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

[D. Showan]

I posted similar questions on a tc switch thread but had no responses so I thought I would try it here



I may try to wire my TC up so I can force the converter to unlock but stay in 4th gear. The way it is from the factory, I can cannot keep fourth without lockup occuring within a few seconds. Fourth unlocked should be like having another gear between 3rd locked and 4th locked. Lockup is great but there is a large space between 3rd and 4th gear.



All of the following results were at 60mph. 1650rpm in 4th locked, I tap the OD button twice to get the TC to unlock temporarily (go to full throttle)and the tach goes to about 1875 at 60mph. 3rd locked is about 2350 rpm at the same speed. I'm thinking that 4th unlocked might be just right for some hills. I am a little concerned about the extra heat build up running unlocked at higher power levels. Maybe it would be better to just use 3rd locked and not worry about the higher RPM but I'm after higher MPG and 2350rpm is more rpm than you need. I believe I read that our 12V engines have the lowest fuel consumption per HP produced around 1700-1800rpm. The efficiency of the engine must be way down at 2300-2400rpm. I know that all the heat produced while unlocked is wasted heat but 2000rpm is a more efficient rpm to run at.



Any thoughts about excessive heat build up in 4th unlocked???

Any opinions about which would be more efficient---???

The two choices--

A. locked in 3rd with less heat build up but at higher rpm

B. unlocked in 4th with more heat build up but a lower more efficient rpm



I know I need more hp so I can stay in 4th locked longer and a fuel plate is in the works.

 

[illflem]

Over 2000 rpms and your mpgs take a big dump. 4th unlocked will burn your transmission up rapidly. Don't see why you can't cruise in 4th locked, are you towing?

 

[D. Showan]

illflem, I should have said I was worried about all this when I tow my 5th wheel this summer. It's about 8-9K lbs. . So you think it will make excessive heat. If that's the case I can live without my gear between 3rd and 4th

 

[FParisi]

Just get a plate and you shouldn't have to worry about all that for towing just 9K. You could just slow down to 55 on hills + I just remind myself of the 6MPG I got with my old gasser motorhome!

Frank

 

[GLASMITHS]

Fourth unlocked

D. Showan - My Dtt 89% with Smart Controller will do just what

YOU want to do. Havn't seen any high temps lately

 

[Snow man]

D. Showan, you do not want to run unlocked in 4th for any lenth of time,your transmission temps will be up there. You can run 4th locked,but it will be hard on your transmission at 16-1700RPM,and the mtor likes 1750 minimum with a big load. THe truck needs an under/overdrive unit to do what you want,or change to 4. 10 gears,and pull in OD. There is no way to get the RPM's you want without speeding,slowing down,or changing tire size or gears.

 

[D. Showan]

thanks for the replies gentleman, I'll reply to your info later today when I can make some time. Thanks again

 

[D. Showan]

Fparisi, I think you're right, a fuel plate will solve most of my hill climbing in OD

locked problems.



Snowman, Your points all make sense to me also. When I add the fuel plate and get more hp and torque at lower rpm's I will be able to stay in OD locked for many more hills than before so I need to decide when to force unlock to save excessive strain on my trans. It now has quite a few upgrades but why make it work harder than it has too. It's not worth excessive lugging just for an extra 1mpg. I'm to cheap to go for a gear splitter.



Glasmiths, It sounds like you sometimes do what I am talking about. How much of a rise in temp do you see, and how long does it take to see it? How much of a rpm rise do you think you would see with your setup doing the following. Starting at 1650rpm locked, unlock the converter and go to full throttle. The more rise the more the slip, and of course the more slip the more heat. Doing this I get about a 225 rpm rise with my stock power. If your rpm rise is lower then you probably won't make as much heat and running unlocked may work better for you. At higher rpm's I have a lower rpm rise when unlocking but I am concerned with the rpm difference at these lower speeds. Please don't mis misinterpret the intent of my comments, I'm not trying to see if my TC is better than yours or vice/versa. I'm trying to find out how much heat a 200 to 250rpm difference (locked vs Unlocked) will make when towing at heavy throttle.



After the comments I received I think what makes the most overall sense it to put in a fuel plate, and when I can't maintain 1700+rpm I will just go to third locked and live with the drop in MPG. With the increase in HP there will probably not be to many hills that will be a problem for 4th locked anyway



Thanks to all of you Dave S.

 

[illflem]

Dave, along with the plate it would be wise to add at least a EGT gauge. I'd go as far as saying it should be mandatory. A #11 plate will give you the power you need without maxing out your transmission.

