The passive system is essentially the hydrogen generator, but not hooked up to any electricity. So basically, the engine is sucking up the vapors from the hydrogenated water. The water4gas manual says this "passive system" has shown improvement by as much as 33% increase in mpg. I have since restarted my experiment due to the need to replace my o2 sensor and catalytic converter. Ever since the repair, my mpg has jumped up to 33ish (previously it was 27mpg combined) mpg combined. So 33mpg combined is my new "baseline" mpg rating to compare the Hydrogen generator to.
