M920 Differential Gears

[M920]

Nice work oldMan99!

I think you are dead on with your calculations and they closely correspond with what I have encountered in the field. All I would like to add to your findings is that
my numbers where based on the tach and speedometer readings in my truck, since I did not own a GPS at the time, and that the tires are definitely 46" tall.....LOL.

I would also like to add, that unlike the new electronically controlled engines, that can make good power and get good milage at low RPM, the Big Cam Cummins in question, does not like to be lugged and needs the higher RPM to maintain good milage.
As a matter of fact, running this engine against the governor all day long, with the 6.17s, versus the lower RPM after switching to the 4.88s, did not make much difference in the milage at all!
The only thing I can think of, as the cause of this, is that the increase in milage that should have occured from the gear change, was offset by the increase of wind resistance from the higher speeds now optainable.

So in my opinion it is only worth to go through the trouble of a gear change, if you want to increase the top speed of your truck. It is not going to do much to increase the milage unless the driver has the "superhuman" capabillities to keep his foot of the throttle! lol

 

[oldMan99]

As a matter of fact, running this engine against the governor all day long, with the 6.17s, versus the lower RPM after switching to the 4.88s, did not make much difference in the milage at all!

The only thing I can think of, as the cause of this, is that the increase in milage that should have occured from the gear change, was offset by the increase of wind resistance from the higher speeds now optainable.

So in my opinion it is only worth to go through the trouble of a gear change, if you want to increase the top speed of your truck. It is not going to do much to increase the milage unless the driver has the "superhuman" capabillities to keep his foot of the throttle! lol

I am guessing that your probably right about that.

--

Not to hi-jack this thread but we have pretty much answered the primary question (Except where to get the parts) please allow me to expand my general knowledge.... (And maybe a question others might have as well?)

So, here is a totally hypothetical question for you, (Disclaimer: I am not a 18 wheeler size engine expert. In fact, know pretty little about their specifics)

I know you have replaced the engine (And trans) in your 920 with a 600hp electronic unit and your already noted you get 6.5mpg (46" tires) with that combo. I have forgotten if I saw what your redline speed with the new set up is??

The below questions are open to all those of you with the experience and knowledge to answer....

First a couple general questions: Of all the 18 wheelers currently on the road what would the average MPG be? Anybody have an educated number to throw at that question?

What 18 wheeler currently is known as the "MPG champion" and what does it get? (I know there are tons of variables including load, driver skill, terrain and so forth but there must be some general numbers that can serve as a guideline.)

So, the million dollar question: Money being absolutely no object - Keeping in mind we want plenty of power, in fact excessive power even... If the quest was to build a 920 with the very best MPG and have a truck that had plenty of power to get any job done without breaking a sweat... What motor, trans and ring/pinion ratio would choose and what would you GUESS the MPG and top end MIGHT be?

 

[Bighurt]

IFirst a couple general questions: Of all the 18 wheelers currently on the road what would the average MPG be?

5.5-7 mpg hauling

What motor, trans and ring/pinion ratio would choose and what would you GUESS the MPG and top end MIGHT be?

I think Soni has got to be close. From what I've read on the 600ISX I can't imagine a better powerplant for the truck.

What kills the M920 and it's the same problem with over the road units of that era. Is we had a horrible understanding of aerodynamics. In fact the units we call aerodynamic are really not aerodynamic at all. And they are getting 7+ mpg.

Another issue when considering mpg, is the height off the ground. There is a point based on speed at which you begin to reduce drag by lowering the vehicle. the closer the vehicle is to the ground the less drag on the vehicle. Usually that is a few inches. Soni's truck is no where near that, he's dragging a sail through the air.

The drag isn't necessarily the height it's the fact that there is all that crap under there creating tubulance. If we had a solid belly pan the vehicle would create less drag. Look at the guys with the 70+ mpg cars. All of them have modified the car to have a solid belly pan.

