I have flown several early model Long-Rangers with the 250 C20B engines equipped with Water Alcohol.
The tank was only 3 gallons [I think] and it was good for about 3 minutes so you had to use it wisely. It did make quite a difference in performance though.On a hot day- Instead of temping out you would Torque out.
To use you would first pull in power until the temperature was in the Yellow arc ( I don't recall the numbers), then activate the switch on the collective. The aircraft would twitch , the TOT would drop down ito the green then you could pull in the rest of the power to achieve 100 %Q. Once climbing out de-activate the system.
One time we had some Isopropyl Alcohol mixed in with the water instead of Methanol. On take-off from the base heli-pad the engine flamed out.