EGT vs RPM's relating to MPG

[SpoonPig007]

Ok ... engineers out there ... please shed some light on this for me.



To achieve best MPG, is it better to tow in 5th and keep the RPM's high (~2200+ RPM/ EGT = ~800-900 / Boost > 10) or to tow in 6th (~1900-2000+ RPM / EGT = ~1000-1150 / Boost <15)?



Logic tells me the higher the egt (and boost) the more fuel is burning, but the engine seems happier running in a lower gear, less boost, more rpm, lower egt's?



For the record, I am not comparing situations where the engine is bogged down, but I am talking running at 60-65 with a 6,000 lb trailer through varying terrain.



Thanks!

 

[JJW_ND]

No engineer here....

From my experience I would not think of a situation where you would get better MPG a gear lower unless you are running to low of rpms.



I believe there is about a 400 - 500 rpm drop from direct to OD (5sp or 6sp). I personally try to keep things running at 1900 or higher more for the running gear then the engine. If you can run at 2200 in direct you would get pretty good mpgs bu they should be better with the lower rpms of OD.



On the extreme end (towing loads) I pulled several stacks of flatbed trailers that I just could not hold 5th (OD) without loose speed so I ran in 4th (direct) the entire 700 mile trip. The next similar load I could keep it in 5th (less wind). Running in 4th 2500 - 2600 rpms I got 4. 5 - 5 mpg. When I can run in 5th I never dropped below 9 mpg.



jjw

ND

 

[SpoonPig007]

I tend to agree, but I am trying to account for the drastic rise in EGT in the higher gear. With more boost, of course there is going to be a rise in the intake temperature, but with only 20 lbs, I can't imagine a rise of over 200 degrees. Assuming this to be true, what else can account for the higher EGT's other than more fuel being burned?

 

[rweis]

Airflow THROUGH the engine? It seems when I run higher rpm's, better air flow, cooler EGT's, better torque, better combustion?



Definitely not a scientific answer, just an observation.



Bob Weis

 

[JLEONARD]

A diesel is a precise machine. Fuel consumption is based upon actual horsepower used and is not rpm dependant.

Theoretically the consumption should be the same in both gears, even though the rpm is different.

But the higher egt indicates some inefficiency perhaps?

Another line of thinking might be that boost is an indicator of horsepower in use..... therefore you are using more ponies in 5th, therefore more fuel?

OK, those are some thoughts based on my experience in the marine arena... .

Next????

Jay

 

[klenger]

Do you have an overhead computer? If so, get on a flat stetch of highway and compare.

 

[SpoonPig007]

The flat stretch comparison wouldn't really work becuase at that point the variables' deltas are minimal.



I am referring more to long uphill pulls, and up and down type pulling, like through the Ozarks. This requires more pedal, and the EGT's, Boost, and RPMs become drastically different from 5th to 6th. Still no problems pulling, just different indicators.

 

[TXDSLSHOP]

I have been thinking of posting my MPG findings but I didn't want anyone to think I was trying to be scientific or anything. These are just my findings. I installed a hand operated throttle in my truck (see sig) recently and have seen about 3 mpg better mileage on average. I deliver trailers and have noticed that my mileage is very predictable dependant on average boost. I will try to explain this a little better. For example, I can pull a single trailer weighing 6k running 65 to 70 in 5th pushing 10 lbs max boost and get 13 mpg. Given the same trailer at higher speeds pushing 15lbs max boost my mpg drops to 10. Now, If I have two of the aforementioned trailers stacked weighing roughly 12k running 65 to 70 and pushing 15 lbs max boost I still get 10mpg. So, my conclusion based solely on my non-scientific findings is that, for me, I can run empty or loaded and I choose what fuel mileage I want! Does this make sense? If I can stand to drive slow enough in the right gear to keep my boost at 10lbs I will get 13 mpg, 5lbs = 17mpg. All my findings were based using calculated MPG topping the tank off to the top of the filler neck and actual odometer readings (no trip computer!) and several thousand miles of driving . If you think I am full of poop I understand. I just enjoy getting good mileage when I am driving for money and I don't care about mileage when I am running around playing, or am in a hurry! Thanks! Jeremy.

 

[SpoonPig007]

thank you ... that's actually pretty informative, as you measured under load.



I think I agree that the boost is the best indicator, as it is directly tied to manifold pressure.



When towing, I think I will aim for whatever gear allows me to keep my intended speed with the lowest boost, and lowest EGT, regardless of RPM (within reason of course).





Does this make sense?

 

[Butch Brandt]

MPG/boost

Killer59 & piglet007



I think that makes perfectly good sense. If (as stated so often) 1 lb of boost = app. 10 HP then it seem to me that higher boost means you are using more horses and one way to get more horses is burn more fuel.



My experience has always been (since 3-96) that for economy,keep the boost down. Sometimes economy just doesn't matter and it's fun to peg the gauge.

 

[JLEONARD]

Sometimes economy just doesn't matter and it's fun to peg the gauge.

"The weight of your right foot is directly proportional to fuel economy":-laf :-laf



That's my theory anyway!!!



Jay

 

[TXDSLSHOP]

I am glad this post read well to others. I can't always say what I mean. I have to say again that I absolutely hate cruise control while I am loaded and refuse to use it! That is why I went to the hand throttle. I just dial it in to the desired boost (MPG) and let 'er eat! EGT's are never a problem and I get good mileage! Jeremy.

