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Old 08-13-2006, 10:44 AM   #1
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Go to Cummins Power Spec and you will find a web based and down loadable file. The both do the same thing. Choose "engine access tool" to download the tool to your P/C. Choose " vehicle gearing" to run the tool via the web site.

Answer the questions and you will receive a summary report of the engine RPM to produce the best fuel economy or power for a given speed. For me, the most interesting part of the report is the #3. #3 gives the minimum RPM, for the engine at the cruise speed you provided. They even put the minimum in <span class="ev_code_RED">red</span>
letters.

By down loading the tool you can get the same report without going online. One could impress folks at the next rally!
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Old 08-13-2006, 10:44 AM   #2
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Go to Cummins Power Spec and you will find a web based and down loadable file. The both do the same thing. Choose "engine access tool" to download the tool to your P/C. Choose " vehicle gearing" to run the tool via the web site.

Answer the questions and you will receive a summary report of the engine RPM to produce the best fuel economy or power for a given speed. For me, the most interesting part of the report is the #3. #3 gives the minimum RPM, for the engine at the cruise speed you provided. They even put the minimum in <span class="ev_code_RED">red</span>
letters.

By down loading the tool you can get the same report without going online. One could impress folks at the next rally!
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Old 08-14-2006, 06:20 PM   #3
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Thanks for the tip, Gary. I tried downloading Power Spec and following your instructions. In the end, I got what I believe is a strange answer. I tried multiple changes in the parameters and it always came back with the same answer.

I have an 1999 vintage ISB. When I plugged in the speed I wanted to go and asked for best mpg, it told me 2100 RPM and not lower than 1900 RPM. At 2100 RPM I'm doing about 73MPH and am getting around 9.0 MPG based on almost 5,000 miles of on the road experience with my rig.

If I drive around 1700 RPM, I'm around 56MPH and I'm getting around 11 mpg, give or take a little for the terrain. To get it up to 2100, I would have to be running most of the time in 4th gear. Somehow, that doesn't seem like what the engineers would have had in mind.

Mine is a lowly 5 speed Allison 1000 so I cannot believe that the transmission gearing is a factor.

Now if I had asked it about the power range, I could see the answers that I got but I asked about MPG.
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Old 08-15-2006, 06:23 AM   #4
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You are correct, the report for my ISC makes only some sense to me. This is my first diesel. I am not sure my skills with gas rigs is always correct with a diesel. My report said:
1. at 60 mph the RPM should be between 1900-2100 with 1900 RPM at the 60 mph point. Cruising at 60 mph at an RPM below 1800 should be avoided. <span class="ev_code_RED">Printed in red.</span>

2. at 65 mph my RPM should be between 1900-2100 RPM with 1950 RPM at the 65 MPH point. Cruising at 65 MPH below 1800 RPM should be avoided. <span class="ev_code_RED">Printed in red.</span>

I can make the above happen by keeping the Allison transmission in 5th gear and not letting it go to 6th gear. I am uncomfortable doing this. Maybe those with years of diesel experience can jump in here.

My skills tell me the best place for an engine to opperate (at highway speeds) is right after the transmission shifts into its' highest gear. This is the best place for torque and fuel economy. For my rig the Allison goes to 6th gear a 50 MPH and about 1600 RPM. 1600 RPM is the top end of my torque curve. I do not feel comfortable cruising at 50 MPH. With my ISC this puts my 60 MPH RPM at 1750. At 65 MPH is is 1800 RPM.

I called Cummins and they explained why the higher RPMs are more economical and better for the engine. There is no way I can explain what they said. Consider calling them and get the explanation directly from the source.

At this time, the bottom line is that Allison and Cummins both have computers in them and are supose to be a smart combination making an efficient power plant. At this time, I will stay letting the Allison tell me what gear it should be in for the given situation.

This is the edit update. The summary report at all cruise MPH below 60 MPH has me keeping the ISC at 1900 RPM. Cummins must figure 1900 RPM is the sweet spot for this engine.
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