• If this is your first visit to the iboats.com Boating Forums, be sure to check out the FAQ. To post a question or comment, begin by signing up. To start viewing messages, select the forum that you want to visit from the selection below.
  • ALL iboats Forum Members and Guests:
    To show our appreciation for your participation in the Forums, we're offering an exclusive Forums-only discount good for an additional 5% off your iboats.com order for a limited time.
    To redeem, simply call our Customer Service Team at 800-914-1123 (Open M-F, 9am - 8pm Eastern Time) and mention the Forums 5% off discount. Offer excludes engines, trolling motors, and electronics.

Announcement

Announcement Module
Collapse
No announcement yet.

Fuel consumption vs. RPM

Page Title Module
Move Remove Collapse
X
Conversation Detail Module
Collapse
  • Filter
  • Time
  • Show
Clear All
new posts


  • Fuel consumption vs. RPM

    Now this may need to be redirected to the engine gurus, but here goes. In a carburated engine, at wot, when we see rpms decrease due to the increase in drop pitch, do we see a decrease in gph? I'm trying to go for fuel econ. in a prop selection. Thanks for replies.


  • #2
    Re: Fuel consumption vs. RPM

    Lug the engine and you use more fuel and the engine doesn't like it at all. Prop the motor for the top of the recommended wide open throttle RPM band with normal load. Best for economy and engine longevity. This is not a car where lower engine rpm = better mileage. Outboards cannot be lugged.

    Comment



    • #3
      Re: Fuel consumption vs. RPM

      So variances in rpm don't have an effect on how much fuel is drawn into the intake manifold?

      Comment



      • #4
        Re: Fuel consumption vs. RPM

        Yes, it does, but picture it this way...If you run a 25 pitch prop and at WOT (butterflies wide open) and engine is turning say 4000RPM, you have a high manifold pressure/low RPM situation.OK, let's say now we are running a 19 pitch prop at 5800RPM...we have less manifold pressure/high RPM.OK, now lets back down to 25MPH, the 25P prop is requiring the butterflies to stay at 3/4-full open to maintain that speed because it is way below the torque curve..the 19P, on the other hand, shows the butterflies to be at 1/3-1/2 open because we are in the middle (or close to) the torque range, but still at 25MPH. We have more RPM with the 19, but lower manifold pressure (some will say vacume) so we are doing less work requiring less fuel.

        Comment



        • #5
          Re: Fuel consumption vs. RPM

          Yamaha has some very nice fuel consumption data on their web site in the Performance Test section. Choose any motor (2 or 4 stroke) and any boat type that they've tested. View those tests and it should become apparent how fuel economy/RPM/load/prop pitch interact.

          Comment

          Working...
          X