Originally Posted by Big Time (Post 2855298)

When the exhaust is open (cap't call off) the exhaust and water is free flowing through the transom, essentially with no back pressure. Once you close the exhaust (cap't call on) the exhaust and water gets re-routed through the prop, which is not as efficient in expelling the water. Once you get over a certain RPM the water being sent through to the prop is not able to expell the water as fast as it is being moved through the engine and therefore you sometimes get reversion.

That is really the very basic and simple explanation, I'm sure someone on the board can give a more mechanically in depth answer.

Originally Posted by gerritm (Post 2855706)

I did have the ECM's reflashed for my 525's and the soot is much less, but still there after a hard run. Which is most of the time. Big Time's explanation is right on. Tbone, everyone knows you are the bestest and smartest. The only reason I knew to run the silent choice below 3K was because there was a sticker or warning in my owner's manual on my old boat with 496 mag ho's that said not to do it.