one liter per minute. There are a whole lot of scams out there involving this but there are a few that are the real deal. Most systems are using a vacum switch so that you aren't wasting current generating gas at idle or when coasting down, so the only time your producing gas is when driving at part throttle and higher. As soon as the engine stops you stop generating gas and most of them use multiple check valves to prevent backflash from occuring