engine "sweet spot" for mileage/auxiliary trans. results?

[cfarrar]

I really have two questions here. I would be interested to know what the engine speed for best mileage is for our trucks. I would think that dropping cruising rpm would help. I am also familiar with the auxiliary gearboxes (Gear Vendors, U. S. Gear, etc. ) that would do that. An auxiliary transmission would introduce some extra power losses through friction, too, that would cut into the gains. Has anyone documented results with one of those in our trucks behind the automatics or the Getrag?

 

[BushWakr - RIP]

Feel free to correct me if this is wrong, but I would think that the RPM is determining factor here.

If you stay within the sweet spot for the engine you'll see your best mileage. . eg: 1800-1900 on an aftermarket and 2100 on a stock system (this is for auto's)

This would also assume you maintain the right conditions in EGT values eg: 600-800*F and under 10psi boost.



yes? no?





Bob.

 

[hdm48]

Wow, I've never even considered this.



My rig, at 70mph is using 4-5psi boost and 550 to 600 egt's.

At 60mph, it is using 3-4psi and 450 to 500 egt's.

This is on level ground and no head wind.



Hummm?



Now at 100+mph I am using 35psi boost and 1250egt's,, Wonder how my milage is now?



Ahh,, oh darn,, forget that last part.





Where did my wife put that DELETE key????



OH, there it is,, right next to the ANY KEY!!!



UH OH-- too late

 

[cfarrar]

I know that there are many variables in this. I think that with other variables excluded, the engine operates most efficiently at about the speed where it makes maximum torque and I thought that that was a little lower than the 1800-1900 that Bob put down.



There is an engine statistic that can be measured in the lab that shows how much horsepower can be produced for an hour from a gallon of fuel. I've seen it published for tractor engines at rated speed. I remember once seeing someone say that the Cummins 6CT (8. 3L) is more efficient than the 6BT (Of course some have the 6BTA. ) that we have.

 

[JLEONARD]

I can get 26 mpg on a trip... mostly at 70 mph and 2000 rpm.

I'm sure slower would get better mileage, but I think it mostly has to do with how heavy your right foot is and how many times it goes thru the firewall (and how many times you "soot" the tailgaters).

Jay

 

[BushWakr - RIP]

I would think there are a couple things to keep in mind though... .

First the actual torque range is lower than 1800'ish... but Second is the fact that an auto in stock trim is loosing way too much to be able to "call" the lower RPM "best"... . does that make sense???

With my 91% tc I showed max "T" at 1480 RPM.

I'd suspect that a stock TC would show max "T" at a higher RPM because the efficiency will improve as the RPM's go up and, you'd expect to find your max "T" up higher as well... ... yes/no???

Now this is not the case when you talk about a stick transmission ...



I have also been told by a "mechanic" that the engines efficiency is at it's peak, in part, based on cylinder temps too... too cold and you can pump out smoke or even "put the fire out",... too hot and well... you know the rest...







pastor bob.

 

[cfarrar]

I think that Pastor Bob's thinking is right regarding the automatic transmission. It's a shame that a unit designed for the speed of gasoline engines was used with the Cummins.



I am most interested in what is best for the truck with a manual transmission. I don't doubt that the combustion temperature is an important variable in this complex interaction of many things. If the engine is asked to do about the same work at a lower speed, the combustion temperature and pressures reached would be higher.

 

[Paychk]

In the book about Clessie Cummins, there are some accounts of cross country trips to prove the reliability of the diesel in automotive applications. On these trips he noted the most efficient use of fuel was when an EGT of 800 F. was maintained, this was on normally aspirated engines, so I don't if it will apply to our turbo engines. It would be interesting to know though.

 

[hdm48]

Ok, now you got me to thinking. What if a guy were to put a rig on a water dyno and have a fuel flow meter in the fuel line. Would this work with a bypass system like we have? Or maybe use a remote fuel tank so one could measure the amount of fuel burned in a given amount of time under various load conditions.

But then, one would have to factor in inclines, wind, parasitic drag from tires, vacuum at the tail, loss of vacuum (passing cars and trucks), throttle repossitioning.

It would be interesting to see.





Dave

 

[BushWakr - RIP]

Hey, you guys are gonna get me into trouble... ... . I promised wildman nascar mark that we wouldn't get in too deep while he was away.....

He just loves to giggle at us when we "go theoretical" on here... .



Pastor Bob.

 

[cfarrar]

A before/after report from someone who installed an auxiliary transmission would be good enough for me!



It do think that I read once where the maximum torque point is for the stock engine.



Clark

 

[dpuckett]

Cfarrar-



You might try posting in the product and accessories forum. May get more exposure. While the later 12 valves had a lower peak BSFC, the idea and general trend should be about the same. I was kind of curious about that as well. My 4. 10s limit me to about 65 or so without shooting my EGTs up and mpg in the toilet. But if I'd keep my foot out of it, that might help, too.



I thought Clessie showed max efficiency with EGTs below 600. I could be mistaken, though. Wouldnt be the first time.



Daniel

 

[sparks57]

Just out of curiosity. Does a 6 speed have a wider gear range or is it the same as a 5 but with gears closer together? Or somewhere in between? Can you even mount an NV5600 in our trucks?



It would be great for mileage if the you had a trannie that had higher gears.



What about gear splitters??? Are trucks with splitters hard to drive (lots of rowing) when empty? Could somebody explain to me the operation of a splitter. Do you row through the first five, then flip the switch and then row through them again or what? Are under-overdrives good for mileage?

 

[cfarrar]

The Gear Vendors and U. S. Gear auxiliary transmissions are two-speed transmissions with one speed "straight through" and the other overdrive or underdrive (but not both). I think that they use a solenoid or electric motor to put pressure on a shifting mechanism when you want to shift. When conditions are right, the shift is made. They can be shifted to give an overall gear ratio that is between the existing ratios in the main transmission and will also provide another "gear" in the form of overdrive or underdrive. To split gears, between 3rd and 4th, for example, you could start from standing still with the auxiliary transmission in "straight through" gear and shift through the gears of the main transmission until you were in 3rd gear. You'd rev it up in 3rd gear and instead of shifting to 4th, you'd shift the auxiliary transmission to overdrive and rev it up. Then you'd shift to 4th in the main transmission and back to "straight through" in the auxiliary. You could use the same process to split other gears. You'd also extend your overall gear range by multiplying the auxiliary transmission ratio by the main transmission ratio to obtain either a lower low or a higher high according to which way the auxiliary was configured. You wouldn't "row" through the gears of the main transmission, then shift the auxiliary and "row" again because the main transmission has a relatively wide gear spacing, ratio-wise. If it were a very close ratio and the uxiliary a wide ratio, you'd shift that way. I think that is how the shifts are made in the big trucks, but it is all built into one unit in modern trucks.



I'd like to read the report of mileage changes in one of our trucks equipped with an auxiliary transmission.



Clark