Fuel Burn on 04/05 390 Sundancer with 8.1s

Let's face it, if you were being a smart-a$$, *I* of all people wouldn't really have a license to complain about it! :D

That mechanical incident happened 17 years ago, so it's not something I think about much. That engine runs perfectly, it just needs a little more fuel. The screenshot is clearly at the upper end of cruising speed for me, so the delta in fuel burn is more pronounced. It would be otherwise be hard to measure because the genny can only pull from the stbd tank.

I watch for changes in consumption though, for sure. IMO, it's the best early-warning indicator for all kinds of issues. Optimizing cruise speed and range is almost secondary.
 
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I agree that it's at least a very good case scenario, but I'd add an asterisk to it. Data points in only 500 RPM increments leaves a lot of questions. They're not putting much effort into finding the sweet-spot.

Even so, their results of .72 are at least in the ballpark. Did you find that before or after you tuned your props?
 
My trip back to the Housatonic River in CT from Block Island RI
 

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So, for someone in the market looking for a 38/40/41 Sundancer, early 2000, I was really hoping for a boat with the 8.1L's because I thought they were a much stronger engine capable of delivering better fuel economy and performance than the 7.4L's. But based on the data provided, my 40 Sundancer with the 7.4L's perforrms very similar, if not better. Does anyone disagree? And I watched a video from Mercruiser about how the 8.1's were supposed to run at higher RPM's, like close to 4000. That explains the poor MPG's in the 0.5/0.6 MPG range.
 
midexp said:
So, for someone in the market looking for a 38/40/41 Sundancer, early 2000, I was really hoping for a boat with the 8.1L's because I thought they were a much stronger engine capable of delivering better fuel economy and performance than the 7.4L's. But based on the data provided, my 40 Sundancer with the 7.4L's perforrms very similar, if not better. Does anyone disagree? And I watched a video from Mercruiser about how the 8.1's were supposed to run at higher RPM's, like close to 4000. That explains the poor MPG's in the 0.5/0.6 MPG range.
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I doubt that any of the v-drive SeaRay cruisers of this size with the 8.1's should be cruising at 4,000rpm.
 
The way I see it the 8.1 engines if you actually read the spec on them the peak of the torque curve is about 4000 RPM to get the most efficiency out of your motor that’s exactly what you’re supposed to do run it at the top of the torque curve I researched all of this before I re-powered my boat with 7.4 the peak For torque curve for that is around 3000 RPM but I tend to run my boat around 3300 I find that’s the best number for the angle of the boat without the tabs . All of this has to do with what type of heads are on the engines that’s what creates the torque curve the 8.1 motor has very large retangle port heads which peak around 4000 RPM I have the small oval port heads on my engine which gives tremendous low-end torque at low rpm RPM range Not really sure what the 8.1 and 8.2 MerCruiser’s were made for but I kind of think they should’ve just been putting them in go fast boats most cruisers seem to run in the low to mid 3k

Woody said:
I doubt that any of the v-drive SeaRay cruisers of this size with the 8.1's should be cruising at 4,000rpm.
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The 8.1S was designed to meet the torque demands of the diesel engine but at a much lower entry cost.
 
The recommended cruising rpm of the 8.1 is as follows:

Horizon 3,200 to 3,600 rpm
HO. 3,400 to 3,800 rpm

I seem to recall the peak torque of the Horizon is at 3,200 rpm.
 
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Woody said:
My 420DA was standard with 8.1's, 370HP, the Cummins was an option. The torque difference is 1,000+ in total.
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390's only came with optional QSB 380's per the factory PDF options sheet..
 
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