Seafari 25 performance

[cdavisdb]

Did an economy run today, 22 miles, F3 stainless props, flat calm, 3300 rpm, 2 adults and full fuel, 2.9 nautical mpg. I am a happy camper. That compares with 2.6 kmpg with aluminum D2s and similar conditions. My old Seabird, shorter, lighter, with the same size engine, did not do this well.

Octane rating apparently makes a huge difference in fuel economy in this engine. The knock sensor enriches the mixture if the engine starts to knock due to low octane fuel. My original fuel test was with mostly fuel from the last owner, who put high octane alcohol fuel in it. I had been putting 90 octane no alcohol gas in. The engine needs 93. Lower octane fuel gave me 2.3 kmpg with the stainless props and somewhere around 2 or less with Bahamian fuel, who knows what octane. Today's run was make with octane booster raising the octane to about 92.

Anybody else have similar results with low octane fuel? Whats the best way to raise 90 octane no alcohol fuel to 93 octane?

[Bushwacker]

. . . Anybody else have similar results with low octane fuel? Whats the best way to raise 90 octane no alcohol fuel to 93 octane?

Good info Connor! Hard to find 93 octane non-ethanol gas these days however! I would expect a richer mixture to hurt mileage. I've heard similar stories about better mpg when towing with premium, and I use premium in my truck when towing for that reason. One guy I know towed a ~30' camper with a V-8 Explorer, all the way from S. Fla. to Canada and back. Burned regular going one way and premium on the way back, and said he definitely got better mileage with premium.

Some octane boosters contain ethanol, so I'd stay away anything like that. I recently had to tow my boat about 30 miles and had about 1/2 tank of 87 octane regular in the truck. Didn't want to fill it up with premium just for that short a trip, so I got a quart of some stuff called "Turbo 108" at Pep Boys. It didn't contain any alcohol but it didn't say how much it would raise octane. Recommended dose was 1 oz/gal, so I added enough to treat 1/2 tank and it seemed to work, as it didn't knock at all in OD, which it normally will with 87 octane when towing. I'm guessing it must have bumped the octane to at least 90-91. It was $8.47 to treat 32 gal, which works out to a little over $.26/gal. Bought some 93 octane premium for the car tonight at local Hess station that was $.25/gal more than regular, so doesn't sound like it's any worse of a rip-off than the gas stations IF it gives a comparable octane boost. Of course there's always AV gas at 100 octane if you want to spend over $5/gal!

[Islandtrader]

Did an economy run today, 22 miles, F3 stainless props, flat calm, 3300 rpm, 2 adults and full fuel, 2.9 nautical mpg.

Nice information,

Just guessing here, but your burn rate per gallon is approx. 7.5 to 8 gph?

If that is true, it seems all 350 cid no matter what they are in @ cruise the burn approx. 8.

Soooooooooo we'll see what the stroker does in the next week or so.

[Blue_Heron]

If that is true, it seems all 350 cid no matter what they are in @ cruise the burn approx. 8.

Soooooooooo we'll see what the stroker does in the next week or so.

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

[cdavisdb]

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?

[Blue_Heron]

Blue Heron: 3000 rpm for best fuel economy? What tests were you looking at?

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

[cdavisdb]

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.