Best MPG at full throttle?

[ZmOz]

I've FINALLY got a new powerhead coming for my boat. (Rebuilt '74 140hp inline 6) I was just reading through the repair manual I have, written sometime in the mid 70s. First it says "the maximum MPG is obtained that will just plane the boat at full throttle". That makes sense. Then it says "A given boat engine usually will get the most MPG at or near full throttle". Is that true? Sounds ridiculous to me...I would thinking minimum throttle to plane would be best MPG, no? <br /><br />Just wondering...never really done any MPG tests with my boat, but I try to keep WOT to a minimum for gas mileage reasons...

 

[JasonJ]

Re: Best MPG at full throttle?

All I know is that I get my best economy at 3800-4000. Anything beyond that and fuel goes away in a big hurry.

 

[Jack Shellac]

Re: Best MPG at full throttle?

The first statement dosn't seem to make a lot of sense and I definitely don't believe the second is true. Most larger motors on a planing hull will get their best milage at around 4500 RPM. We've all probably read a jillion boat tests and this is where it generally seems to work out.

 

[ThomWV]

Re: Best MPG at full throttle?

It sounds like an awful lot was lost in translation on that manual.<br /><br />I have a FlowScan so I know just how much gas my engine is sucking down. I can absolutly assure you that WOT is not where the milage is. My best is at about 4,200 RPM. By going from there to full throtle I gain about 5 mph and it costs me a doubling in fuel use from my cruise use of about 11 GPH to more than 20 GPH at WOT. So if the first 3/4 throtle gets me 11 GPH and right around 40 mPH and WOT gives me something like 45 MPH and burns more like 22 GPH it should be pretty clear where to run the engine.<br /><br />By the way, when can we expect that "Jawboneing" we were promissed to begin?<br /><br />Thom

 

[Solittle]

Re: Best MPG at full throttle?

OMC used to inform it's customers that the best fuel economy for any of their motors was WOT less 10%.

 

[swist]

Re: Best MPG at full throttle?

Look in any magazine or Powerboat Reports that gives fuel consumption vs engine RPM. The engine will still be lugging a bit at the absolute miminum RPM to stay on plane - so the best mileage is usally a bit above that (and better for the engine too). I've run a lot of outboards - it's always been somewhere between 3800-4300 rpm.<br /><br />And you know, once you get used to the feel and sound of your engine, you almost don't need a Floscan, you can feel when the engine is very happy - unscientific I know but other people have said the same thing.

 

[JasonJ]

Re: Best MPG at full throttle?

That is true Swist. My motor sounds very different when it is in its "happy area". I fall below that and it gets a more rattly raspy sound, in the sweet spot it sounds smooth and more quiet, and above that the shrieking of my wallet drowns out the motor altogether..

 

[BoatBuoy]

Re: Best MPG at full throttle?

I agree with swist and JasonJ. There is a sweet spot. I always thought that it was where the throttle increase called for full timing advance. Any throttle beyond that was diminishing returns.

 

[Bobby O]

Re: Best MPG at full throttle?

As a person who has had flow scan's on twin 1985 Merc in line 6 90hp and 1995 twin 225 merc carbed engines, and now twin 150 hp Honda 4 strokes, and loran and gps, just over planning speed by 25% of maximum will give you the best range. P.S. If you do not use flow scan's, you will never know. I'm hooked on them for the last 12 years or so.

 

[Ronald1]

Re: Best MPG at full throttle?

How does a flow scan work? Is this an elaborate or a simple device? Where do I get one and how much does one cost? Sorry to stray from the Topic. Thanks

 

[Dunaruna]

Re: Best MPG at full throttle?

Its a devise that measures the amount of fuel used, it gives GPH and when coupled to a GPS it will give you very accurate MPG. Its a foolproof way to find the 'sweet spot'.<br /><br /> http://www.iboats.com/mall/index.cgi?*******=093863251&keywords=flowscan&submit.x=27&submit.y=8 <br /><br /> http://www.iboats.com/mall/index.cg...man&current_category=navman&*******=093863251 <br /><br />Aldo