I don't suggest either for the truck, our best solution is gearing and lowering top speed. The government (if you trust them0 did a study (55 mph national speed limit). In the study they concluded that for every mile over 55 that a truck traveled it used a certain precentage more in fuel. Now of course we all now speed has an effect and I can't recall the actual numbers but they do play a part.

 

[oldMan99]

5.5-7 mpg hauling



I think Soni has got to be close. From what I've read on the 600ISX I can't imagine a better powerplant for the truck.

What kills the M920 and it's the same problem with over the road units of that era. Is we had a horrible understanding of aerodynamics. In fact the units we call aerodynamic are really not aerodynamic at all. And they are getting 7+ mpg.

Another issue when considering mpg, is the height off the ground. There is a point based on speed at which you begin to reduce drag by lowering the vehicle. the closer the vehicle is to the ground the less drag on the vehicle. Usually that is a few inches. Soni's truck is no where near that, he's dragging a sail through the air.

The drag isn't necessarily the height it's the fact that there is all that crap under there creating tubulance. If we had a solid belly pan the vehicle would create less drag. Look at the guys with the 70+ mpg cars. All of them have modified the car to have a solid belly pan.

I don't suggest either for the truck, our best solution is gearing and lowering top speed. The government (if you trust them0 did a study (55 mph national speed limit). In the study they concluded that for every mile over 55 that a truck traveled it used a certain precentage more in fuel. Now of course we all now speed has an effect and I can't recall the actual numbers but they do play a part.

You know, instantly as I read your response I remembered all of that same stuff...lol...

I had completely forgotten about the underneath airflow issue. This is why the new "Anteater" trucks all have fairings that go pretty close to the ground and extend all the ay down the sides of the tractor. I remember seeing pictures of complete rigs that had the same fairings all the way down the trailer. I can't remember sever seeing one like that on the road though. Some even install those skirts on the tractor to within an inch of so of the highway to try to control the underneath air.

When looking at the 920 there is so much crap that will screw up what little aerodynamics there is it is not even funny.

Soni is getting 1 to 1.5 MPG more with his new engine that he did with the stock set up. Your going to have to drive a LOT of miles to use the increase in MPG to justify that swap... LOL... Still, it is an interesting question....

I also remembered the 55mph studies you mentioned. I wonder if Soni ever did a trip at 55 with the stock setup just to see what the MPG was?

I do know the effect of drag is not constant it is exponential. The increase in resistance from 55 to 60 is way more than the increase from 50 to 55 and the higher you push the speed the worse it gets, so it does make good sense that as Soni noted, the improved efficiency in the engine is likely absorbed by the increase in drag from driving at higher speeds. I wonder what MPG he would get at 55 with the new motor? (I wonder if he has the patients to burn a whole tank of fuel at 55??? LOL)

 

[Bighurt]

I do know the effect of drag is not constant it is exponential. The increase in resistance from 55 to 60 is way more than the increase from 50 to 55 and the higher you push the speed the worse it gets.

No offense to Soni, but sometimes he seams to just have money to spend... maybe preferred chassis pays his MV addiction.

Anyways, yes it is exponential but there are quirks.

For example my F350 when stock sucked mileage bad at 70 mph, so I drove 65. It was a toss up between pay way to much at the pump vs be late to work. And I hated getting passed at 55.

Now my F350 is modded and tuned, she gets way better mileage than stock but driving 65 doesn't help as it once did. The reason for my truck was that the rpm's changed enough between 65 and 70 to increase the boost the turbo's put out.

It's just a coincidence that the increase in boost helps the mileage of a tuned (increase fuel pressure) truck.

So I drive 70 again...

Now if I drive at 55 I'm sure it will change significantly but who wants to drive 55.

The effect was also noticed when i haul my RV again I used to tow at 65 getting about 9 mpg, now I tow at 70 and get almost 12 mpg. Sure if I slow down it will get mildly better but then we have that same problem...