 

[D. Showan]

thanks illflem, I remembered to put in my signature on my original post but I forgot on the others, sorry about that. Why can't our sigs come up automatically. The very first thing I did was put on a set of gauges. I was thinking for my 96 auto I would go with either a #8(230hp-605ft/lb) or a #6(250hp-635ft/lb) plate. From what I read on the TST web site the 250hp plate is about the highest I should go without making additional engine mods. Does this sound right?

 

[phoekema]

4th lockup

You probably don't want to hear this, but when you start on the upgrade path, you have to be carefull what you do and in what order. When you upgrade the engine so you have enough power to pull most hills in 4th lockup, you start another problem. The stock converter is barely up to the task with a stock engine and when the horsepower and torque go up very much the stock lockup clutch is almost certain to start slipping. I upgraded only the injectors on my truck and already had slipping problems in the 50 to 65 mph range. BD has a device to fool the transmission into providing more pressure to the lockup clutch which may postpone problems for a while, but eventually you will be replacing the converter. After doing a lot of research, I went with the ATS tripple lock converter. After babying the truck all last year to keep the slipping to an absolute minimum, it great not having to worry about that any more. Because of the design, the ATS converter can be used effectively with stock line presures. Each upgrade improves the towing ability, but nothing is cheap! One way to look at it is the upgrades cost less than a new truck. Good luck on you upgrade path.

 

[D. Showan]

Phoekema, What you say makes sense. I have already upgraded my trans with very good stuff from Goerend Bros. I'm quite confident that the way Dave Goerend has it set up it will hold the kind of power I plan on making. I'm trying to do my mods in a systematic way so they all work together. Thanks for the info.

 

[D. Showan]

Glasmiths, Are you out there????????

 

[Boldt's Wagon]

Showan: I have been thinking of the same thing, towing in 4th unlocked to increase milage. I tow an 11. 5 ft tall, 11,000 lb fifth wheel. I can tow at 67 mph/1800 rpm in 4th(OD) to protect the transmission or tow at 58 mph/2250 rpm in 3rd. Either way is not optimum for milage. I figure that towing in 4th unlocked would result in 58 mph/1700 rpm and better milage.



The first generation trucks don't have lockup and they tow. They seem to be able to tow without excessive temperature problems. If we are pulling a hill we can shift to 3rd/locked to control temperatures.



If there are any first generation owners who tow in 4th(OD) reading this, what are your transmission temperatures on the level while towing 10,000 lbs or so and at what speeds?

 

[D. Showan]

Bolt's wagon, That's a very good question. How do the non lockup trucks tow without overheating? Stock they don't have much power, so maybe that's the key.

 

[Extreme1]

I would suggest a fuel plate, a GSK 3k kit and a free flowing exhaust to help address the EGT's. The GSK kit will make it pull very effectively past 2500 RPM. Hills won't be a problem.



Also, if you haven't removed the air duct between the fender and the air box, do it!

 

[phoekema]

first generation w/o lockup

I had a '93 with an auto trans before the '98 I now own. While the first generation converter didn't have lockup, the stall ratio was MUCH lower than the newer converters that do have lockup. There was much less slippage in the first generation converters than there is in the newer ones when the newer ones are not using lockup. I don't know why they increased the stall ratio (slippage) in the new ones other than to take advantage of the torque multiplication off the line, knowing the converter could later be locked up to increase highway efficiency.

 

[Boldt's Wagon]

I checked with a transmission man, and the 1st generations had an oil flow through the torque converter to the cooler of 3. 4 gpm. The 2nd generations have 2. 8 gpm or 20 percent less. Depending on the efficiency of your torque converter, your temperature rises will vary. The transmission man said you could run all day if the exit temperature from the transmission was 185 or less.



I did some calculations to estimate the milage increase of 4th unlocked over 3rd locked. I estimate 0. 5 to 1. 0 mpg increase. This looks like it might be worth trying when you also consider the reduction in cab noise/racket you have to listen to all day.

 

[D. Showan]

Boldt's wagon, that makes sense to me. If the trans temps don't go over 180-190 degrees I can't see how it can hurt the trans. My temp sensor is in the outlet line going to the converter so I would see any temp spikes fairly quickly.



The fluid coupling in this Goerend converter is much better than the stock one so maybe the temps won't go up to much (at least on some hills)



I'd still rather pull in 4th locked so a fuel plate will probably solve most of my concerns but I may wire it up to control lockup any way and do some experimenting this summer while I travel

 

[D. Showan]

Extreme1,

I understand the 3K gov spring kit will help the engine pull over 2500rpm but won't running RPM that high hurt the MPG quite a bit??? or does the spring kit somehow help the efficiency at higher speeds????