EGT as it relates to MPG ???

[Mick2500]

I'm getting acceptable (17-18MPG) on my 24V ETH 6-Speed but I was wondering about where to cruise for the best mileage. I just did a quick 1,000 miles to CA and back this past weekend. Got 17. 3 going over and 17. 8 coming back. I was running 78-82 about 90% of the time and my boost was running above 10psi at all times. My EGT was 550-600 degrees ([post turbo) most of the trip.
I've noticed that if I run in 5th gear (direct), I can keep the EGT's down around 425-450 degrees. If I run in 6th at the same speeds, the EGT climbs to 600'ish.
Is the best mileage obtained at low EGT regardless of the boost psi?

Thanks in advance for any wisdom on this subject.

Mick2500
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2001. 5 2500 ETH/DEE, QuadCab, 4X4, SB, LSD, 3. 54, SLT+, Sport, Intense Blue PC/Agate, Tow Package, Camper Package, 'DC' Bed Cover, Sliding rear window. Totally STOCK!!
On Order 4/28/00
Arrived 9/14/00 !! #ad

My Truck

[This message has been edited by Mick2500 (edited 01-15-2001). ]

 

[Mr. B]

stock truck 75mph 550* 22. 5mpg overdrive
double overdrive 550* 22. 5mpg

stock truck 85mph 600-625* 20mpg overdrive
double overdrive 700-775* 17. 5mpg

Milage kept under 600* city/hwy 14/22 (16/21)
600-950* 12/20 (adv. per tank 14/16. 5)

Information is based on 99 auto 3. 54 w/ 22% dbl overdrive. temps are pre-turbo.
Information gives (2) answers.
(1)temps over 600* will reduce milage.
(2)extra gear will not give you added milage

 

[jkern]

but doesn't higher EGT's imply that you are burning more fuel hence higher temps? i think that higher EGT's are a product of more fuel, therefore lower MPG. also in some higher gears the engine of course turns slower, thus probably running lower than in the maximum powerband, thus the driver feeds it more fuel knowingly or not to maintain speed, thus again using more fuel and creating higher EGT's. i have a gasser car that gets better MPG in fourth gear (direct) than in fifth (OD), (except on the interstate cruising about 75 mph then 5th gets about 3-5mpg greater).

one more question... what are you defining as double OD?

please help if my logic is all wrong!

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Jeremy Kern
1999 2500HD 4x4 QC,SB, ISB, 5spd, 3. 54LSD, 33x12. 50 cooper STT's Denver off-road bumper with Ramsey RE12000, PIAA series 520, Rancho RS9000's, heavy ft. springs, JVD Torque Enhancer, DiPricol boost and pryo gauges, and a bunch of other little things! www.geocities.com/phrn30 for pics!

 

[HC]

My truck is similar to yours. At any speed where 6th would not be lugging the engine, the trip computer will report much better mpg in 6th than in 5th. But the trip computers are only an estimate, so it could be fooling me. To be sure, you would have to just drive a couple of tanks over the same stretch of road, not using 6th at all and see what happens. And, weather (temperature) would have to be about the same, since that impacts mpg quite a bit. Unless the difference is a big one, it will be hard to tell without running several tanks each way.

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2001 ETH, QC, LWB, 4WD, 3. 54, SPA Pyro & Boost, K&N RE-880 w/foam pre-cleaner, ARE Z-series Cab-high shell, Rear ARB locker, and all kinds of synthetic fluids...

 

[Alan Reagan]

EGTs are not an indication of more fuel if you are indicating a "rich" fuel condition. The high EGT condition of course is a result of more fuel being added but more work is being accomplished. As you add fuel you go faster and the engine has to work harder and it gets hotter and actually is running leaner. The turbo spools up and more air is added (boost)making the mixture leaner which tends to raise the EGT. If you could get more fuel in at the higher air pressure the exhaust temp should go down but then you would be putting raw fuel out the exhaust. In other words as the engine works harder (pulling a load, going faster, going up a hill) the exhaust gas is not removed so efficiently and heat builds up rather quickly. The turbo boost helps cool some but is really there to provide oxygen to all the fuel you are burning. I tend to see the best mileage when I'm running 6th gear, 7 lbs or less boost with the EGT in the 550 to 650 range. What was the question again?

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2001 HO 6 spd 2500, 4. 10 LSD, Towing Package, Electric Brakes, SPA EGT/Boost Gauges, Running Boards, Fog Lights, Real Backup Lights, Running Boards

 

[Mick2500]

HC, Dane, Alan and Mr B,

Thanks for all the input. I've REALLY tried to do some running at 65 MPH but I find it impossible. #ad
Here in AZ with the 75 MPH speed limit (don't think I don't appreciate it, I DO!) it's hard to travel at 10 under. 10 over seems safer since that is about what the traffic usually runs.
I'll play a little more with it and keep watching. I never run under about 1900 RPM on the highway, either in 5th or 6th. I don't believe I'm lugging it since boost is up and I never haul anything. Call it my 8,000# toy. #ad


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2001. 5 2500 ETH/DEE, QuadCab, 4X4, SB, LSD, 3. 54, SLT+, Sport, Intense Blue PC/Agate, Tow Package, Camper Package, 'DC' Bed Cover, Sliding rear window. Totally STOCK!!
On Order 4/28/00
Arrived 9/14/00 !! #ad

My Truck

[This message has been edited by Mick2500 (edited 01-16-2001). ]

 

[HC]

it's hard to travel at 10 under. 10 over seems safer since that is about what the traffic usually runs...