 

[Ramafid]

I have always wondered about the boost issue as well. Here is my question. What if you change just the exhaust housing on the turbo and leave all else the same? With a smaller housing you will have higher boost and lower EGT's ( not WOT ). The larger housing lower boost and higher EGT's. So which is better? I would think that more air for a given rpm would give increased fuel mileage. Right? More air plus the same amount of fuel ( to a point) should give slightly more power. Yes or no? That's the whole idea behind a diesel's advantage over gas. It operates at very lean fuel mixtures. Any engine efficiency experts please chime in.

 

[Hohn]

I am no expert, but here's a few things to think about...



Fuel economy is primarily a result of ONLY two things: how much fuel the engine has to burn, and how the power produced is tranlated into motion (basically, gearing and driveline efficiency). Everything else relates to one of these or the other: tire pressure, wind resistance, towing load, etc etc.



So FUEL FLOW is the best indicator of MPG. Unfortunately, you can't really measure how much fuel the engine is using from one rpm to another, because the bypass fuel increases with RPM as well. IOW, you will see low fuel pressure at high RPM even when the engine is burning almost NO FUEL.



Boost and EGT are NOT indicators of fuel efficiency! They simply tell you HOW the fuel was burned (under what conditions) NOT HOW MUCH fuel was burned.



The relationship of boost to EGT to what throws everything off, because it's a circular relationship. More EGT means more boost, more boost lowers EGT. It's like it's a self-correcting thing.



All EGT will tell you is the RATIO that the fuel is burning at. Generally, we think higher EGT means lower mpg. But if we increased the air volume MORE than we increased the fuel volume to the engine, we would see LOWER EGT, while also getting LOWER MPG-- this is the opposite of what seems to happen at first glance.



So MPG, BOOST, and EGT ONLY correlate for ONE truck, with a given fueling and turbo setup. Comparing one truck to another will not do you any good wen it comes to details, only trends are observable.



Rule of thumb: maximum fuel ecnonmy will come at the LOWEST rpm (within its operating range) the engine can operate at without lugging, given a certain condition of loading.



Want to see something interesting? Run a whole tank in OD at 1400-1500 rpm with no cruise control. Yep, your truck can get 25mpg easily. Because at these speeds (50mph or so) there is LITTLE load on the engine, and you are getting the most bang for the buck by running in OD.



Som the short answer is this: the only time you will get better MPG by downshifting is if the engine simply doesn't have enough power to pull OD w/out lugging. Ignore your boost and EGT gauges for MPG of one gear over another. Run in OD if at all possible.



BTW-- a bombed engine in OD will almost ALWAYS get better MPG than a stock engine in DIRECT, pulling the same load at 60-70 mph.





Justin

 

[Loren]

I agree with Justin's analysis. Here is some additional information.



In my experience, the best thing you can do to improve your mileage is slow down.



While researching an article and evaluating the mileage improvements of an overdrive transmission I ran some tests with the overhead mileage computer. Mine is no longer accurate, but I can still use it to compare different situations. Here's the short version of what I found while comparing gearing.



70 mph - 6th gear - 1875 rpm - 20. 8 mpg (from computer)

70 mph - 6th gear plus overdrive - 1550 rpm - 23. 6 mpg (computer)



I also tested the combination at 75 mph and got similar results. My experience leads me to belive that as long as the truck can pull the gear without lugging, your best fuel mileage will be obtained at the lowest rpm.



This is exactly what Justin said, but I have some numbers to back up his conclusions.



As an aside for those who may be curious, the 23. 6 mpg on my overhead computer works out to an actual fuel mileage of around 19-20 when calculated by miles driven divided by actual fuel used.



Loren

 

[SpoonPig007]

What is the definition of lugging then? The point where the desired speed canot be maintained given the current gear ratio?



Can it be defined as the point where that engine can develop full boost? Uphills at slower rpm, I cannot attain full boost, but still have plenty of power from torque?

 

[JJW_ND]

A followup....

I totally agree with the highest gear lowest rpms but I do find the boost is a great mpg indicator in my case. Whether I have a load of trailers in tow or running empty the lower the boost (regardless of the speed) the better mpg. When running empty at 70 mph or slightly over 2K rpms with calm winds and OAT over 50 I run about 4 lbs boost and would expect 21 mpg. Just the slightest wind will increase or decrease that. With a tail wind I can run 75 - 80 and still keep 4 lbs boost with the same mpg.



Lugging! I am not sure you can lug our trucks without a load either a trailer or substantial load in the bed. This is assuming one accelerates up through the gears normally to the crusiing speed ... no shift from 3 to 6th at 40 mph.



In my previous post I mentioned loads were I could not hold 5th (OD) I would call this lugging if I tried to keep it in 5th (OD). I was well above peak torque rpms but yet on a flat stretch I would slowly loose speed. I expect to loose speed on a hill but not on the flats.



The fuel consuption tables Cummins provides for the B series all show the best fuel economy at or slightly above peak rated torque rpms. If you dyno you will have a better idea of were your trucks peak torque is. Mine is at 2K rpms.



jjw

ND

 

[Hohn]

What JJW said is correct. You can note trends based on boost. But you can't compare one truck to another. Especially if turbos are different, trannys are different, etc. A larger housing turbo will give you less boost for the same amount of fuel burned.



So you should do what JJW did. Take notes on the conditions you are driving in, then relate that to the MPG you get.



Justin