Need Advice Immediately! MPG Towing Nightmare
03-16-2011, 02:50 PM
#31
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BSFC curves on a B series engine are generally flat (practically speaking) from 1600-2100 rpm, the then rapidly degrade from there.
But there's more than BSFC difference that can explain a given MPG performance. Tire pressure, tire size, tire design, wind resistance, etc all factor in much more than the the differences between a couple thousandths of BSFC.
03-16-2011, 03:03 PM
#32
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Mine too.. the 6.7 likes the 1800 ish rpm or lower range empty for good mileage.
Interesting note in the testing here was how the mileage was slightly improved even though you had the truck downshift more. Lugging the engine down in higher gears doesn't give better mileage. Its more about what RPM the engine is most efficient and keeping it there for optimum economy.
Interesting note in the testing here was how the mileage was slightly improved even though you had the truck downshift more. Lugging the engine down in higher gears doesn't give better mileage. Its more about what RPM the engine is most efficient and keeping it there for optimum economy.
With the H&S Overdrive software, my Witech scanner is showing 165psi line pressure on the tranny....not sure what stock was?
Chris
03-16-2011, 03:39 PM
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Mine too.. the 6.7 likes the 1800 ish rpm or lower range empty for good mileage.
Interesting note in the testing here was how the mileage was slightly improved even though you had the truck downshift more. Lugging the engine down in higher gears doesn't give better mileage. Its more about what RPM the engine is most efficient and keeping it there for optimum economy.
Interesting note in the testing here was how the mileage was slightly improved even though you had the truck downshift more. Lugging the engine down in higher gears doesn't give better mileage. Its more about what RPM the engine is most efficient and keeping it there for optimum economy.
03-16-2011, 03:53 PM
#34
Chapter President
BSFC curves on a B series engine are generally flat (practically speaking) from 1600-2100 rpm, the then rapidly degrade from there.
But there's more than BSFC difference that can explain a given MPG performance. Tire pressure, tire size, tire design, wind resistance, etc all factor in much more than the the differences between a couple thousandths of BSFC.
But there's more than BSFC difference that can explain a given MPG performance. Tire pressure, tire size, tire design, wind resistance, etc all factor in much more than the the differences between a couple thousandths of BSFC.
03-16-2011, 06:58 PM
#35
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BSFC curves on a B series engine are generally flat (practically speaking) from 1600-2100 rpm, the then rapidly degrade from there.
But there's more than BSFC difference that can explain a given MPG performance. Tire pressure, tire size, tire design, wind resistance, etc all factor in much more than the the differences between a couple thousandths of BSFC.
But there's more than BSFC difference that can explain a given MPG performance. Tire pressure, tire size, tire design, wind resistance, etc all factor in much more than the the differences between a couple thousandths of BSFC.
03-16-2011, 08:19 PM
#36
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Cummins maps BSFC across the entire torque curve and different percentages of load. So, you might map BSFC at a 10% throttle sweep, then at 90%, then at 80% and so on.
Differences between curves are then interpolated to produce a 3-d response surface.
Differences between curves are then interpolated to produce a 3-d response surface.
03-16-2011, 08:50 PM
#38
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OK, I found some B-series BSFC data for a 2007 and newer engine. Not sure which calibration this would have been-- probably 08 ish.
Anyway, BSFC is lowest below 1800rpm. The complete BSFC map is "cummins confidential" so obviously I can't publish the data.
But it decays above 1800rpm. Keep it below there for best BSFC.
However, it's important to note that BSFC plays a very little role in fuel economy overall in the real world. BSFC always improves with load-- the more power the engine is asked to make, the more efficiencly it will make that power. Obviously, that's hollow comfort for MPG. If you are making more power, you are burning more fuel and killing MPG.
So the real trick to MPG is simply using the least amount of power you can. All of the suggestions in this thread can be summarized as means to this end.
JH
Anyway, BSFC is lowest below 1800rpm. The complete BSFC map is "cummins confidential" so obviously I can't publish the data.
But it decays above 1800rpm. Keep it below there for best BSFC.
However, it's important to note that BSFC plays a very little role in fuel economy overall in the real world. BSFC always improves with load-- the more power the engine is asked to make, the more efficiencly it will make that power. Obviously, that's hollow comfort for MPG. If you are making more power, you are burning more fuel and killing MPG.