Thankful I really only spend fuel money in the winter as I ride the R6 about 4500 miles during the summer months, both of them!

 

[oldMan99]

No offense to Soni, but sometimes he seams to just have money to spend... maybe preferred chassis pays his MV addiction.

Anyways, yes it is exponential but there are quirks.

For example my F350 when stock sucked mileage bad at 70 mph, so I drove 65. It was a toss up between pay way to much at the pump vs be late to work. And I hated getting passed at 55.

Or...... Leave earlier??




Sorry... just could not resist that....

 

[jamboly]

He guys can you dumb down a little for me. You guys are way beyond me with trying to improve the mileage, speed etc. I am just trying to see what it would take (if parts are available), to fix my M920. The problem I have is that when I went to pick it up, the fluids had been drained in engine and transmission because of EPA concerns with leaks. We got a wrecker to roll it on the low boy trailer (brakes were already caged), back end first. As the rear wheels were going on to the trailer, we heard a loud pop. The pinion gear shaft broke just inside the seal of the differential. Aside from trying to understand why it broke, what are my odds of finding the parts to fix it? Who would I check with for parts availability and cost?

 

[NDT]

He guys can you dumb down a little for me. You guys are way beyond me with trying to improve the mileage, speed etc. I am just trying to see what it would take (if parts are available), to fix my M920. The problem I have is that when I went to pick it up, the fluids had been drained in engine and transmission because of EPA concerns with leaks. We got a wrecker to roll it on the low boy trailer (brakes were already caged), back end first. As the rear wheels were going on to the trailer, we heard a loud pop. The pinion gear shaft broke just inside the seal of the differential. Aside from trying to understand why it broke, what are my odds of finding the parts to fix it? Who would I check with for parts availability and cost?

Ouch, what on earth could have caused that? As far as parts, I would write down all the numbers on the axle data plate and head down to all the drivetrain parts houses on Houston's east side and see what they come up with for you.

 

[Nonotagain]

We got a wrecker to roll it on the low boy trailer (brakes were already caged), back end first. As the rear wheels were going on to the trailer, we heard a loud pop. The pinion gear shaft broke just inside the seal of the differential. Aside from trying to understand why it broke, what are my odds of finding the parts to fix it? Who would I check with for parts availability and cost?

Parts are readiably available for the rears in the M915-920 series truck as well as complete drum to drum units from wrecking yards.

If it were my truck, I'd pull the drive shafts and axles and see if it's the center section or if the drums are rusted to the brake pads. Then check the transfer case to see if there were any problems found.

I've seen axles broken and one member here found that his truck had a broken axle when picked up from the GL lot.

 

[oldMan99]

Parts are readiably available for the rears in the M915-920 series truck as well as complete drum to drum units from wrecking yards.

If it were my truck, I'd pull the drive shafts and axles and see if it's the center section or if the drums are rusted to the brake pads. Then check the transfer case to see if there were any problems found.

I've seen axles broken and one member here found that his truck had a broken axle when picked up from the GL lot.

Stock parts may be available but after a few weeks of goggling and phoning I have yet to find a set of 4.88 or 4.89 (I have been told by various sources each was made) for the 920. Anybody with any leads much appreciated.

 

[jamboly]

Parts are readiably available for the rears in the M915-920 series truck as well as complete drum to drum units from wrecking yards.

If it were my truck, I'd pull the drive shafts and axles and see if it's the center section or if the drums are rusted to the brake pads. Then check the transfer case to see if there were any problems found.

I've seen axles broken and one member here found that his truck had a broken axle when picked up from the GL lot.

I am not sure what you mean by "seeing if it is the center section". Keep in mind, this truck rolled good both forwards and backwards until the rear tires started going on to the trailer. Then there was a distinct pop. The break is a fresh brake (shiny metal across entire shaft), no signs of previously being cracked.

