[ursa minor]

the 4.5 V6 is as far as I know Mercruiser's first completely original design but it's not their first self produced engine.

Back in the late 1970's when GM stopped making the in line 6 that had been the midrange (165 HP) sterndrive at that time, Mercruiser developed the "470" which was a roughly 225 cubic inch inline 4 cylinder with an aluminum block / cast iron liners and a closed cooling system. I believe they used one of the cylinder heads and possibly the connecting rods and pistons from a Ford 460 V8. Originally it was 170 HP with a 2 barrel carb, later versions did 185 HP and I think they even did a 4 barrel version that was 190 HP. It had an outboard style crank driven alternator (no belt) as well. My parents put around 1500 hours on one over 30 summers without too much drama. It didn't like cold starts (electric choke) and wasn't the smoothest idling engine but it was pretty economical. I do remember hearing some horror stories about them and the alternators were prone to quitting but ours did OK. When the 4.3 V6 became available, they stopped producing the 470. Mid-1980's or so.

Mercury has a lot of high tech casting experience in aluminum, at one point they made the block for one of the high end Corvettes (ZR-1 maybe?) because they had so much experience in aluminum casting with the large outboards. Mercury Racing group makes some pretty extreme engines in house for the racing and sport boat markets, I'm sure they've done their homework.