"ToneJohn" wrote in message

only becuase in all the mercruiser diagrams and manuals they show
"optional" riser...

Generally speaking, the only reason you need a riser is if, in your
particular installation, the manifold or elbow outlet is at such a height
that the final exhaust tube to the transom exhaust outlet has a very gentle
pitch, so that there *might* be a chance of backflow into the manifold in
pitching seas. If the original factory installation did not include a
riser, then you probably don't need one.