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Discussion Starter #1
Is it possible to get lower boost psi at higher speeds when just cruising down highway,but when in need of power it be there?I get 22.5-23 mpg at 60mph with 6psi on boost!!!I do alot of traveling and would like lower boost at 70-75mph!!!ANY HELP WOULD BE APPRECIATED!!!!
 

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I don't think you'll be able to lower boost psi at highway speeds unless theirs some kind of mod to do it is your intake baffle still in? Most of us on here have either a c.a.i. Or removed that baffle I've never had gauges with the baffle in but I'm sure it lowers it some .
 

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Lower boost pressure means less power, which means you'll have to give it more skinny pedal to keep the same speed, which means less mpgs for you more than likely.

That all depends though on BSFC of the engine at a given boost pressure and rpm.

You would literally be shooting in the dark and guessing. And why the hell would you want to change things up when you are already getting phenominal gas mileage?
 

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Discussion Starter #4
Well I know my mileage is good but I always want better!!!haha I can tell though at 70-75 mph I am at ab 9-10 psi of boost and mileage drops drastically!!not really complaining just woundering if its possible to get better mileage at 70-75mph than 14-15 mpg!!!!
 

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The mileage is dropping due to increased wind load which is why the motor needs more boost to make more power to push harder against the increased resistance. Gearing would probably help more than anything to get the motor in a sweeter spot if it isn't already. Take off one of your rear tires per side. That would help :)
 

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Discussion Starter #6
The mileage is dropping due to increased wind load which is why the motor needs more boost to make more power to push harder against the increased resistance. Gearing would probably help more than anything to get the motor in a sweeter spot if it isn't already. Take off one of your rear tires per side. That would help :)
These figures are with my truck empty not pulling the two trailers!!!im running 373 gears also!!!
 

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Well I know my mileage is good but I always want better!!!haha I can tell though at 70-75 mph I am at ab 9-10 psi of boost and mileage drops drastically!!not really complaining just woundering if its possible to get better mileage at 70-75mph than 14-15 mpg!!!!
The more boost you've got the higher your volumetric efficiency is. Therefore, more boost is good for economy. Jomobco's right, your problem is aerodynamics. Drag is an exponential equation. When speed doubles, drag quadruples (or there about)!!!
 

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Discussion Starter #8
The more boost you've got the higher your volumetric efficiency is. Therefore, more boost is good for economy. Jomobco's right, your problem is aerodynamics. Drag is an exponential equation. When speed doubles, drag quadruples (or there about)!!!
That's is true but im not pulling anything when im checkingmy milage!!!!
 

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It doesn't matter if you're pulling or not.

The speed difference is what matters.

It takes more power (fuel + boost) to push through the air at 70-75 MPH than it does at 60-65 MPH.
 

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Everyone's missing the biggest issue here.

CTD do excellent on fuel until you start creeping past the 1800 RPM mark. Once you cross that line (around 70 MPH) you're starting to dump fuel into the engine. Add that up with your increased drag, and walah! Instant fuel economy reducer.

On the freeway, I just set my cruise for around 1750 RPM. I average 19-20 MPG depending on the hills and prevailing winds. If I bump it up to 1850-1900 RPM my mileage drops down to 16-17 MPG.
 

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Everyone's missing the biggest issue here.

CTD do excellent on fuel until you start creeping past the 1800 RPM mark. Once you cross that line (around 70 MPH) you're starting to dump fuel into the engine. Add that up with your increased drag, and walah! Instant fuel economy reducer.

On the freeway, I just set my cruise for around 1750 RPM. I average 19-20 MPG depending on the hills and prevailing winds. If I bump it up to 1850-1900 RPM my mileage drops down to 16-17 MPG.
Eh.... not really.

If you're logic were completely true then if 1800 is good, 1700 would be that much better, and 1500 even better than that. Truth be told (and look in any physic books (that discusses aerodynamics) or aerodynamics book and you'll see that drag plays a much bigger role. At 1900 RPM sure there is more gas (only because there are 100 more cycles per min than 1800), but there is also more POWER, thus the reason the formula for HP is a function of time. Take aerodynamics out of the picture (for example, running the truck in a test cell) and you'll see the mileage and RPM are pretty linear. However, when you run the truck above about 60mph (coincidentally about 1800, which is where the engineers designed them to run), you'll see the mpg getting exponentially worse.

Revolutions per min are a factor, but only a SMALL factor in the whole equation. Another example, take a truck runnin 4.10s and one running 3.43s. If gearing and rpm were the biggest factors you would see a more substantial mpg difference in them at the same speed than you would running each truck at 60mph and then 80.

That being said, when making a blanket statement that 1800rpm is the best, you are forgetting that rear end gearing also has a say so in the optimum RPM.

Just some food for though.
 

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Well I know my mileage is good but I always want better!!!haha I can tell though at 70-75 mph I am at ab 9-10 psi of boost and mileage drops drastically!!not really complaining just woundering if its possible to get better mileage at 70-75mph than 14-15 mpg!!!!

If your serious about getting better mileage at those speeds look at a gear vendor overdrive unit. They are pricey but they will get your rpms back down and your mpg back up.
 

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Everyone's missing the biggest issue here.

CTD do excellent on fuel until you start creeping past the 1800 RPM mark. Once you cross that line (around 70 MPH) you're starting to dump fuel into the engine. Add that up with your increased drag, and walah! Instant fuel economy reducer.

On the freeway, I just set my cruise for around 1750 RPM. I average 19-20 MPG depending on the hills and prevailing winds. If I bump it up to 1850-1900 RPM my mileage drops down to 16-17 MPG.
My auto runs at a lower rpm at 75 mph than my stick. 1900 vs 2100. My auto gets worse mileage. My auto is a long bed which is (heavier) the only difference other than the intercooler which I think changed from 07-08. Both are 4x4, SRW, 6.7, sliding bed covers, same side steps, same tire type and size, etc.
 
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