BF8 question

[yzchamp]

half way up the leg on the back there is 2 holes that allow exhaust gasses to escape. why are they there? when i plug them all of the exhaust goes under water, and motor is more quiet. can i plug them permanently or do they have a purpose?

 

[Dockside Marine Repair]

Thing is most of the exhaust comes out of the prop. When you're under way and producing more exhaust its easy for it to get out because of the void in the water caused by the lower unit as it cuts it's way through the water. When you're at idle or trolling there is no void, only backpressure. These holes give the exhaust and easy route to escape and provide proper exhaust back pressure which is important, to some engines it's everything. Plugging these holes may cause the pressure to build and eventually stall the engine. Almost all outboards have them, they're engineered and fine tuned via decades of trial and error. You could try it. I doubt it would hurt the engine but be mindful af any changes. It may result in harder starting, poor idle and acceleration, and fouled plugs. Or it might not have any significant change. Just pay attention to any changes use good judgement. If it makes it run poor it could eventually hurt the engine.