A customer asked me to add an O2 sensor to his boat's riser manifold, since they don't sell such manifolds anywhere. This type of modification is for the racing or performance enthusiast who wants to squeeze every last bit of horse power out of an engine. On a carbureted engine the O2 sensor can be attached to a gauge to allow for fine tuning of the air-fuel mixture. On an engine equipped with fuel injection the computer will be able to make automatic air-fuel adjustments based on the data it receives from the O2 sensor (open loop); without an O2 sensor the engine would run at a fixed base air-fuel mixture (closed loop).      See pics below, click to enlarge.