I have not done a specific E-25 conversion.
I have assisted with some conversions for E-85 for racing purposes.
So, what is about to follow is some experience, some theory, and some guessing.
First, your 10 percent figure would be mathematically close.
[(0.25 * 67000) + (0.75 * 116700)] / 116700
[16750 + 87525] / 116700 = .893 or 89.3 percent energy content, so 10.7 percent lean.
The problem with this calculation is assuming 100 percent fuel burn.
Ethanol has an AKI of 116. Great for a 15:1 compression race car, not so great for a GMC with what, maybe 8:1 ratio?
All of the ethanol will NOT burn. I currently use a figure of 7~8 percent lean for E-10.
So calibration CAN be made, but it is going to be trial and error for any specific vehicle.
Other considerations:
(1) fuel valve orifice will need to be increased to handle the extra fuel
(2) leather (or in the case of early 1950's Zenith - metal) accelerator pumps will be required
(3) one should start buying spare carbs, as long term, the ethanol is going to eat the bottom of the bowl.
(4) Older brass floats should be used (solder used on the new reproduction floats from the other side of the world doesn't like ethanol in any mixture). I am not a fan of phenolic floats.
Jon.
