HHO is Brown's gas, created by seperating by electrolosys hydrogen from water. Water is H2O; Two parts hydrogen and one part oxygen. When the hydrogen is seperated from the water that leaves pure oxygen. When the seperated gases are then run through the air intake on an autombile engine, it creates Brown's gas, and therefore supplements the fuel and helps it burn better in the cylinders, causing the vehicle to get better gas mileage. But, if only the hydrogen is put through the system, it will not make Brown's gas, and therefore will eliminate problems like rusting exhaust and catalytic converter from the water going back out through exhaust manifold.