Its a home made system. I have it so 1 switch arms the system and using a Hobbs pressure switch in the intake, the fuel lock opens at 5 lbs boost. You can set it to anything you like. I also have a manual overide that I can hit and inject gas at any time regardless of boost psi. The propane really helps on those long uphill runs.. Heres some photos Mike

http://pg.photos.yahoo.com/ph/westkill1/album?.dir=9adc&.src=ph&store=&prodid=&.done=http%3a//photos.yahoo.com/ph//my_photos