Fuel economy question?? 4th unlocked vs. 3rd locked

[Boldt's Wagon]

I have done some calculations with Excel spreadsheet and it appears the only time operation in unlocked OD might be effective is with a 12 valve engine, 3. 54 rear end, and between 55 to 65 mph while towing. This is the operating range where the rpm would be less than 1800 rpm and known to be harmful to the transmission torque converter clutch. Over 65 with 3. 54 gears you can operate in locked OD (over 1750 rpm). 12 valvers with 4. 11 gears can operate at 57 mph or over in locked OD.



The 24 valve engines have better fuel economy than the 12 valves at higher rpms (see TDR issue number 29). The 24 valve engines are more economical to operate in 3rd locked than OD unlocked. The fuel consumption data in issue number 29 shows the 24 valve uses less fuel than the 12 valve over 1800 rpm, supporting the observation that the 24 valve sweet spot is 2200 rpm.



The major cause of poor fuel milage while towing is air resistance or drag. Increased air drag and lower milage is caused by increased speed, increased frontal area, and increased drag coefficient. With my 11. 5 ft tall fifth wheel, a 5 MPH increase results in about 1. 5 mpg decrease. That 5 inch height increase for flipping the axles will reduce the milage about 0. 3 mpg. A low profile fifth 9. 5 ft high would increase my milage about 1. 75 gpm.

It appears the drag coefficient can be reduced with a wing or by reducing the distance between the truck and trailer. A 1000 lb reduction in weight is good for about 0. 2 increase in mpg.



Best milage is obtained by gearing to operate at the optimum rpm at your preferred towing speed. The optimum fuel consumption rpm is 1500 to 1800 for the 12 valve and 2000 to 2200 for the 24 valve.



As I have one of the 3. 54 geared 12 valves, I will rig a unlock switch and test towing in unlocked OD for the next few months.

 

[GLASMITHS]

Bolt's Wagon

If you are towing a heavy 5er, and the rpms drop below 1900 you are lugging the 24V. Your mileage will be less. I never let mine go below 2000, unless I am empty. You would be better off in third locked, IMHO. Your CTD and transmission will be

 

[D. Showan]

Boldt's wagon,

Your data sounds to be just about right on. I have the exact combo that you described: 12V, 3:54 rear and without using 4th unlocked I can't tow at 55-60 and stay in the sweet spot. I usually tow 65-70 so I'm up around 1750-1800rpm but can't maintain that on a lot on hills so I go down to 3rd locked at about 60mph and 2300rpm, I know fuel economy goes way down. I too am going to rig mine to try fourth unlocked and see what the temps do this summer, will let you know.



Glasmiths,

Is your smart controller expensive and what exactly can it do? When you run in 4th unlocked what kind of temp rises do you see??

 

[DR]

I tow a 7500 lb boat normally at 65-70 mph. I leave the TC unlocked until I get up to speed, then lock it up. If I hit a grade that requires me step into it, I'll unlock the TC until I'm over the hill then re-lock. The trans temp may go to 190 but as soon as I re-lock they're back down to 160-170.



My truck is a 95 auto with the #6 plate(a little foward) and the 3k springs. I've also turned the line pressure in the trans to 62 psi. Plenty of power and no slippage.

 

[GLASMITHS]

Smart Controller

Actually, the DTT Smart Controller does not keep the rpms up; it is my personal "186" memory If you try to read most of the threads on the tdr you will come to the place that higher rpms, within the limits of the engine, are better on the engine and transmission.



I saw a table for the 24V that stated that the best fuel consumption was at about 2050 rpm. So 1900-2200 would appear to be the ideal range, with the higher range preferable when towing. Someone asked Cummins how long you could run 2800rpm in third locked on their web site. The answer was all day all night. Yea, it is noisy, but you could get a chebby or pherd



As to the Smart Controller, it mainly controlls the lock-up on the t/c. I don't know what DTT recommends, I personally would not put it on a stock t/c or valve body. You can lock the t/c 2nd thru 4th. It does not shift locked to lock. It will not let the transmission shift into o/d until about 50mph and down-shift when you slow to this speed. Also, if you are using the PAC brake it will downshift locked to lock, you talk about engine braking:--)



