If you run meth, remember that meth has a much lower stoich point than gasoline. If you do not know the actual quantity of meth and the quantity of fuel injected, its better to use lambda than AFR. If you do know the quantity then you can use this equation to calculate the AFR to match your lambda.
[desired lambda*% gasoline in decimal form*14.64]+[desired lambda*%meth*6.4]= AFR
For 20% meth at a 0.8 lambda (11.7) the calculation will be
[0.8*0.8*14.64+0.8*0.2*6.4]=10.4
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