First- Thanks Bushwacker, for fleshing out my abbreviated post. I hadn't had time to come back and explain what I was thinking. And of course it is still just a guess on my part.
A comment on the Suzuki, insofar that I think that it isn't so unusual in terms of peak performance compared to car engines, although comparing cruise in a car to a boat is a whole different thing.
The DF140A specs look like small car engine specs. 2 liter 16V engines that hit 140hp were not that uncommon as far back as 1990, and those typically had (I think) 9.5-10.1:1 compression ratios, port EFI, knock sensor and catalyst. Sporty cars like the Nissan Sentra SE-R or VW GTI.
I have a suspicion that modern 87 octane might burn just slowly enough that at high speed and WOT, a knock sensor can pull timing to deal with pinging. At cruise, the boat shouldn't be at WOT, so effective compression ratio is lower due to throttling. And jabbing the throttle probably splashes in more fuel for accel and that might also reduce the tendency for pinging because of the cooled fuel rich mixture. This can also be tweaked a little with intake runner design to help/hurt cylinder filing at different engine speeds.
As an aside- a fast burning fuel that has decent "octane" or AKI is probably ideal for a boat. We want max torque at max speed- as this is what the prop needs. In this case, if you can get a very fast burning fuel, you retard timing and make more power. That way you aren't starting combustion while the piston is still coming up. A high tolulene fuel can do this, and the results show up on a dyno almost automatically with a knock sensor equipped engine that has the authority to pull ignition timing. *
But we get E10 most of the time. Or maybe REC 90.
* And you can bet that I will try this on the knock sensor equipped 350 MAG MPI bravo 3 on the 25 to see if I can touch 50MPH. Just to try.
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