a term that is utilized when tuning. In basic terms think of it as an adjustment to your stoich rating when tuning AFR. For example with pump gas tuning .80 lambda, 14.7 x .80 = 11.76 AFR target.
Lets assume your car blower car has a nice tune that gives a perfect 11.8 AFR under WOT with pump gas.
So now it is race day. We add some timing to our tune, drain the fuel tank, and fill up with some high octane unleaded VP109.
We go for a blast down the 1/4 mile while datalogging and see the A/F is reading 12.8 then make a quick trip to the bathroom only to find out they are out of toilet paper.
There are a few factors here to consider.
The stoich rating of VP109 is 13.41.
Most wideband A/F modules are calibrated to 14.7 pump gas stoich.
So this is the formula to determine the true A/F for the run.
AFR/Wideband Stoich x Race Gas Stoich
12.8/14.7*13.41= 11.7 AFR on Race Gas
So you may think 11.7 sounds safe but we must remember the VP109 stoich is 13.41. So 11.7 AFR = .87 lambda. For the most part a safe lambda value is .80 to .82 for a forced induction mustang. I prefer .80
So we take VP109 stoich of 13.41 x .80 lambda = 10.73 AFR
This is the AFR (10.73) I am tuning for.
Keep in mind this is not what will display on your wide band as it is programmed to 14.7 stoich. You must convert the AFR.
AFR/Race Gas Stoich*Wide Band Stoich
10.73/13.41*14.7 = 11.76 AFR is what I want to see on my wideband when running VP109.
Just to reiterate 11.7 on the AFR display with VP 109 is really a 10.7 AFR.
Ok so hopefully you are not totally confused.
There is a simple solution to all of this madness. The SCT Advantage software has a Scalar for Stoichiometric Air Fuel Ratio. You simply enter the correct value for the fuel you are using and all lambda fuel calculations will be based on this value. Then just do the first formula conversion to determine what you should be reading on the AFR display. Some AFR modules may be able to be programed to the correct stoich.
Or leave the stoich in the tune and wideband at 14.64 and calcualte what AFR you need to read on the wideband for the fuel that is used. (This is how I did it in the past, but I find it easier to change the stoich scalar)
This is why you have to exercise caution when mixing race gas and pump gas as you do not know the true stoich of the mixture. The leaded race fuels have a stoich that is closest to pump gas. I always drain my tank before putting in the race gas. When I mixed fuels in the past I didn't notice a drastic change in AFR, but when I have 100% unleaded race gas it really leans out with a 14.64 stoich scalar in the tune.
The other caution is the winter (oxgenated) gas we get in Arizona. I noticed the car leaned out in the winter when it was tuned on the better summer blend. A 14.1 stoich scalar works for the AZ winter gas if your initial tune was based on the summer blend.
Race Gas Stoich Ratings
Sunoco MO2X UL ? 14.5
Sunoco 260 GTX ? 14.4
Sunoco 260 GT ? 13.9
Sunoco 260 GT Plus ? 13.7
Sunoco Standard ? 14.8
Sunoco Supreme ? 14.9
Sunoco MO2X ? 14.5
Sunoco HCR Plus ? 14.8
Sunoco Maximal ? 15.0
Sunoco MaxNOS ? 14.9
Turbo Blue Unleaded (100 octane) -13.9
Turbo Blue Unleaded Plus (104 octane) - 13.7
Turbo Blue 110 - 14.7
Turbo Blue Advantage - 14.9
Turbo Blue Extreme - 15.0
VP Street Blaze 100 - 14.16
VP C10 -14.53
VP C16 - 14.77
VP 110 - 15.09
VP MS109 - 13.41
E85 9.765