serious discussion of 5-ton speed increase via gearing (transmission, differential)

[OPCOM]

I learned today that there is a 4.92 ratio for the 800 series differentials and at at least the ring and pinion were available. Yes someone out there in truck parts land is sitting on a few. Did not check for the spur and bull gears. Diff gears is the right way to do it of course. I also learned that the cost of parts for that 68 MPH redline is >3000. per axle.

 

[rwbrown72]

Yikes!!!!!

 

[73m819]

I learned today that there is a 4.92 ratio for the 800 series differentials and at at least the ring and pinion were available. Yes someone out there in truck parts land is sitting on a few. Did not check for the spur and bull gears. Diff gears is the right way to do it of course. I also learned that the cost of parts for that 68 MPH redline is >3000. per axle.

just remember, going to 4.92 will hurt you on hills with the 250, now a 400 would power you over hills like a moto-cross racer

 

[kastein]

3000 per axle... good lord!

Stock gears are 6.64 right? That's a 35% increase in ground speed at redline holding tire size equal, for ten grand.

I'm almost certainly throwing some 395s on mine (~10% increase in ground speed) and if I can find an overdrive aux transmission for a decent price with .63-.74 or so gearing I will likely do that too. Should get around a 75mph redline (instead of 50) but I will keep it at 60 or so and let the engine rest, I'd rather go a bit slower and keep my connecting rods unstretched.

 

[jwaller]

Just remember that as you stack up the gear ratio and the rpm's come down; that nhc250 is not going to be able to hold road speed empty, let alone pull anything.

To keep it moving your going to have to keep the engine at redline which means using all the gear you put in it.

 

[jwaller]

Honest john missle trucks had a different ratio too but I don't think it's going the right way for what you are trying to do.

 

[OPCOM]

The 400 Cummins suggestion is great. That wil have to wait though. I'm just getting to know the 250. Does anyone have the torque curve for the NHC250?

I bought that transfer off epay. The seller ck975 does not seem like a sketchy person and has some trucks for sale as well. This will be the least costly route and is my second choice after finding out how much differential gears cost. Shipping it is going to be $200.

The main area now where help is needed is flange sizes.
Memphis told me the flange sizes are different from the T-138 found on the M818.

Can anyone comment on the difference in input, rear, and front connection differences between the M818's T-138 case and the M939's T-1138 case?

Since this has been done before, someone ought to know what coupling hardware I may need. haha please advise!!

The 400 Cummins suggestion is great. That wil have to wait though. I'm just getting to know the 250. Does anyone have the torque curve for the NHC250?

I hope that as this project continues, the members here will keep helping fantastically with answers and advice.

 

[Hammer]

That looks, uhm, HEAVY!

 

[OPCOM]

Just remember that as you stack up the gear ratio and the rpm's come down; that nhc250 is not going to be able to hold road speed empty, let alone pull anything.

To keep it moving your going to have to keep the engine at redline which means using all the gear you put in it.

The TM says 1500-2100 for the operating range but no other data. For sure the beans drop off real fast. I wish I had the torque curve for it. I've been working on a spreadsheet but can only estimate the torque. No way to measure it.

I had though the torque curve for a diesel was pretty flat above a certain RPM and went on out to redline but what do I know.

I used a pyro on the M35 and was able to improve it greatly by properly using that. I have no experience with the 250 at all except a looong drive in it at 12 degrees F with a ruined PT pump.

 

[jwaller]

I am very interested to see how this turns out for you bc if it works out like I think it will, I'll be next in line to swap xcases. Somewhere I do remember seeing the torque curve for a 250. I think it's in the -20 or 24 stuff for the motor, not the truck.

 

[Hammer]

Torque curves on a diesel with turbos are pretty flat.
Naturally aspirated are not the same. Even though they are not that bad, they still don't act the same.
Isn't the peak torque rated at 1,500 rpm?

 

[OPCOM]

The gentleman at CSI was kind enough to count the gear teeth for me. The input gear has 41 teeth and the output gear has 30 teeth. The OD of the case is therefore 0.73170731707317073170731707317073 to 1

I have to give kudos to CSI for taking the time to check a case they had sitting apart in the shop and answer a hobbyist's question. If you look at their site, they are in business to take care of the big guys. What I am saying is that I called all of the big vendors and this one is who came through for us.