I agree, 10 under wouldn't be safe. I'm estimating you turn about 2250 rpm at 80 mph in 6th, about 3100 rpm at 80 mph in 5th. Are you sure you meant to say running at the same speed, or did you mean at equivalent rpm, 5th vs. 6th? If you really meant same speed, put my money on 2250 rpm in 6th, if you meant rpm, I'd bet on 58 mph/2250 rpm in 5th as the winner over 80 mph/ 2250 rpm in 6th.

Either way, don't you just love the way that engine purrs at 2250 rpm? It sounds like it wants to drive to China and back! #ad

 

[Rambling]

JKern

I believe MrB was referring to the use of his auxillary overdrive while using his standard overdrive gear (manual or automatic).

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<font color=#990000>~<font color=#990000> '99 2500 SLT <font color=#990000> ~ QC ~ 4X4 ~ LB ~ Auto ~ 4:10 LSD ~
<font color=#000000>*<font color=#990000> BD E-brake<font color=#000000> *<font color=#990000> BD Autolock<font color=#00000> *<font color=#990000> Gear Vendor OD<font color=#000000> *<font color=#990000> Jordan Research 2020 T-brake <font color=#000000> *<font color=#990000> Bullhide spray-in Line <font color=#000000>*
<font color=#000000>*<font color=#990000> 2k Komfort 5'er - 9,700 lbs <font color=#000000> *
<font size=-2>... Lookin' for the 'RE' to go in front of my 'TIRED' so's I got more time to Play!... <font size=-1>

[This message has been edited by Rattlin (edited 01-17-2001). ]

 

[Dane]

Mick, Just did a 2000 mile round trip to Baja and back. During the trip I was able to do some fuel mileage VS RPM testing with my ETH/6 speed. I found there is a BIG difference running in 5th and 6th at 1800 - 2000 rpm. Anything over 65mph in 6th and your fuel mileage drops off real fast. At 1800-2000 RPM carrying a Cabover I was getting 15MPG. At 2000-2300RPM I was getting 12MPG. Just remember that at 1800 RPM in 5th or 6th is real close to lugging the engine, so you gotta keep your ears and butt listening to the music the Cummins makes and down shift before it starts lugging #ad


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2001 HO 6 speed Regular Cab SLT 4x4 3. 54 anti spin 2500. Used for the daily grind and sneaking away to some secret Baja beaches toting a cabover

 

[MikeR]

Keep your EGT's under 600deg. That's where you'll get the best mileage. I've have read that in every article written by diesel specialists,and so far,don't have reason to argue it.

-Mike

 

[Scott Gray]

Wow, maybe I'm missing something.

New to this but I tow at 55+MPH using 6th gear when on the highway. Ideally I run at about 65 MPH and am at 1850 RPM. The turbo spins up as required to make hills but I never think I am lugging the engine. If it is a big hill (remember I'm in east Texas) I drop to 5th gear but this is rarely necessary if I'm on the highway.

As I read into this thread I think I'm hearing that I need to keep my RPMs above 2000???? Why?? Seems to have enough power at 1700+

Don't have gauges yet.....



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2001. 5 2500 ETH/DEE, QuadCab, 2WD, LWB, SLT+, 3. 54, Tow Package, Camper Package, Jake e-brake and more...
Pulling a 30', 12. 5K lb Alpenlite 5'ver

 

[LSMITH]

FWIW, I was told by the service manager at Cummins that luggging the engine is anytime you go to max throttle and cannot accelerate, whether that is 900 or 2900 RPM. Either case is just as bad for the engine.

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99 2500 QC 4X4 AUTO SB 24V LARAMIE SLT 3. 5 LSD 285-75-16'S

 

[Mr. B]

Originally posted by jkern:
but doesn't higher EGT's imply that you are burning more fuel hence higher temps? i think that higher EGT's are a product of more fuel, therefore lower MPG. also in some higher gears the engine of course turns slower, thus probably running lower than in the maximum powerband, thus the driver feeds it more fuel knowingly or not to maintain speed, thus again using more fuel and creating higher EGT's. i have a gasser car that gets better MPG in fourth gear (direct) than in fifth (OD), (except on the interstate cruising about 75 mph then 5th gets about 3-5mpg greater).

one more question... what are you defining as double OD?

please help if my logic is all wrong!

I have tried all different ways of getting better milage with my GearVendors Aux. Overdrive.
The end result was that it could give me better in combined city/highway if I used it for coast mode only. highway/interstate milage could only be matched at 70-75mph. Exhaust temps went only slightly up from there but milage was reduced. I thought that more power would increase the powerband to give better milage in DBL overdrive. (everybody was braging that the 275 injectors increased milage and power) "That was B. S. " I guess when you are already getting 22+ highway stock you should be happy.
The only benift from the OD other than bragging rights was the engine ran quieter and had speed! 130mph@2700rpm (2K was95-97mph)

 

[Mick2500]

HC,
I meant the same speed not the same RPM. Say on county roads, running 50 MPH in 5th
I'm spinning 1900'ish, in 6th, I would drop to 1200'ish. At 75 MPH, I'm spinning 1900 in 6th (35" BFG's). I watched the EGT and MPG display closely today. Up to about 45 MPH, 5th is better but after that 6th is better for fuel mileage (at least the ECO display thinks that's true). I'm beginning to believe that as soon as I can get into 6th without loading the engine too much, I'm getting the very best mileage I can get. RPM's eat more fuel, no way around it.

Mick2500