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Old 05-21-2012, 08:36 PM   #29
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Here's a technical paper from Cummins:
Secrets of Better Fuel Economy

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Old 05-21-2012, 08:49 PM   #30
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This is bizarre. To read this with any expectation of learning anything is totally futile. Just drive your unit at whatever speed makes you feel secure. Absolutely nothing else matters. An Allison trans will do all of the thinking for you. So you get an extra mpg...if that is an issue with you you should have never bought a motorhome in the first place. Maybe a chevy volt would make you happier. Jeez...just do it. This is not rocket surgery.

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Old 05-21-2012, 10:45 PM   #31
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Originally Posted by Mike Canter View Post
Got hold of my mechanical engineering friend at the diesel manufacture and this is what he gave me.
"Engine's sweet spot is the RPM where the brake specific fuel consumption (BSFC) is at a minimum" It is a real term. Based on that definition I did a further search on the internet and found this which puts it more in laymans terms.
The Sweet Spot
OK, read that article. Didn't quite understand all of it.

If the sweet spot is when the BFSC is at a minimum, that presents a problem for anyone with an RV. The article states that point is determined by the engineers. Where do you find that point in your RV owners manual.

The General Performance Data for my engine states that BSFC is measured in LB/BHP-HR. From 1200 to 1800 RPM the BSFC is 0.31 and from 1800 to 2100 RPM it is 0.32. So, that bit of info is just about useless.

The article further states "Then they(engineers) determine the rpm range the engine should be driven. For some, itís 1,200-1,400 rpm. For others itís 1,350-1,550, and so on. Your ownerís manual should list the ideal range for your engine. Roughly at mid-range is the sweet spot, where youíll get your best fuel economy". RV owners do not have access to that info.

Engine torque curves, that I have seen, reach a peak and remain there for a couple of hundred RPMs then start to drop. On My engine the torque is almost flat from 1450 LB.FT. @ 1200 to 1428 LB.FT @1500 RPM then starts to drop. So in conclusion, I would think operating at the lowest RPM on the flat portion of the torque curve would result in the highest MPG. Of course it goes without saying you must be in the lowest ratio gear. I didn't see anything in the article that disputes this conclusion.

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Old 05-21-2012, 11:14 PM   #32
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Pairajays......I think you might be looking for too technical of a description of a "Sweet Spot". As an example:

My boat use to CRUISE (reasonable speed to get somewhere), bow up, smooth ride, with best fuel consumption (verified by a fuel flow meter) at about 32 mph. It was burning 7.2 gallons an hour of fuel. If I slowed down, the bow dug in and fuel consumption went up. If I sped up, fuel consumption went up. 32 mph was my "Sweet Spot".

My old Class C had a "sweet spot". At 62 mph, it cruised smmoothly, got reasonable mileage, and didn't struggle on slight inclines. I could almost feel it in the seat of my pants running at it's optimum at that speed.

Even though everyone is talking about a pretty precise rpm range. The "sweet spot" can also include a speed/rpm where the coach just settles in and runs smoothly along. That can include the rpm running at it's optimum (as described by others) wind resistance just right, and relatively level and smooth terrain.
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Old 05-22-2012, 02:14 AM   #33
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I have yet to see anyone mention the speed/rpm's that I am seeing which was the initial intent of this post. It would seem obvious to me that if I need 1800 rpm's to achieve 62 mph and everyone else is achieving that speed at 1450 rpm's, it must be the 4:88 rear end and 70 tires. I would think I would get better mileage at 1450 than at 1800. If I can increase the mileage from 7 to 8 mpg, I would gladly take the approx 15% in fuel savings. Might not mean anything to some of you, but it means something to me.

Again, thanks to all the responses (save one) and I think I might know a little more than when we started this thread.
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Old 05-22-2012, 06:01 AM   #34
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The lower RPMs of around 1450 at 62 MPH can only be achieved by larger displacement high torque motors. This is one of the main reasons for these big motors. For my Detroit Diesel to do this I have a displacement of 14.1 liters or 855 cubic inches. Your higher rpm of 1800 maybe perfectly correct for the RPM range of your motor. My max HP is at 1800 rpm and my max torque is at 1200 rpm and my max rpm is only 2000. What I am trying to say here is that I believe your peak RPM is probably way above mine or a Cummins ISM-500, ISX 600 or a CAT C9 or C12. If your lower your RPM and try and maintain the same speed then your fuel consumption may go up because it is not making the HP it needs to push that weight through the air at 62 mph. You can see this problem occur in Jeep Wranglers if one installs large diameter tires. The motor slows down in rpm so one is below the power curve on the higway and the mpg gets worse
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Old 05-22-2012, 06:05 AM   #35
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The whols sweet spot thing is overrated, while i understand the concept I drive to suit conditions.
1st class "C" ( 460 ford) ran real nice @ 62-63 best ride quality and best handling
1st class "A" 84 coachman (chevy 454) was about the same
2nd class "A" (ford v10) liked to run around 64-65 for ride and stability/ease of handling
3rd class "A" workhorse 8.1 chevy liked to run 65-66 for what felt right
HR DP neptune35, ISB 340 likes 64-67 depending on road for best ride and engine operation (feels good & solid/steady) approx 1850 rpm in sixth.

I read most of the post and next time out I will probably try 62-64 and see what happens.

Cummins says my motor is most efficient @ 1900 -2400 rpms but that gets me to 70mph starting and I will not run in 5th gear unless climbing etc.

Find the spot where you are comfortable (can relax a little) and the MH feels best/good/most stable to cruise in and that is your sweet spot!

dont worry about it, find your comfy spot and enjoy! a gain/loss of 10-20 minutes on 4-6 hour trip is nothing.
Relax and enjoy

Mike B
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Old 05-22-2012, 06:47 AM   #36
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Mike I totally understand what you are saying but without us realizing it the designers of these mhs put the correct gearing and tire diameter to make the engine sweet spot to be in that 62 to 64 mph region. So we are already driving in the correct range. Yes, I agree that there is also a "Comfort Zone" for a motorhome where you feel comfortable driving at but that is not the purpose of this discussion. Some of us are just interested in what the engine "Sweet Spot" is and how it is derived. It is educational for some of us.
Mike Canter
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Old 05-22-2012, 08:29 AM   #37
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As luck would have it, my aluminum wheels are 7.5" so the idea of an upsize to a 80 tire goes by the wayside. Guess I'll live with the original size. Thank You for your information.

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Old 05-22-2012, 10:05 AM   #38
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This whole obsession with RPM is a bit silly. Sweet Spot, as used in marine terms, is the RPM at which you get the best speed per Gallons Per Hour (GPH) of fuel consumption.

In a Motorhome, it would seem to me that GPH @ MPH would determine Sweet Spot. On a windless, flat road, the speed at which I get my best mileage is the sweet spot. Unfortunately, that might be at 35 MPH, so I have to sacrifice some MPG to get where I'm going in a timely fashion. Thus, MY sweet spot would be determined by the rate I'm comfortable driving at the fuel consumption I'm comfortable paying for. An MPG meter would be much more important than a Tach to determine the sweet spot. Using the two gauges together ensure the reduction in stress on my engine and wallet.

Notice I didn't use torque or horsepower in my discussion. An engineer will use them to determine drivetrain gearing to design an efficient coupling of a particular engine to it's use. We drivers need to understand where our engines develop peak torque and horsepower to select best gear and speed for the terrain, but it doesn't have a lot to do with sweet spot.


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