[ryker137]

If a person isn't concered with top speed or holeshots and would rather have improved MPG, what kind of prop would a person look at changing to?

My intial guess would be change the ratio in the outdrive and match it to a prop. I am sure that would loose top end speed but might allow the engine to turn less rpms with more load. Letting the engine torque work instead of RPMs.

But if a person just stayed with stock gearing what should a person look for with prop selection?

[WetWilly]

Hi ryker137, if your looking for just MPG, I recommend sticking with the stock prop. I find that in most circumstances, the factory prop gives a good balance between takeoff, top end and overall cruising. You want a prop that allows the motor to achieve the listed max RPM at WOT.

I suggest you try waxing the hull (less friction) and keeping the boat weight down and trimmed properly if you're looking for a few more MPG, these three things go a long way to helping get every last mile out of a gallon of gas.

I suggest you contact Mercruiser to find out at what RPM your motor makes the best mileage, and then you can determine whether you need to spend any money to squeeze out a few more MPG. Since boats typically rely on engine RPM alone to determine speed (no multi-gear transmission), the question is: At the best MPG, how fast do you want to go? This may determine what you will need or can do to achieve that speed.

Good luck,
WetWilly

[ryker137]

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If changing the gear out isn't that costly or labor intense - dropping down to the 1.47 ratio like the v8's have would be nice for top end and perhaps MPG or GPH.

The 4.3l v6 is the same engine thats sold in S-10 trucks. So there are high performance intakes, camshafts, and carbs that would allow the 4.3l to make almost as much torque in the upper RPMs as the weaker tuned 5.0l v8.

I've seen 4.3l trucks with 220hp - 240hp.

My overall speed would be average slighly over plane out - since that nets the best MPG in most cases. But it would be nice to outrun the rain o make a beeline to the restroom (for the wife).

[SEMIJim]

Originally Posted by ryker137

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If changing the gear out isn't that costly or labor intense - dropping down to the 1.47 ratio like the v8's have would be nice for top end and perhaps MPG or GPH.

Maybe. Maybe not. I don't know power-boats well, but I do know something of gasoline-fed internal combustion engines. Here's the deal: They have this thing known as "torque curve." It is an expression of engine torque as a function of engine speed. To make a long story short: Increasing gear ratio in an attempt to increase top speed or gasoline milage can actually decrease both, because the engine never gets to that sweet spot where it runs most efficiently.

Years and years ago I owned a little Yamaha Twin One Hundred motorcycle. It was under-torqued, IMO. So I dropped the gearing a bit. Not much. Just a few teeth. Not only did my low-end torque improve, my top end improved even more!

Originally Posted by ryker137

The 4.3l v6 is the same engine thats sold in S-10 trucks. So there are high performance intakes, camshafts, and carbs that would allow the 4.3l to make almost as much torque in the upper RPMs as the weaker tuned 5.0l v8.

Yes, but those things tend to decrease gas milage, IME.

Jim

[ryker137]

ICE operates at its most efficiency when under larger loads. Keeping that throttle body open and decreasing pumping losses while turning as few RPMS as possible. Which is why the 6.0l Corvette gets better MPG than a 2.0l Honda S2000.


correct?

The torque curve for the 4.3l is terrible for anything past 3500rpms. A better camshaft profile and intake design will offer a more flat torque band up to redline. With camshaft and intake there can be a 20+ft/lb gain over the stock profile at 3500+ rpms. I think the stock Mercruiser uses a single plane intake, so going to a double plane intake design is a huge improvement in top end torque.

[SEMIJim]

Originally Posted by ryker137

ICE operates at its most efficiency when under larger loads. Keeping that throttle body open and decreasing pumping losses while turning as few RPMS as possible.

Not quite correct. ICE operates at best efficiency when operating under the load for which is was designed and at some optimal RPM.

Originally Posted by ryker137

Which is why the 6.0l Corvette gets better MPG than a 2.0l Honda S2000.

Not on my planet, it doesn't . Less than 22 MPG for the 6.0L Vette, slightly more than 25 MPG for the 2.0L S2000. (These are combined average values.) (Ref: True Delta real-world results from surveys.)

Originally Posted by ryker137

The torque curve for the 4.3l is terrible for anything past 3500rpms.

Define "terrible." I'm reading it flattens-out at about 4000 RPM. IIRC, recommended WOT on these is 4800 RPM?

Nonetheless, it looks like you've researched it more thoroughly than I.

Jim