Fuel consumption: set throttle, lower RPM or higher RPM?

[STV_Keith]

OK, so without the flo-scan, I'm trying to figure out where to cruise my boat. If I go down the lake and set the throttles at a fixed spot, it will cruise at, say, 3600rpm with the jackplates all the way down. If I go up on the jackplates, RPM goes up and so does speed, but I haven't changed throttle position.

With more RPM, does that also mean more fuel consumption, or because the throttle plates are at the same setting and the RPM is up, the vacuum should be lower (more negative) so it shouldn't be drawing on the carbs as hard.

FYI, 3600rpm with the plates down is about 39-40mph. With the plates up I can gain about 600rpm or so and 5mph. What I'm trying to figure out is if the fuel mileage is better with the plates up and more RPM or the plates down with less RPM.

Any thoughts/suggestions?

Remember, these are carbureted V8's. :)

[Darkside's Calling]

I believe that the speed increase is from reduced drag by raising the motor, and more speed also means the boat is riding higher in the water, should be more efficient as it would be running at a reduced load with higher vacumn.

[bigboy]

I believe that the speed increase is from reduced drag by raising the motor, and more speed also means the boat is riding higher in the water, should be more efficient as it would be running at a reduced load with higher vacumn.


I agree!!!

[T-REX]

:iagree::iagree:

A unloaded mota will git better fuel economy than a loaded mota...anykind!!...

az long az your rpm rise, AND your speed increases, U are still use'in tha HP to push the boat forward and not just raising tha props out tha water, and waste'in fuel....

I find that ever mota's "Happy place" is differnt...Props, geers, weight, hull shape all play tha big deel az ta wher tha "Best rpm" iz for best effeciency, which translates to best economy!!!....Juss my unprofessional opinion!!

Tha best ecomnomy for a pair ov V-8's would be ta leeve it in tha shed, under tha cover!!!!:eek::eek:......Sorry....I couldn't hep myself:D:D

[STV_Keith]

I believe that the speed increase is from reduced drag by raising the motor, and more speed also means the boat is riding higher in the water, should be more efficient as it would be running at a reduced load with higher vacumn.

That's pretty much what I was thinking...thanks for validating my logic. :) I will play with this this weekend if I get any clean water.

Rexy - I figured you biased Chizzler guys would say something like that! :D

[T-REX]

Rexy - I figured you biased Chizzler guys would say something like that!


Sorry...I'm juss an ole *******...I practice a lot...But I am good at it!!..Lol....:thumbsup::D:D

[Instigator]

the old Sonic. Was 5 - 10 MPH's in trim/height at same throttle setting.
Have been shopping for fuel flow gauges on e-bay for new boat.
Can get a set up for twins for a couple hundred bucks and figure I can make that up in the first few tanks of gas.

As to what rev range to target, I believe I've read that optimum is at torque peak which on stock OMC V-6's (and probably 8's) are around 3500 as I recall and you would have that on your 8's from Montys dyno sheets.

[j_martin]

I think carbs have a vacuum controlled "economizer" or "power" valve that richens the mixture when the vacuum goes down, The sweet spot is just before that valve opens.

Back in the good ole days, I used to greatly improve the midrange economy of a quadrajet by clipping a coil or two off'n the spring that opens that valve, in this case a mixture needle in front of the bores, thus moving the sweet spot up the torque curve almost to the point where it didn't feel good, but not quite.

I'm sure it raised cylinder pressure and NOX, but I really didn't give a chit. It ran good and didn't break the bank at the petrol stop.

hope it helps
John

[Riverratt]

I just got a new Consumers Marine catalog and they have some cheap fuel flow meters in there for around $150.00 includes 2" gauge and the sensor. I am considering one. Did anybody ever try the cheaper ones?

[GTO GEORGE]

More rpm means more fuel. You might go farther, cuz your going faster, but you'll use more fuel.

[STV_Keith]

For those looking at fuel flow meters...take a look at the Lowrance LMF-200 gauge. It can do two motors, looks at flow, measures GPH, etc. It's bus can be connected to a Lowrance GPS/chartplotter and then you can get MPG as well.

[HighVelocity]

The true test of economy on a boat is being able to maintain a given speed,,,,, and being able to pull back on the throttles through trim, etc.

So if 30 mph = 3000 RPM
40 mph = 3500 RPM
50 mph = 3700 RPM--I would stay here to cruise
60 mph = 4500 RPM
70 mph = 5800 rpm

[kimswang]

Keith, have you figured an RPM that will get you the best economy? I will follow this thread with great interest as it is 85 miles from my house to the fishing spot here and needless to say I am burning a ton of fuel... On the average I am getting about 0.4 miles to the gallon. A comparible boat with trip Sukis is getting 1.84.... I know the first step is to raise my outers and get them dialed in....