blending fuel - octane rating

[prkaye]

if you blend 100LL with 91 auto fuel in 50/50 proportions, what is the resultant octane rating? Is it 95?

 

[BobTurner]

if you blend 100LL with 91 auto fuel in 50/50 proportions, what is the resultant octane rating? Is it 95?

There are two answers, and they are both no!
1. Avgas and autogas use two different measurement systems, both unfortunately called ‘octane’. Av gas octane is “motor octane”, while auto gas octane is the average of “Research Octane” and “Motor Octane” (look at any car gas pump, some where you’ll see ‘R+M/2’, that’s the average of the two different tests).
2. Car gas 91 is about 87 motor octane. So the question should have been, is a 50-50 mixture of ‘91’ car gas and 100LL have 87+100/2 = 93.5 motor octane? The true answer needs testing, but most likely the mix is a bit higher than 93.5 motor octane. That’s because the octane increase with more tetraethyl lead is very non-linear. The first little bit of lead provides a big octane increase, the next bit an increase but not as much, and eventually you reach a point where more lead results in no further octane increase.

 

[prkaye]

Thanks Bob!
To actually blend fuels, is there a special process for pre blending, or do you just dump the two fuels in the tank and let nature blend them?