Probably the best part is the fact that each of these features are adjustable, up or down. You can downshift from 3rd to 2nd manually and it will remain locked. I just removed a "dumb" BD Autolock. Everytime you touched the o/d or GV the t/c unlocked and you had to hit the throttle to lock it back up. You just cain't be doing this going down a hill All of them are in the 200-300 range. Oh, it also has the t/c saver. It unlocks when the throttle is pushed to a PRESET position; usually 80%. AND always at WOT. My temps have never been over 180 while towing; this was on a long, about 5% grade, I mean long, 4-5 miles, and pulling 26k, SMOKING a brand new powerstokeOo. The wife said "did you have to do that"



EDIT: I'm sorry. I do not tow in o/d. I have a Gearvendors and I lock out o/d. At 60 mph it is about 2200 rpm. Thats where I like it, as long as there is no headwind. Over all my mpg is 12. 5-13. Have gotten 14. 5; also 7 thru West Texas with a 50 mph head wind

 

[D. Showan]

thanks for the reply Glasmiths, You must have a 24V. I have driven them and they definitely do like to rev higher than the 12v's. If I towed more weight (only 8K), or had a 24V I would tow in 3rd also.

 

[Boldt's Wagon]

Well, I installed a switch to force unlock of the torque converter. Spliced the switch wires into the orange wire with black stripe from the middle connector of the PCM.



Went for a test drive and it didn't work. Figured the switch must be bad. Got back and looked and there are two wires orange with black stripe from the middle PCM connector block. Checked the service manual and the other wire is for the "Wait to start" light. Reconnected the first wire and spliced the wires/switch into the correct orange/black wire.



Made a second test drive and was switching from stock (TC locked) to no TC lock and between 3rd and OD with and without lock and came to a stop light. As I slowed down I got the herky jerky's that are characteristic of trying to stop with a mystery switch and the TC kept in lockup. But I was in the unlocked position. The engine died, with restart everything seemed OK. I turned the corner and went down the road a piece and unhooked the power and ground to the "PressureLoc".



Last year when I got the DTT conversion, Bill Kondolay and I observed a severe shifting(?) cycling while doing a test pull with the trailer. I went to BD and had the "PressureLoc" "adjusted". This adjustment involved stretching the spring. Later on the way home I experienced the shaking again, I disconnected the power to the "PressureLoc" and the problem quit. Later last year I hooked up the "PressureLoc" 12v power suppy again and observed no problems until today.



The TC lockup solenoid is activated by a ground in the PCM that ground the TC lockup solenoid. The voltage supply to the TC solenoid is about 5v. The "PressureLoc" vacuum solenoid is activated with 12v and uses the ground established in the PCM for the TC lockup solenoid.



What I figure happened is the PCM got to a condition that allowed the 12v "PressureLoc" current back feed via the "PressureLoc" selonoid and ground through the selonoid windings for the 5v TC lockup selonoid, thus locking up the TC. The switch I installed prevents grounding through the PCM when the switch is open (no TC lockup).



I continued the testing with the "PressureLoc" electrically isolated from the transmission and had no repeats of TC lockup while stopping.



I guess I will remove the "PressureLoc", an intermittent confusion and uncontrolled lockup of the TC with potential transmission damage is not worth the advertised advantages.



The nontowing test run showed about 160-165 transmission temperature on a grade with 7 lbs boost and 160 rpm increase at 60 mph over locked OD. A slight grade with 4 lbs boost had 100 rpm increase over locked OD at 60 mph.

 

[D. Showan]

Bolt's wagon,

Thats great info, I'm anxious to see your towing results. Thanks, for the update

 

[Snow man]

Boldts,thanks for the info. you can tell your running a nice tight TC. Do you have the 89%TC. You hardly gain any RPM with the DTT,I know its not uncommon to drop 5-700 RPM going into lockup with the stock TC.

 

[Boldt's Wagon]

I have the DTT 89 percent. The rest of the mods are in my signature below.

 

[D. Showan]

Snowman,,

I've noticed that there is a big difference between going into lockup at 48-50mph and going into and out of lockup at 60 mph. All the converters must be much more efficient at higher speeds. My stock converter dropped 700rpm going into lockup at 48mph and relatively light throttle. No matter how I try, the most I can get now is about a 350-400rpm drop and it is usually about 200-250. Even with my stock converter at 60mph (I don't remember the exact numbers) going into and out of lockup at light throttle there was a fairly small RPM change. I never tried it at full throttle with the stock one but with my Goerend TC the rpm rise is approx 225 RPM at 60mph at full throttle ( I didn't notice the boost but will check next time).