So the real trick to MPG is simply using the least amount of power you can. All of the suggestions in this thread can be summarized as means to this end.
JH
03-16-2011, 09:25 PM
#39
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Thread Starter
I wanted to add a few notes on the RPM.
68mph = 1900rpm (approx)
70mph = 2000rpm (approx)
73mph = 2250rpm (approx)
These numbers come from only flat ground with load. Obviously with an incline they fluctuated a bit.
As far as the discussion about the downshifting: One thing I wanted to add was this.... On the lower tunes, where downshifting was frequent, the tranny temp increased quite a bit. On the higher tune, which netted slightly less mpg, the tranny seemed to stay cooler. I cannot tell you guys how much cooler but I want to say it was about 30deg. Yes, I have the transmission tuning.
Also to note, my truck never rolls smoke. I have never seen even the slightest smoke come out of my truck. (Don't know why I added this comment)
68mph = 1900rpm (approx)
70mph = 2000rpm (approx)
73mph = 2250rpm (approx)
These numbers come from only flat ground with load. Obviously with an incline they fluctuated a bit.
As far as the discussion about the downshifting: One thing I wanted to add was this.... On the lower tunes, where downshifting was frequent, the tranny temp increased quite a bit. On the higher tune, which netted slightly less mpg, the tranny seemed to stay cooler. I cannot tell you guys how much cooler but I want to say it was about 30deg. Yes, I have the transmission tuning.
Also to note, my truck never rolls smoke. I have never seen even the slightest smoke come out of my truck. (Don't know why I added this comment)
03-17-2011, 12:44 AM
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Late to the party and not trying to stir things up, but my '03 7.3L tows a 5K trailer all the time (it's my jobsite trailer, goes everywhere my truck goes nearly every day) and it gets 13mpg all day long.
Totally stock truck (250hp/525tq). 68,000 on the odo.
Totally stock truck (250hp/525tq). 68,000 on the odo.
03-17-2011, 07:00 AM
#41
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True, but a Dodge diesel of similar year and calibration will do about as well.
You can't meaningfully compare mileage from a 2g NOx engine to a 0.2g NOx engine calibration (2011 CTD). EGR vs non EGR, after treatment vs non aftertreatment.
It's apples and brain surgery.
You can't meaningfully compare mileage from a 2g NOx engine to a 0.2g NOx engine calibration (2011 CTD). EGR vs non EGR, after treatment vs non aftertreatment.
It's apples and brain surgery.
03-17-2011, 07:37 AM
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As an aside, a concrete supplier I am good friends with just bought a 2011 Dodge CTD to replace his '05. I asked him about it and he said it was a nice truck, but his '05 was more fun.
03-17-2011, 08:04 AM
#43
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OK, I found some B-series BSFC data for a 2007 and newer engine. Not sure which calibration this would have been-- probably 08 ish.
Anyway, BSFC is lowest below 1800rpm. The complete BSFC map is "cummins confidential" so obviously I can't publish the data.
But it decays above 1800rpm. Keep it below there for best BSFC.
However, it's important to note that BSFC plays a very little role in fuel economy overall in the real world. BSFC always improves with load-- the more power the engine is asked to make, the more efficiencly it will make that power. Obviously, that's hollow comfort for MPG. If you are making more power, you are burning more fuel and killing MPG.
So the real trick to MPG is simply using the least amount of power you can. All of the suggestions in this thread can be summarized as means to this end.
JH
Anyway, BSFC is lowest below 1800rpm. The complete BSFC map is "cummins confidential" so obviously I can't publish the data.
But it decays above 1800rpm. Keep it below there for best BSFC.
However, it's important to note that BSFC plays a very little role in fuel economy overall in the real world. BSFC always improves with load-- the more power the engine is asked to make, the more efficiencly it will make that power. Obviously, that's hollow comfort for MPG. If you are making more power, you are burning more fuel and killing MPG.
So the real trick to MPG is simply using the least amount of power you can. All of the suggestions in this thread can be summarized as means to this end.
JH
Kinda tells me why I can still get 15mpg @ 100mph
And from what you're telling me HOHN, I made the right choice with 3.42's and the tall OD of the Auto!Chris
03-17-2011, 10:20 AM
#44
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Absolutely you did. It takes X amount of power to move that load under a given condition. The engine makes that power with different levels of efficiency at different points in the RPM range. It is most efficient beneath 1800 rpm.