Omc cobra exhaust bypass

[matthysone]

Replacing the inside trim lines on my 1990 OMC 3.0 Cobra. I removed the engine and the inside transom plate and the upright exhaust pipe. I see that the exhaust can either go through the bellows or before the bellows there is a large opening to the outside. My question is what is the purpose of having this opening as opposed to running all the exhaust through the bellows and out the prop. I am assuming this large opening is by design to help prevent any back pressure.is that correct? Why not have just one or the other?

 

[Scott Danforth]

its an idle relief port. at idle, the engine wont run with the back pressure that running thru the drive and out the prop has.

at WOT the exhaust flow out the prop is helped by the movement of water which also provides some scavenging of the gasses.

 

[matthysone]

Makes sense after you explained it. Ty. Also why when running on muffs the exhaust and water exits there and it’s not a torn or disconnected bellows after all.