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Old 05-20-2012, 09:57 PM   #15
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This is articke below is pretty imformative on Sweet Spots for lots of OTR diesels. Ken it states that the CAT C9 Sweet Spot is at 1650 rpm. If so what speed is that in your MH? It backs up my Detroit Series 60 Sweet Spot being at 1450 rpm.The article states that trucks try and gear for the engine's Sweet Spot to be at 65 mph

Gearing For Fuel Economy
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Old 05-21-2012, 10:27 AM   #16
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This is articke below is pretty imformative on Sweet Spots for lots of OTR diesels. Ken it states that the CAT C9 Sweet Spot is at 1650 rpm. If so what speed is that in your MH? It backs up my Detroit Series 60 Sweet Spot being at 1450 rpm.The article states that trucks try and gear for the engine's Sweet Spot to be at 65 mph

Gearing For Fuel Economy
I started to read this article but gave up. It had too many words. Are you and this article saying the most efficient operating point of a diesel engine is something other than at the peak torque?

I have a performance data chart on my engine, 425 hp Cat C-12. It includes, among other things, the fuel rate (GPH) and engine torque (LB.FT) from 1200 to 2100 RPM. At 1200 RPM the engine torque is 1450.04 LB.FT and fuel consumption is 14.5 GPH. As the RPM increases, the HP goes up and torque goes down. HP peaks, 425BHP, at 1800 RPM and fuel consumption is 19.1 GPH. From 1800 to 2100 RPM both HP and Fuel consumption decrease and torque continues to decrease. At 2100 RPM, BHP is 410, torque is 1025.21 LB.FT and fuel consumption is 18.68 GPH.

Jim E
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Old 05-21-2012, 10:40 AM   #17
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I drive mine at 62 MPH which is around 1700 rpm's. Anything above that and the buffeting begins and the coach just start to sound different. At 62 it just sounds right and quiet so whatever you want to call it then it sounds good and feels good to me. I call it my feel good spot. OOOOOO Baby Baby!!
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Old 05-21-2012, 10:40 AM   #18
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So Ken at what rpm is your peak torque at and do you think you could efficiently drive it at that rpm? My peak torque is at 1200 rpm and my advertised Sweet Spot is at 1420 rpm which is 62 MPH. There is no way I can drive it at 1200 rpm. My torque curve is pretty flat but my transmission will not let me drive at 1200 rpm at a steady speed
My peak torque (Cat C9 400HP, 1100 lb-ft torque) is exactly 1400rpm. My coach runs 64 MPH at 1400 RPM, and it will sit there all day long. Absolutely no problem setting the cruise control and letting it work. In economy mode it will lug down to 1275 or so sometimes before shifting.

Ken
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Old 05-21-2012, 10:58 AM   #19
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This is articke below is pretty imformative on Sweet Spots for lots of OTR diesels. Ken it states that the CAT C9 Sweet Spot is at 1650 rpm. If so what speed is that in your MH? It backs up my Detroit Series 60 Sweet Spot being at 1450 rpm.The article states that trucks try and gear for the engine's Sweet Spot to be at 65 mph

Gearing For Fuel Economy
1650 for me would probably be over 70 mph. I don't generally drive at that speed so I've never spent any time watching. When passing I will get above that for short times.

I wonder if that article is comparing apples to apples. For example it talks about 10 and 13 speed transmissions in heavy duty aerodynamic streamlined trucks so I'd believe they mean OTR's at a conservative figure of 80,000 pounds. Compared to a flat faced MH turning a 6 speed automatic and 45,000 pounds.

I still have to believe that Roadmaster chassis did their research and Cat did theirs as to what would be ideal for a specific MH or a specific engine. Since my actual observations match their written words, I think I'll stick with 1400 rpm.
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Old 05-21-2012, 11:27 AM   #20
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Jim E and Bucks. The way I understand it torque is only helpful in accelaration. To maintain speed is a combination of HP and torque and climbing steep grades is all HP. Like I said there is no way the trucks operate at just peak torque and there is no way I can operate and maintain a constant speed in my mh at the 1200 rpm peak torque point. One thing is my transmission will not allow it and will downshift. Those performance charts are pretty hard to relate to actual performance on the road and are off a dyno run. That article is talking about heavier vehicles but that is all gearing because the motor remains the same. It doesn't know what it is pushing or pulling down the highway. The weight of the vehicle it is pushing or pulling does mean that to operate at a set speed requires more or less throttle/boost to hold it there so more or less fuel consumption. Bucks I am surprised that 1650 rpm would be 70 mphs with a C9 but I learn things everyday. So what RPM are you at at 62 mph in sixth gear? I understand another term for Sweet Spot is Cruise RPM for that motor that it likes to run at all day.
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Old 05-21-2012, 01:11 PM   #21
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I'm beginning to believe that "Sweet Spot" is an imaginary term. It is a term that is not definitive. We all drive at a speed that is comfortable for us. Some say they drive about 55 mph others say 65 mph. A lot have quoted around 62-63 mph. As has been mentioned before, the drive trains of OTR trucks and MHs are designed to be as economical as feasible to get the intended job done. MH chassis manufacturers have settled on a road speed of somewhere between 60 and 65 mph. That range is determined by state speed limits and fuel economy. I personally believe the most economical point is the speed necessary for the xmission to shift into the highest gear ratio. My MH with a 425 hp Cat C-12 will do that at about 55 mph. I am comfortable at that speed but i am also comfortable at 62 mph. Currently, I prefer 55 mph since I have no reason to get anywhere any sooner than what 55 mph will achieve. I travel mostly on the West Coast which means California and 55 mph is the speed limit for MHs in California. I have no reason to change when in Arizona, Oregon or Washington. The big benefit of this speed limit is better mpg.

