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RPMs at 60 MPH?

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New to me 2004 Dutch Star model 3809 on a Spartan chassis with 370 hp Cummins / Allison 6 speed.

I keep reading about people running 62-63 mph at 1650 rpms. They keep talking about 1650 rpms being the sweet spot. I am not seeing anything close to that as I see 1800 at 62 mph according to my gauges. I did not have the trans in "Economy Mode", but I don't think that changes the rpms.

I read on another forum where poster claimed they had to get to 68mph to shift into 6th gear and then could back off to 65 mph. If that is the case, I never got to 68 as I'm of the 60-62 mph comfort zone type of driver. Perhaps I was in 5th gear? I will call Spartan on Monday to see if they can tell me what rear end ratio I have.

Am I the only one looking at those kind of numbers? What is your targeted rpm for maximum economy?

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I have to agree with the 1650 range and about 63MPH. I have found on the C7 Cat. 330 HP on my 2005 Fleetwood,Freightliner Chassis with Allison 6 speed that this RPM works best on 4% to 5 % grades and is where I get the best fuel economy. Less shifting down as long as the grades aren't very long. I do use my Cruise Control a lot but find that in hilly areas Cruise Control doesn't seem the best for economy.

Guido1942

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Quite a lot of "sweet spot" RPM depends on the engine. Normally it is 100-200 RPM above your engine's PEAK TORQUE RPM. You can look that up for your ISL.

And gearing involves tires (expressed in revolutions per mile) and rear axle ratio. Most rear axles have a tag on them with the ratio stamped on them.

The transmission is not a factor as all Allison 3000 series have the same ratios with 4th being 1:1, 5th being .75 and 6th being .65.

The formula for RPM/MPH is:

(RPM X 60) divided by (rear axle ratio X transmission ratio X tire revolutions per mile)= MPH

Get the tire revolutions per mile from your tire manufacturers spec sheet (also on line).

The smaller the engine, the higher the peak torque RPM, so comparing engines of different sizes can be misleading.

Brett

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The peak torque RPM should be on the engine dataplate (if same as ours, the dataplate is on the side of the engine opposite the exhaust side). I could not get to the one on our 330, so called Cummins to confirm and the peak was 1400+ (don't remember exactly). Mine is an ISC and 2000 model year chassis. I have heard that later model years run higher RPMS, but thought it was later than 2004.

If you call Cummins with your ESN they can tell you what the tech who bolted the exhaust manifold on had for lunch the day he did it. :)

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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

__________________

This article gives the engineered Sweet Spot for a lot of motors. The motor doesn't know what chassis it is sitting in so a DD S-60 is a DD S-60 in a mh or a truck.

http://www.truckinginfo.com/hdt/archives/2007/06/068a0706.asp

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Mike,

Great links.

There IS a difference, though between an 80,000 pound truck with the same engine as your 40,something or 50,something pound motorhome. Because of your "light weight", you can run lower RPM for a given road speed/remain in the "sweet spot" at higher speeds. Because with heavier weight, as mentioned in the articles, one has to compromise between economy and ability to pull the load.

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Brett, I don't want to get too technical here and bore people BUT the BSFC can only be computed on a engine dyno (We do this all the time with our big racing motors) and the best BSFC or the lowest fuel consumption versus the best HP can only be computed on a dyno which is the named the Sweet Spot. This best BSFC POINT (Sweet Spot) does not change depending on the weight of the vehicle. The amount of fuel used to maintain the speed based on weight at that lowest BSFC point may change but not the RPM of the best BSFC point. In other words if on my Detroit Series 60 the best BSFC vs HP is at 1450 rpm for a 80,000 lb truck it still remains at 1450 rpm with my 45,000 lb motorhome BUT the amount of fuel used at the best BSFC point is higher for the 80K lb truck than it is for the 45K motorhome.

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Thanks, Mike. Interesting information.

If you know of a source for the "sweet spot" on the common Caterpillar, Cummins and DD used in RV's over the last 20 years or so that would be a great information resource for all of us.

But, to make sure I understand this, the sweet spot is that RPM at which the engine produces the most HP/gal of fuel-- correct.

Depending on gearing, that may not be the most efficient RPM/road speed. I am thinking of a lot of coaches with low numerical rear axle ratios that don't get into 6th gear until 65 MPH or so. So, if their "sweet spot" puts them at 72 MPH, the "sweet spot" may not give the best MPG. Please correct if this is not correct.

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•That link I included above gives the Sweet Spot for most big motors but one has to be very careful because they make numerous versions of the same engine with different power ratings and Sweet spots. You are 100% correct in that one's MH may not be able to be driven at the Sweet Spot because of other influences. I believe that most mh owners drive at the point their mh's just shift into 6th gear. Mine will shift in at 58 mph and will stay there if I am in the Economy mode and 62 mph just happens to match my Sweet Spot RPM so that is where I drive. The biggest contibuting factor to MPG is the drag coeificient of the MH and the second biggest factor is the person behind the wheel. So yes again the Sweet Spot may not be the most efficient point in which you can drive because of these other factors. If the MH designer that decided on the overall gearing and engine RPM did not take into account the Sweet Spot then that presents a problem. I also find that smaller engines run at higher RPMs to make power which also make it hard to have the Sweet Spot at the average 60-62 mph point.

Brake Specific Fuel Consumption (or BSFC) is the ratio between the engine's fuel mass consumption and the crankshaft power it is producing. So yes, the motors Sweet Spot is the point where the BSFC is the lowest which is the point in which you make the most HP with the lowest Gallons Per Hour fuel flow.

A lot of people confuse Sweet Spot of the engine with the Comfort Zone at which speed they feel good driving the MH at.

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Mike, et al:

Here is another "good read" on fuel consumption and performance in a motorhome-- Written by Caterpillar Corp, but physics is physics, so the information applies to all heavy vehicles-- gas or diesel. It helps set reasonable performance expectations and covers many of the factors affecting fuel MPG.

http://www.catrvclub.org/PDF_Docs/Understanding_Perf.pdf

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