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Have not tried anothre impeller. The water coolant stream is pretty heavy so at leaast the impeller seems to be functional. It looks like there may be three ports for exhaust gasses to escape. One high on the back side of the motor, another slotted port on the back about 8" below the upper port and then the exhaust through the prop. But that lower exhaust through the prop should be attenuated slightly by the water I would think but it sure is noisy.Exhaust is water cooled.----Have you tried a new impeller?----4 strokes are quieter and thus more popular.-----More expensive to own but that does not seem to matter.