# Why does this exist?



## southerncannuck (Jun 27, 2016)

The gasket under this plate gave way and created an exhaust leak. I replaced it but I was wondering why there would be an exhaust port apart from the main one in the lower unit. Possibly to stop water syphoning up into the power head? It’s just a guess but if someone knows please let me know. BTW, it’s off of a mercury 9.9 4 stroke.


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## MatthewAbbott (Feb 25, 2017)

Emissions?


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## Tow-mater (Sep 28, 2020)

Not sure exactly but my guess is so that the motor isn't pushing against the water when trying to start. On my 25hp there is also 2 holes on the exhaust pipe that id guess allow the exhaust to flow thru. A path above the water line basically. My bet is just for starting the engine.


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## firecat1981 (Nov 27, 2007)

I think it's for when you are in reverse. Going backwards pushes water against the prop exhaust, if there wasn't another exit the backpressure might stall the motor.


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## southerncannuck (Jun 27, 2016)

My wife thinks it’s to break any potential siphons that might introduce water into the cylinders.


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