FAO: Justyboy,
Im in the process of building a DIY Nitrous kit now. All Im waiting for is the solenoid for the Nitrous. It has to be able to cope with the high pressure. It is possible to run a 6v solenoid at 12v to increase the power of the coil (the gas will keep it cool) and make changes to the seat size and plunger tip, but I wanna use one designed for the job initially. You can pick them up for about £45. Ive fitted a kit supplied by the Wizard of NOS, and although incredibly good quality, Im sure I can make one equally as functional for a fraction of the cost. The fuel solenoid is just an axial 12v solenoid valve (about £25). Im using twin 4lb CO2 fire extinguishers for the gas (must be CO2 ones to cope with the pressure). Nylon tubing is easy to come by and all of the compression fittings can be picked up quite cheaply. The switches can be picked up for pennies at Maplins and the jets can either be made easily or you can use webber carb jets (pre-sized). I have all of the Internal diameters for jet sizing, and the calculations for matching the fuel jet to the gas one at given fuel pressures. I also have access to a RR and gas testing equiptment.
The benefit of making my own kit is cost, and I can make a very stealth kit that can be hidden easily from the missus and removed easily for servicing (the cars under warranty).