Very few outboards have much in common with there car engine counter parts, there are a few exceptions but not many. The reason why is simply due to duty cycle. Outboards see what would be considered extreme duty in the automotive world. Mercury's durability testing use to require a 500 hour run at WOT, and that was like 10 years ago. Most car engines would never survive. Think of it this way, even when cruising, my outboard is running between 3,500 and 4,500 rpm. On your car or truck, even pushing hard down the interstate doesn't yield much over 2k rpm, maybe 3k rpm on a gas engine powered pickup towing something. Yeh, there are some short stints of higher rpm but on average they lead a pretty easy life compared to most outboards. Duty cycle is the same reason when you buy a 4500-5500 series pickup, they are de-tuned compared to the 2500-3500 series. Same reason GM still uses the 6.0 in the heavy duty trucks instead of the 6.2. Sure the 6.2 make more power and torque, but under severe duty it won't survive like the old cast iron 6.0. Frankly I think outboards of today are pretty impressive, your basically getting a mild race engine that's made to survive several thousand hours and has excellent emissions and drive-ability.
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Thinking out-loud, last year I made a 30+- mile (one -way) run seven weekends in a row to a honey hole that I pretty much had to myself on the Sakakawea, 600 hp and a big gas tank sounds nice, lol! FYI, for all the complaining I do about how windy our state is, that fact I was able to make the run that many weekends in a row tells me we had a pretty decent summer! And that's considering I don't have a 21' tournament boat in my garage!