Please lets not get into another TC battle. I sounds like were in the same ballpark in our rpm changes and efficiencies.



The question for me is how long will it take to excessively raise the trans temps with the approx. 200 rpm rise in rpm when going unlocked? (that is full throttle with my current stock power and at that engine rpm, approx 1875, it should be producing enough power to climb most hills at a safer rpm for the engine and trans.

 

[GLASMITHS]

t/c percent

D. SHOWAN:



If you are showing (no pun) 250 or so rpm change, your t/c ain't very tight (?). My old BD would show 350, like almost a whole gear! On an earlier thread several guys with the DTT 89% reported only 75 rpm change. Mine is 89%, but it has a livestock valvebody whick may account for the 125 increase or drop. That is one of the features with the DTT, lock-up is no big deal. It slips a little unlocked, but not enough to cause a lot of heat.

 

[D. Showan]

OK DTT is the best. Lets move on from there.

 

[GLASMITHS]

HATEFULNESS

Sorry to ruffle your feathers. I was merely posting information. I thought this was the purpose of these threads. Just don't get your drawers in a knot because of it. I won't post anymore if it will make you feel better. DTT, JUST GET OVER IT SON.

 

[Snow man]

D. Showan, if your raising only 225 RPm at full throttle,you have a tight TC. You are very right about RPM's and amount of slippage,all TC's will slip less at high RPM,but due to stator design the amount can vary due to how its built or milled. What gear are you in at 60 ,when you do the test,and how do you test it,do you stomp it out of lockup,and hit the mytery swithc at 60 to lock it,and note the rpm change,or do you lockit,stomp it,then unlock the TC and note the change in RPM?As far as the DTT being best,thats like saying Dodge is the best,its a matter of opinion,what matters is whats best for you,and your budget,keeping in mind your intended use. I hope we can keep this thread open and clean,since were getting good comparisons here. When I get my race 91 in,ill post the slippage,with my 89,at 400+hp ,if i rol on and let the Tc saver work it will pop the rpm's up about 250,this is at 70 mph in OD with 4. 10's,It is difficult to tell though because it takes a second for the TC to fully charge going from lock up to fluid coupling. it is hard to watch. i notice with the comp off there is less than 125 RPm slippage at any give time no matter what gear,or what speed.

 

[D. Showan]

Snowman, Thanks for injecting in more civility. I have done these tests in 4th gear. 4th locked is only about 1650rpm at 60mph. I can't yet keep it out of lockup in fourth for more than a couple of seconds (I didn't wire it to control lockup yet) I started this thread to get peoples opinion to see if I was wasting my time doing so. If the trans overheats quickly it's not even worth the trouble to wire it up. The way I'm getting my unlocked numbers is this: I slow down to about 56-57mph tap the OD switch twice to temporarily unlock the TC but keep 4th gear, go to full throttle, and as I reach exactly 60mph my tach is at 1850-1875, thats a 200-225 rpm increase over locked. I don't know about the rest of you, but my stock tach is only marked every 250 rpm so I'm doing my best to guestimate some of these rpm changes. I'll try this same test at 70 mph tomorrow, I know the rpm rise will be less.



Glasmiths, Sorry about my flip response. I'm trying to accuratly report my info and the interpretations can come down any way people see fit. I can also honestly report a approx. 50 rpm difference between locked and unlocked(about the width of the needle) if I do it at light throttle and 60+ mph.

As I'm sure we are all aware, it is very hard to accurately compare TC's because of the the multitude of variables involved. Please don't try to compare my 200 rpm rise from 1650 to 1850 at full throttle to someone saying they get only a 50-75 rpm change at light throttle because I get that too.



Again I repeat, This thread was started to learn more about the possibility of towing in fourth unlocked, not for people to cheerlead their favorite company and cut down others by using unfair comparisons.



Bolt's wagon and others have been very informative and helpful and soon there will be more data for us to interpret (hopefully fairly). I will get mine wired to control lockup also but don't have any towing trips planned for a month or so.

