Re: Seafari 25 performance
 

Just about all the engine tests I've looked at for Mercruiser 350/377 CID motors show best fuel economy at 3000 rpm and 7 to 8 gph. Haven't found anything on the stroker, but it should be pretty close to the 377. Put that prop on her and let us know what she does.
Dave

Re: Seafari 25 performance
 

If hi octane fuel makes that much difference, maybe AV gas isn't so expensive after all. I'll look for the Pep boys stuff. I've been using Valvtect Octane boost. MMT is the active ingredient, no alcohol, comes in quarts, the most price efficient thing I could find. Definitely worth it if it raises mileage like it seems to.

Blue Heron: 3000 rpm for best fuel economy? What tests were you looking at? What I've seen, boat tests of 350s, look more like 3300-3400. What's the difference?

Re: Seafari 25 performance
 

Connor,
Go to Mercruiser Engine Test and pick any of the boats under 30' with a 350 Mag MPI or a 377 Mag. Scroll down til you see the graph that shows mph, gph, and mpg. The graphs only have data points at 1K, 2K, 3K, 4K and WOT rpm, so maybe I'm reading too much into it. But just about all of them hit the sweet spot for economy at idle and then again at about 3000rpm. Your Volvo could also be programed with different fuel or timing curves. If you have real world measurements, they've got to be more accurate than these tests for your application
Dave

Re: Seafari 25 performance
 

Thanks Dave, good data source. Looks like those are consistent with my memory of boat tests that examined smaller rpm increments, like 200 rpm. 3000 is better than 4000, but peak efficiency lies somewhere between 3 and 4. I'm remembering usually about 3300, but that might not be exact. Looking at my vacuum readings, my boat might be best at slightly below that.