Maybe the oil had nothing to do with the difference at all. I am usually right in tune with changes in the way my cars run and possible engine problems but I seem to have little to go on in the boat other than WOT test. Not sure I trust it and here's why if you want the details.
Mine also seemed to have lost a little WOT rpm the last time we ran full speed on the lake. It behaved better than I expected with newly installed Smart Tabs but indicated rpm was down about 200 or so right at the same 4200 range you saw. I used to see about 4400 with a 21 pitch prop on mine which is more or less the lower limit and closer to 4900 with the 19 pitch prop I've also run. With the Smart tabs and the 21 prop I was hoping I would really break through the elusive 40 mph for my little 3.0 L
It seemed with the 19 pitch I was ALWAYS under 40 and with the 21 I could hit 40 if I could find the right spot on the lake with a slight downhill grade.
But one reason for the rpm might simply be the tachometer itself
My tachometer has had a tendency to get stuck to the point it has not buldged a few times after winter storage. So the following spring including this one I had to peel the bezel off and work the needle to get it moving again. Out of warranty so why not see if I can fix for free. It's a bit difficult the first time but once the bezel and glass is off the tach, the needle can twirl around and around 360+++ degrees freely. If it's sticky this frees it up again. But sometimes I'd flick it to spin it and it sticks a little below 0 and/or right about 4200. These two points are basically 180 degrees to one another and it might be the design of the windings in the tach itself that cause this. So maybe it sticks around 4200 on each of ours and has trouble getting just above that?
While my tach was out last and still set for a 4 cylinder I hooked it up in parallel to my old 88 M6 and ran it a little. Well the M6 can easily turn 7000 rpm which x 1.5 (6/4) is over 10,000 rpm. I did not run it up that high but did reach full scale on the Mariah tach and also noticed this slight tendency to stick/stall at 4200. I also switched the Mariah tach to 6 cylinder setting and it was actually very close to the BMW tachometer readings so I don't think I've changed the calibration or response of the tach from my fiddling around. But if I remember to I intend to check it with a scope sometime out on the lake because I think it still hangs slightly at 4200 and 4300-4400 isn't enough more to make it budge.
So maybe it's just the tach itself. And maybe using it more often will free it up and maybe that is what you've experienced the last time out. I would expect a 4.3 with more or less the same boat size as mine and 50% more hP to easily hit 4800 rpm with the right prop and easily run closer to 50 mph. But others will know if that is what to expect.
Low RPM
Re: Low RPM
Re: Low RPM
