Also thanks to Swatski for reply.
Seems strange that MR-1 'regular' and MR-1 "HO" are not compatible ( my words, not yours) or rather "non-HO is not upgradeable to HO".
1) Same displacement in both types; yes? Same engine blocks, for both non-HO and HO, correct ?
2) Manufacturing efficiencies / economy of scale / economics would push Yamaha to use the base block, and upgrade 'breathing', maybe camshaft, and Engine Control Unit to squeeze more RPM + HP out of HO version.
If I look at the air intake 'throttle body' that has valve that opens to let more in, closes to reduce RPM, that feeds intake manifold, it looks like it is a 'chokepoint' for air coming into engine. I am talking about the thing that the hose from air cleaner clamps to. Possible to get larger diameter throttle body? Very small increase in diameter would mean more cross-sectional area for air to enter, better breathing.
Another aside question - wondering when the rev limiter kicks in for the non-HO MR-1 in the SX210.
Stated another way: what would curves of RPM vs Horsepower look like? i.e. as long as we keep below rev limiter boundary, is there any advantage to reducing impeller pitch such that max RPM at Wide Open Throttle is, say, 8500 or 9000 RPM? Or, does power output rise, and then start falling above 8000 RPM or so, for this non-HO engine?
Air cleaners: I did experiment with some simple K&N air cleaners, to try to reduce intake restriction:
1) removed rubber tube to throttle body in stock setup (remove hose clamp, pull off big black rubber tube that feeds engine air intake)
2) Fit short, cylindrical - shaped K&N air filters onto the engine air intake or throttle body (bought generic ones from K&N and picked largest / tallest ones that would still fit between air intake on engine where you hose clamp it on, and vertical fiberglass wall between engine compartment and cockpit)
Result: according to tachometers, and when trying to hold boat weight relatively constant (same 2 adults and same full gas tank each case), I managed to pick up maybe 100 - 200 RPM with the K&N's. BUT the noise increase was horrendous ! Much more engine noise. So went back to the stock long rubber hoses with stock air cleaners.
My apologies if all of this stuff has already been covered.
Anyone know, what max typical non-HO MR-1 engine RPM is, at sea level, with wide open throttle, moderate load, full gas tank? 8500 RPM ?
j/k I will be calling the insurance though and see if my insurance goes down.