Could it be that when the back wheels were going on that they were trying to turn at a different rate than the axle in front of it?

I'll get the numbers off of the axles and see what I can find in Houston. If I get lucky, I will let you know.

 

[eldgenb]

No offense to Soni, but sometimes he seams to just have money to spend... maybe preferred chassis pays his MV addiction.

Anyways, yes it is exponential but there are quirks.

For example my F350 when stock sucked mileage bad at 70 mph, so I drove 65. It was a toss up between pay way to much at the pump vs be late to work. And I hated getting passed at 55.

Now my F350 is modded and tuned, she gets way better mileage than stock but driving 65 doesn't help as it once did. The reason for my truck was that the rpm's changed enough between 65 and 70 to increase the boost the turbo's put out.

It's just a coincidence that the increase in boost helps the mileage of a tuned (increase fuel pressure) truck.

So I drive 70 again...

Now if I drive at 55 I'm sure it will change significantly but who wants to drive 55.

The effect was also noticed when i haul my RV again I used to tow at 65 getting about 9 mpg, now I tow at 70 and get almost 12 mpg. Sure if I slow down it will get mildly better but then we have that same problem...

Thankful I really only spend fuel money in the winter as I ride the R6 about 4500 miles during the summer months, both of them!

I would not bark up that tree if it were me, The reason Soni did the engine transmission combo that he did was not to blow money but to achieve a truck that could sustain a significant speed while pulling heavy over any terrain, any increase in mileage was unexpected and only a bonus. The 400 is a great engine but with the sheer mass of these trucks pulling heavy it does not have the grunt needed to maintain any real speed "when geared as Soni has his geared" 4.88 with 46" tires and 400hp with a unreliable tranny is no combination that I would want to drive as much as he does anyways.

The needs of one man can not be judged by another. Soni uses this truck more than many of us will ever get an opportunity to do and also as a display case as to what he is capable of in his profession as a professional fabricator and Scorpion manufacturer.

 

[Bighurt]

I would not bark up that tree if it were me, The reason Soni did the engine transmission combo that he did was not to blow money but to achieve a truck that could sustain a significant speed while pulling heavy over any terrain, any increase in mileage was unexpected and only a bonus. The 400 is a great engine but with the sheer mass of these trucks pulling heavy it does not have the grunt needed to maintain any real speed "when geared as Soni has his geared" 4.88 with 46" tires and 400hp with a unreliable tranny is no combination that I would want to drive as much as he does anyways.

The needs of one man can not be judged by another. Soni uses this truck more than many of us will ever get an opportunity to do and also as a display case as to what he is capable of in his profession as a professional fabricator and Scorpion manufacturer.

I think everyone took it as the joke it was supposed to be...

I've had a couple conversations with him via the phone and he hasn't mentioned the comment yet. I apologize if taken incorrectly.

We have discussed his truck and what he's done to it, and why. Yes Soni uses his truck... a lot!

I think Soni and I are very similar, and I would conjecture that many of us are. The need follows the have. If he didn't want the truck he wouldn't need the truck but the fact that he has the truck means now he needs the truck. More importantly it's using the truck that pays for the truck.

I plan to use my truck, point of fact I hope to use my truck to pay for itself and others. My full time job pays for me to eat and live but I need something to pay for my toys.

 

[cundupa]

oldman99, you send me a check for the amount you're going to spend trying to get a 2mpg increase out of a 1979 truck designed to haul huge loads on and off road and I'll pay for the fuel you think you will save. I'll bet I have quite a bit left by the time you're sick of driving that thing!

 

[oldMan99]

oldman99, you send me a check for the amount you're going to spend trying to get a 2mpg increase out of a 1979 truck designed to haul huge loads on and off road and I'll pay for the fuel you think you will save. I'll bet I have quite a bit left by the time you're sick of driving that thing!