That's my story and I'm sticking to it. Ask anyone who agrees with me and they will tell you the same thing.

Jim E
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Old 05-21-2012, 01:44 PM   #22
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I myself don't believe it is imaginary term. I believe it is there and we just don't understand how the rpm is derived. The term has been used for a long time and OTR even base their gearing on it and it falls right in place for my engine MH combination. I do have a close racing friend that is an engineer for a major diesel company and I will try and contact him and ask him if he can shed some engineering light on it. Right now he is off on a trip supporting that company in a far off land but he does check his email once in a while. Maybe if he doesn't know he can point me to another diesel type engineer that can help us.
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Old 05-21-2012, 04:16 PM   #23
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Planocat,
In my case the aluminum wheels were good for the tire change. Fender clearence was no problem. Speedo can be recalibrated at most Cummins shops with a laptop if you know the revolutions per mile of your selected tires.

Also check your wheels. If they are 8.25 inch, that appears to be recommended by Michelin for their 275/80's but it also lists 7.50 as acceptable. Spacing for the duals is 12.2 inches center to center. Perhaps your wheels are the 7.50 inch which may cause a spacing problem for the duals. You will have to do your homework.

With that said and done, if your wheels are 8.25 inch, you have 12.2 inch spacing on the duals and the fender clearence is the same for the '04 as the '05, I would not know why the 275/80 Michelins would not work. They work well for me, plus they ride better than the 70 series tires.
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Old 05-21-2012, 04:44 PM   #24
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The sweet spot is typically somewhere between peak torque rpm and peak horsepower rpm, with the peak torque end of the range being most common. Just where it falls will vary a lot depending on the engine torque/hp curve, coach weight and air resistance, transmission and rear axle gearing, tire rolling resistance, head/tail winds, and probably a host of other small variables.
According to CAT it's around 200 RPM past torque peak and probably much the same for our Cummins

On ours that would have been 1650 but with the Banks kit I'm not sure anymore, it widened the torque/HP curve so much.
I still run 1625-1675 and it's really at home there.
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Old 05-21-2012, 04:52 PM   #25
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I read about the most efficient speed for my 260 HP ISB with a five speed trans and also experimented. Both lead me to drive with the engine at the lower end of the maximum torque band. That means 55 MPH and 1600 RPM. I get 11 MPG and which is definitely better than driving 65 MPH.
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Old 05-21-2012, 05:33 PM   #26
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I myself don't believe it is imaginary term. I believe it is there and we just don't understand how the rpm is derived. The term has been used for a long time and OTR even base their gearing on it and it falls right in place for my engine MH combination. I do have a close racing friend that is an engineer for a major diesel company and I will try and contact him and ask him if he can shed some engineering light on it. Right now he is off on a trip supporting that company in a far off land but he does check his email once in a while. Maybe if he doesn't know he can point me to another diesel type engineer that can help us.
I certainly don't want an argument to start, I'm no mechanical engineer or anything else. But up to this point, no one has defined "Sweet Spot". That's one reason I think it is imaginary. I've done a little searching on the web and found nothing.

Jim E
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Old 05-21-2012, 06:12 PM   #27
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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
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Old 05-21-2012, 09:09 PM   #28
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From Mike Canter's, The Sweet Spot cite: "The engineers test, tweak, measure and test again until they certify the engine for release. Then they determine the rpm range the engine should be driven. For some, it’s 1,200-1,400 rpm. For others it’s 1,350-1,550, and so on. Your owner’s manual should list the ideal range for your engine. Roughly at mid-range is the sweet spot, where you’ll get your best fuel economy."

So this seems to agree with driving the Cat C9 engine at peak torque like the manual says to do? The manual repeatedly says the most efficient rpm is peak torque, the Spec sheet clearly says peak torque is 1400 rpm. This also agrees with someones speculation that MH manufacturers often spec for 65 mph. (at least in my MH it does)

62 MPH for me results in downshifting to 5th gear unless I'm in economy mode when it will lug down a little further before downshifting. Once it downshifts, if the cruise is set at 62 it will not shift back to 6th under normal conditions. I believe it does if the cruise is on, and we go down a hill without the engine brake on. If the engine brake is on it will go into downshift mode and never upshift.

Ken
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