 

[D. Showan]

DR,

Thanks for your response also. That is exactly the kind of info I am looking for.

 

[Boldt's Wagon]

Well, I went out and did some testing pulling the fifth wheel. The trailer is 11. 5 ft high and the GCVW was estimated at 17,500 (light condition, no water, copilot, groceries, or beer). The temperature was 60 degrees and there was no wind. My EGT probe is mounted in the elbow after the turbo. I have 235x85 R16 tires. Put on 48 miles testing 3rd locked, 3rd unlocked, and OD unlocked at 55, 60, and 57 mph.



The runs at 55 mph were first and the power useage (boost pressure) looks high compared the the 57 mph. I attribute this to the tires and rearend not being up to temperature yet. You need a minimum of 20 miles to bring tire and rearend temperatures up to equilibrium.



The second series of runs at 60 mph showed significantly higher boost pressures than the 57 mph runs (14 psi vs 9. 5 psi in 3rd locked). In the past I had observed fuel milage went in the dump if I pulled over 60 mph. I had learned by trial and error that my towing setup liked to operate at 57mph. It appears that I may have an air separation and increased drag between 57 and 60 mph. May have something to do with the gap between the cab and trailer or I was on a slight grade I did not notice. I will have to check this out again and see if there is an aerodynamic fix.



The final runs at 57 mph were on the return 24 mile leg when the tires and rearend were up to temperature. The 24 miles out and return follows the pool behind McNary dam so there is no net elevation change but has some slight rises and declines.



3rd locked - 2220 rpm, 9. 5 lb boost, 440 EGT, and 140 tran temp



3rd unlocked - 2280 rpm, 10. 5 boost, 450 EGT, and 160 tran temp



OD unlocked - 1725 rpm, 8 boost, 525 EGT, and 170 tran temp.



The DTT 89% TC showed 60 rpm increase in 3rd unlocked and a calculated 190 rpm increase for OD unlocked to locked. On a slight grade with the 60 mph run in OD unlocked, the boost went to 25 psi and the rpm to 2050 (a 425 rpm increase over the calculated locked rpm).



The TC gets tighter at higher rpms and the rpm increase is less for the same boost at higher rpms. Thus, you see a 190 rom increase in OD at a lower starting rpm vs the 60 rpm increase for 3rd.



Using a rule of thumb that each lb of boost is equal to 10 hp and 20 hp hr per gallon, the three runs at 57 mph would indicate 12. 0, 10. 9, and 13. 4 mpg for 3rd locked, 3rd unlocked, and OD unlocked, respectively. These numbers support the predicted trend with the 12 valve 175/180 hp engine having the highest efficiency at 1600 rpm (as reported by Cummins, TDR issue no. 29).



As should have been obvious, for maximum towing economy, operate your rig at a speed and rpm that results in 8-9 psi boost and less than 550 EGT (post turbo). This should be 4 to 4. 5 gallon of fuel per hour.



It appears that the 12 valve, automatic transmission, 3. 54 rear ratios can squeeze out a 10 percent gain in fuel milage in the 55 to 60 mph range by operating in OD unlocked. This needs to be verified by some long distance testing.

 

[D. Showan]

Bolt's wagon,

Man I 'm impressed. That's a lot of great data. So if I read your info correctly, on relatively level ground you get a 190rpm rise going unlocked in 4th and the trans temps stabilize at around 170 degrees on a 60 degree day. It sounds to me that you could run that way, save some fuel and it shouldn't hurt anything.



You also said you got a 425 rpm increase going unlocked on a grade. What were the trans temps doing then??? Maybe 4th unlocked is best for the flats. I was hoping to use 4th unlocked for the hills. Logic would tell us that a 425 rpm difference would make lots of heat, was it????



Thanks for taking the time for all these experiments, I hope the other members appreciate it as much as I do

 

[Boldt's Wagon]

My notes say the small hill in unlocked OD had 190 transmission exit temperature, not a long enough grade to get a steady temperature. I suspect that a longer hill would have had the temperature climbing still higher.



For a hill when using unlocked OD, a slight grade could use 3rd unlocked and save a TC lockup, a larger grade would also shift into 3rd locked.



I saw 190 manuvering through traffic and lights when I returned home and 230 while backing the trailer into its storage position.