Our Engineers designed a new air intake system for these engines. The intake system serves to quiet engine noise. An idle air by-pass tube supplies air to the crankcase at low rpm. There are no holes machined into the throttle plates, therefore no whistle sound at idle. Innovative baffling in the air silencer box cancels resonant noises. Unlike other manufacturers' outboards, there's no distinctive "pucka-pucka-pucka" sound of an air compressor, or any belt whine, or any cam chain noise - we don't use them.