Not so much the MPG gain that I am after (But I'll sure take any and all MPG gain I get!) I'm more interested in being able to cruise the interstate at 70 and not have the engine running at max RPM the entire time.

I wonder if anybody makes an axillary overdrive unit that I could slide in between the transmission and the transfer case? I'm not sure there is enough room there for one. If not, I could put it between the transfer case and the rear end. Of course in that location it could only be used when in rear wheel drive only, (Not with the front end locked in) but, since I only want/need it for good roads in good weather that will not really be an issue.

With an Aux. OD unit then I could keep the stock gears which will be much easier to locate replacement parts for if I should ever have a problem inside the axles.

Does anybody see any issue with using an OD unit on the 920/916?

 

[eldgenb]

Not so much the MPG gain that I am after (But I'll sure take any and all MPG gain I get!) I'm more interested in being able to cruise the interstate at 70 and not have the engine running at max RPM the entire time.

I wonder if anybody makes an axillary overdrive unit that I could slide in between the transmission and the transfer case? I'm not sure there is enough room there for one. If not, I could put it between the transfer case and the rear end. Of course in that location it could only be used when in rear wheel drive only, (Not with the front end locked in) but, since I only want/need it for good roads in good weather that will not really be an issue.

With an Aux. OD unit then I could keep the stock gears which will be much easier to locate replacement parts for if I should ever have a problem inside the axles.

Does anybody see any issue with using an OD unit on the 920/916?

just find yourself a fuller RTO or RTLO tranny with the taller OD and swap the whole tranny. If that is not enough put some 16-20's on it, take out the 5th wheel spacer and that should be more than enough to get you the desired outcome with a tranny that you can rely on with minimal cost and proven components.

 

[Slicks]

Does anybody see any issue with using an OD unit on the 920/916?

Yes I think the problem you will find with a double ovedrive and 6.17's might be too fast of a driveshaft speed. It would be a good question to ask a good driveshaft shop about before just doing it.

 

[oldMan99]

Yes I think the problem you will find with a double ovedrive and 6.17's might be too fast of a driveshaft speed. It would be a good question to ask a good driveshaft shop about before just doing it.

Yeah, I thought about that (driveshaft/pinion speed). But... Assuming the max speed is no higher than stock (58mph) the shaft/pinion speed remains the same, just the engine RPM is less.

I will however stop in at the local custom driveshaft shop and see how much of a shaft/pinion RPM increase would be going from 58 to 68/70 and see if that would cause problems in the shafts or rear ends.

I would tend to think that the shafts/hanger bearings would be fine, I am more concerned with the goings on at the pinion/ring gear. That would not really be an issue for the driveshaft shop though. Anybody have any recommendations as to a professional rear end shop that might have real info about this?

 

[mslogr]

Yall know if you want to lower rpm and keep the speed relatively the same, you could flip the gears in a 13 over and make it a double over. We've done that for years, to trucks that came out of the oilfeild. Most of them had 4;63 gears and a 13 so we'd flip the gears to reduce rpm on the top end. The only problem was you lost some on the bottom, but with 6;17 gears you would'nt notice much. I'm currently working on putting a 13 double over in my 920 now. I also thought about high shaft spin, but I'm not looking for a speed demon, just a little more with less hammer on the gov.

 

[jamboly]

I am not sure what you mean by "seeing if it is the center section". Keep in mind, this truck rolled good both forwards and backwards until the rear tires started going on to the trailer. Then there was a distinct pop. The break is a fresh brake (shiny metal across entire shaft), no signs of previously being cracked.

Could it be that when the back wheels were going on that they were trying to turn at a different rate than the axle in front of it?

I'll get the numbers off of the axles and see what I can find in Houston. If I get lucky, I will let you know.

I found the pinion gear and ring gear. Alan, of Houston Drive Train 713-675-6375 was able to find two pinion and ring gear sets, of which I got one. Tell him you need 6.17:1 gears for a Rockwell/ SUHD rear end.