Yes there is an on-off switch inside. Depending on the tune, you can use various strengths of methanol/water. Most (but not all) windshield washer solutions that are used to prevent freezing have at least 30% and up to 49% methanol. You can buy "Boost Juice" from Summit in 4 gal packs. If you go with straight methanol, your local Drag Strip can supply it (price varies greatly) or a speed shop can sell it to you in a metal container.
As for amount you will use, it depends on how often you are doing WOT runs, on the street or at the strip. In normal driving you will likely not use much at all. Then there are people like BPD who go through it like an alcoholic goes thru bourbon!!!!
How W/M works is that it cools the air charge allowing your tuner to increase the timing and maximize the A/F ratio under WOT conditions. When the percentage of Methanol goes above 50% it becomes flammable, which adds a second dimension by effectively increasing the combustible charge. This means that you can add more boost and still keep the A/F at safe levels.
The more radical the tune and higher methanol amount may mean that you should not switch it off, if you have a habit of flooring it randomly on a regular basis, especially if you have installed a 3 bar sensor which allows for boost of higher than 14 psi.
Remember that adding a W/M injection system alone, without tuning for it actually will slightly decrease performance. It does however add safety to any tune, including the stock one, as it lowers the air charge temperature which is heated up when being compressed by the turbos.
Lastly, even though my tunes were quite radical, in daily driving as well as on long trips, my W/M system rarely sprayed, so I would use more of the solution cleaning my windshield than I did thru the engine. And yes... a 75% solution is an expensive way to clean your windshield, but it WORKS GREAT!!!! I would say that over 12 months, I used about 4 gal of boost juice and a couple gal of pure methanol (that I mixed with the boost juice to get a 60-75% concentration).
One thing I did to my simple system from Torrie, was to add a light that told me when the system was spraying. That means that when the boost went to around 8 psi, the pump started spraying, and the light comes on. The second the boost dropped below the set level, the pump quit and light goes off. In normal driving I hardly ever saw the light.
