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Peon Extraordinaire
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Discussion Starter #1
So my question is fairly simple. Which one hurts mileage more EGT or RPM? I drive 37 miles one way to work and am able to cruis for 20ish miles at 50-60MPH. I have noticed that doing 55 in overdrive my egt's sneak up above 600 sometimes up around 900. I will be cruising at 1500 RPM. I can lock the OD out and my EGT's drop to ~450 but my RPMs jump to ~2300.

So which is better:

the low EGTs with higher RPMs

Low RPMs with Higher EGTs?

Sam
 

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IMO if you're driving light, Rpms should make a bigger difference.
I base this on No scientific data, comparison or thought. Just an opinion.
 

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Yeah, personally, I'd feel that higher RPM's are going to hurt.

I feel that way for a couple of reasons. First, friction. Second, the aerodynamics of moving parts. I would think that those items increase exponentially as RPM's go up.
 

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Genie144:

Why don't you do a test and tell us?

A couple of tankfuls with low RPM/high EGT note the MPG.
A couple of tankfuls with high RPM/low EGT note the MPG.

Not a tough test to do. Enlighten the masses.
 

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sounds like a good idea to me.
But you need a baseline first...
 

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Yup. Two weeks baseline. That also gives you a chance4 to standardize your driving.

Then two weeks one way (high RPM/low EGT)

Then two weeks the other way.

Don't sweat the fact it is winter. The percentage change is what you are looking for. Maybe repeat in mid-summer.
 

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as long as all the tests are done on winter fuel, it won't matter...
 

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Peon Extraordinaire
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Discussion Starter #9
Dave

Normally I would do the testing and report. A single tank load each way (100 gallons). But I won't be able to perform the testing for a month or two. I have some things I am tweaking with my fuel system that would make it impossible to establish an accurate baseline. Not to mention the validity of any data would be suspect with the change of fuel pump and additional filtration in the middle of it.

I was curious if anyone had done the testing - I know you always say keep heat down and RPMs low. I can see an argument for each being more important.

As soon as I get the fuel system locked up I will do the testing.

Sam

On edit - not to mention that my fuel additive choice (SVO) makes my truck less than an ideal test bed for the masses :D
 

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Junior Mint
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If it's not hilly run in OD for the best mpg in this case.

EGTs are higher but that is a function of less boost, not more fuel being used as you might have suspected.
 

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Based on some training at work , higher rpms will get worse mileage.

At a lower rpm you burn more fuel per rotation becuse it takes more tq to make the same hp it needs to go the same speed. But you make far less revolutions per mile.
If it takes a third more fuel at lower rpm per revolution to go 55, but you turn half the rpms you still gain. And it shouldnt take a third more , so you gain still more.

Make sense? As long as you are not sending a ton of raw unburned fuel out the tailpipe to do this it should prove true. I guess in an extreme situiation it might not work, but for a stockish truck Id bet money on it.

What makes your NOx emission higher? Higher rpms or or higher EGT?? (trick question, has absolutley nothing to do with this thread)
 

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What makes your NOx emission higher? Higher rpms or or higher EGT?? (trick question, has absolutley nothing to do with this thread)
Hmmm, I'm unsure, but I'd say higher EGT because of the burn of everything.

What do I win!!!...or lose... :D
 

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ahhhhttttt, Both increase Nox. Higher the cylinder temps are the more Nox it makes, more rpms produces it more often. Also-- the longer the cylinder temps are at that temp the more it makes.
 

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I tend to think RPM matters more, but since I have a stick shift and can keep RPM and EGT down at the same time, I tend to not think very much about which matters more. I could be wrong. Good testing is the best way to know for sure.
 

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RPM......


A diesel engine is governed. It takes xx amount of fuel to maintain XX RPM(throttle position) at XXX horsepower level. The higher the load(weight or head wind or up hill) more fuel(HP) it takes to maintain the speed desired so throttle positions change. Take the same TPS position in OD and direct at the same RPM, say 1900, the computer will deliver the same mm^3/stroke all things being constant. It's just that in direct you are not traveling the same distance that you are in OD.


As long as you're not blowing black smoke...leave it in OD and keep the revs below 2000rpm.
 

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RPM......


A diesel engine is governed. It takes xx amount of fuel to maintain XX RPM(throttle position) at XXX horsepower level. The higher the load(weight or head wind or up hill) more fuel(HP) it takes to maintain the speed desired so throttle positions change. Take the same TPS position in OD and direct at the same RPM, say 1900, the computer will deliver the same mm^3/stroke all things being constant. It's just that in direct you are not traveling the same distance that you are in OD.


As long as you're not blowing black smoke...leave it in OD and keep the revs below 2000rpm.
While I agree with the end conclusion here theres a small problem. If you are running 2500 rpm in drive at say 60 mph, and you shift to OD, it will most likely take more Throttle to maintain the same speed. Unless its an unloaded truck going downhill or something like that. Also, on an electronic controlled engine, TP is not the only thing controlling the amount of fuel. Far as that goes, not on a mech injected one either. Boost will also control the fuel somewhat too.

And because it DOES take the same amount of hp to move the truck xxx mph with yyy amount of load, then by reducing rpm --to make the same hp requires more tq to do it, so fuel delivery HAS to go up per revolution.
But I still agree that lower rpm's will get better mileage.
 

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I thought the op was talking about an unloaded commute to work. I never mentioned anything about maintaining MPH :confused: except where I said TP would change going up a hill(increased load) and MFD would change. I also know what all affects fuel on both mech and electronic injection systems.


Inferred throttle position......TP AD counts, RPM and MFD functions. 1900rpm with the same TP in different gears is not going to yield the same MPH, that's why I said he's not goind the same distance. 2500 to OD in my 6 spd would be no prob and would definitely have less TP input on the programs I was running when the truck was running. My stock program has a "value" of 8mg/[email protected] 1800rpm and TP AD counts 300(WOT is 1023). I say value because 8 mg is not what the injectors are injecting due to other mechanical mods. Anytime at the throttle position and that RPM the PCM will want that MFD.


Took me awhile to compare that graph and functions when I first looked at them....maybe I'm still confused on them.
 

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when pulling a load(wheather in the bed or behind the truck) the use of a lower gear to keep the engine in the power will increase mpg, but when empty luggging the engine as low as possiable will increase mpg...i have done this , i drive a lot(200-300)miles a day, so it is easy to compare.i can lug down to 1000-1100 rpm and gently pull away on almost any hill(in high gear).i know nothing about egt, but if your under 1100 it is safe...
 

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peterbilt'in
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not quite shure if this is an anwser.. but on my own driving...

650 miles almost non stop...

50-60 MPH gets me about 20 MPG and 15 hours of driving or so... almost 40 gallons of fuel....

driving 60-70 gets me 18 MPG and 34 gallons of fuel.... and about 10 hours of driving with little to no stopping as before.

at the end of the month ill be doing another test like that about driving, over the 650 mile trip. 1 tank of fuel... cruise controll.. and keep'n on keeping on

around 70 mph with my 02, 7.3 Auto, extended cab, 8 foot, 4x4 with tool box (level with bed rails) is the sweet spot. @ around 18.## mpg and making the trip in 10 hours.

-Rob
 

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Rob-
More data is always welcome, but your observations are mostly due to aerodynamics.
the EGT/RPM debate mostly comes in for driving around town and freeway entrances, where you're accelerating from a stop.
although, RPMs do play a factor in your MPG at freeway speeds too.
 
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