Most in the truck rebuilding business are not aware of the OD status of these cases. I doubt this will drive up the cost though because in the normal use, breakage is high for reasons already discussed here in this topic. I was told by one of them that due to so much breakage under abusive conditions, the majority of the value of a T-1138 case is the case housing itself not the innards.

One discussion with a vendor came away with the breakage is mostly due to the very deep reverse gear in the Allison automatic transmission, plus the higher torque provided by the torque converter. The circumstances cited for breakage was where the vehicle was fully loaded and the driver gave it the lead foot on low reverse.

For my part I will make it a rule never to use low reverse anyway. I have never had to use it and any use of that combination in my case would only be necessary for the most extreme or emergency circumstances.

OK well I need to go look for that NHC-250 TM and find the torque curves. I have only the parts manual for the newer type 250 used on the 900's.

 

[doghead]

info. Cariboo 6x6 Trucks: M-812A1 and the M-945 specifications

http://www.mil-veh.org/archives/03-08/0724.html

https://www.fbo.gov/?s=opportunity&mode=form&id=372486b2a81b054b8bc973280948c34a&tab=core&_cview=0

 

[OPCOM]

The Cummins NH250 engine is 855 c.i.d. small (2 in.) camshaft engine rated
250 hp @ 2100 rpm with 654 lb-ft torque @ 1500 rpm and clutch engagement
torque at 600 lb-ft @ 800 rpm.

yeesh 600 Ft Lb @ 800RPM. Just for clutching.. not a working speed no doubt. I had no idea it had any beans down there and didn't want to find out.

Unlike a car engine though it ought to inject less fuel at low RPMs, so as to limit the torque.

Attached is a detail of cummins part numbers for the manuals and where to get them if they are still around. I cleaned up that old post.

Thanks Doghead!

 

[OPCOM]

3000 per axle... Good lord!

Stock gears are 6.64 right?

6.44

 

[nitro_rat]

You can turn the governor up to 2400-2500 rpms on the cummins without hurting it. Pulling trucks do more. You'll damage it faster by lugging it than by revving it. I wouldn't hesitate to turn 2400 rpms.

 

[73m819]

You can turn the governor up to 2400-2500 rpms on the cummins without hurting it. Pulling trucks do more. You'll damage it faster by lugging it than by revving it. I wouldn't hesitate to turn 2400 rpms.

as long as you rev your 250, thats cool, though I dont see it lasting long, a over rev for a built pulling truck is one thing, so you get a few second runs out of it, fine, you turn one of these 250s to 24 or 2500, going down the road, I hope you have another motor in the shop. you will have valves hitting pistons, then down hill from there

 

[OPCOM]

It will be after the first of the year before I can get the shop to assist in this. It's a matter of all the 18-wheeler truckers wanting their stuff done over the holidays, and that work pays much more than what they will be making off me.

Other work on the DieselFitter project is on hold until the transfer is done. I am going to pick up a couple of A/C units in NM over the holy days. Also going to stop at the Black Hole of Los Alamos, to feed my science diet.
Los Alamos Sales Company - Home Page
The Black Hole

 

[The PIG Smith]

I need to go back and reread these 8+ pages/78+ posts to fully understand all that has been said.
Some of this over my head.

Anyways, one concern I have about owning a M-818 is the ability to maintain highway speeds.
Sure a M-915 or greater would be better suited for my needs, but I perfer the looks, sounds and attitude of a tactical truck.

My solution is a comprise and that is to seek out a XM757 8x8 tractor or take a XM656 and convert it into a tractor.
Why?
These 8x8 rigs have the looks, sounds and attitude of a tactical truck that I desire.
From what I understand they can perform safety at 60+ MPH and offer a smooth ride.
So best of both worlds...the kewl factor of a MV vehicle and not clogging the highways for other folks.

 

[rwbrown72]

I need to go back and reread these 8+ pages/78+ posts to fully understand all that has been said.
Some of this over my head.

Anyways, one concern I have about owning a M-818 is the ability to maintain highway speeds.
Sure a M-915 or greater would be better suited for my needs, but I perfer the looks, sounds and attitude of a tactical truck.

My solution is a comprise and that is to seek out a XM757 8x8 tractor or take a XM656 and convert it into a tractor.
Why?
These 8x8 rigs have the looks, sounds and attitude of a tactical truck that I desire.
From what I understand they can perform safety at 60+ MPH and offer a smooth ride.
So best of both worlds...the kewl factor of a MV vehicle and not clogging the highways for other folks.

When you find the xm757, get two of them and I'll pay you for the second one!!!