More Power From NHC 250 in M939 Series Trucks With Allison Transmissions

[patracy]

A little more food for thought.

855 Cummins duel fuel line mod? - The Combine Forum

I'll try it as well, seems to make sense. I question if it will really make 25hp. But it should help with fuel supply.

My thoughts are this...
Turbo setup of course
Smaller button
Adjust the throttle shaft
Dual line setup (I'll just have the local shop make one for me)
Pull timing out with the rocker boxes
Shim the governor spring for a little more fueling
Boost and Pyro gauges

 

[Beerslayer]

Thanks to the link to the Combine Forum, there are good photos of a dual fuel line setup on the third page of the post. Looks simple, think I'll try it.

 

[OPCOM]

About tires and RPM. When I drove the M818, which is normally a 52MPH/2100RPM truck, back from IL with 16.00x20's on it, the tach showed 2100RPM at 59MPH GPS speed. With 11.00x20 tires on it and having exchanged the T-138 1:1 t-case for the T-1138 0.73 OD T-case from a 939 truck, it can run 69MPH at 2100RPM. I usually do 60 in it and its very happy at 60. It is at its limit on power, for sure, and it's not redlined. I could be wrong but I was advised to stay off the redline on the 855 or NHC250.

I'm guessing the 939 type of truck has different transmission -a 1:1 lock up, the Rockwell T-1138 t-case, and maybe different rear end gearing. Maybe it can go a little faster with a tire change and accelerate better with the mods, but the thing with the continuous engine power (as in highway use) seems like it is there to stay.

I also noticed the front tires get much warmer than the rears after a run at 60MPH. More so than they did on the M35. No big deal but its one of the things that ought to be mentioned. I don't much care for used military tires on the front on the highway, prefer semi tractor steer tires.

 

[Csm Davis]

About tires and RPM. When I drove the M818, which is normally a 52MPH/2100RPM truck, back from IL with 16.00x20's on it, the tach showed 2100RPM at 59MPH GPS speed. With 11.00x20 tires on it and having exchanged the T-138 1:1 t-case for the T-1138 0.73 OD T-case from a 939 truck, it can run 69MPH at 2100RPM. I usually do 60 in it and its very happy at 60. It is at its limit on power, for sure, and it's not redlined. I could be wrong but I was advised to stay off the redline on the 855 or NHC250.

I'm guessing the 939 type of truck has different transmission -a 1:1 lock up, the Rockwell T-1138 t-case, and maybe different rear end gearing. Maybe it can go a little faster with a tire change and accelerate better with the mods, but the thing with the continuous engine power (as in highway use) seems like it is there to stay.

I also noticed the front tires get much warmer than the rears after a run at 60MPH. More so than they did on the M35. No big deal but its one of the things that ought to be mentioned. I don't much care for used military tires on the front on the highway, prefer semi tractor steer tires.

Okay where are you getting 16.00 x 20 steer tires? Also isn't the Spicer 5 speed a overdrive in 5th or is that just the one in the deuce? Think your M818 and the M939 have the same axle ratio so I would think that your 818 would be the faster of the two.

 

[73m819]

818 has the od trans

 

[KaiserM109]

Subscribe me; I pickup my M923 on Nov. 19, 2012 at Ft. Benning, GA.

I'm moving up from a non-turbo M109, so I expect to be blown away.

hasbeen, your depth of knowledge is greatly appreciated. When I understand it better I will definately refer to it. I have bookmarked this thread.

 

[alignv3d]

Now I need to add this. Bought a whole truck with a 350 small cam

 

[bugwugger]

I'm just going to cut the tamper seal on the pump and crank the screw on my 923 and see what happens. Would like to find the tm or info on screw cranking !! Lol

 

[OPCOM]

"Okay where are you getting 16.00 x 20 steer tires?"

I'm not getting 16.00x20's. It had them before I changed the T-case, but i didn't like them and went to 11.00x20's and changed the T-case to the T-1138.

I think at this point in time my m818 is the faster of more than the M939. But I don't drive it fast and I enjoy a lower RPM instead, which was the point.

The arrangement is: NHC250 -> M818 original Spicer OD 5-speed -> T-1138 OD transfer case -> stock M818 axles -> 11.00x20's.

I was just saying the fronts get warmer than I like and if I am going to be running 60 on highways instead of going slow offroad I want to put better front tires on for that application.

 

[OPCOM]

"Now I need to add this. Bought a whole truck with a 350 small cam".

What's the difference between that and the MHC250 small cam? I though the 250 block and the rest was maxed out on power. Does the 350 have cooling jets?

 

[GHall]

Talking about shimming the throttle spring? Is that the spring on the bracket attached to the IP? What is the best way to shim it?

thanks for any help!!

 

[WillWagner]

Talking about shimming the throttle spring? Is that the spring on the bracket attached to the IP? What is the best way to shim it?

thanks for any help!!

No. All the tuning is done inside the pump. The spring on the outside is just a return spring.

 

[73m819]

Well I install the 22 button (stock is 27) have not been on the road yet, waiting to install the pyro., will let all know the results on the raising fuel by the lowering the button number

 

[sandcobra164]

I've got an M923 hanging out in the front yard that can't keep up with a turned up Deuce. The M923 is now on 14's, the Deuce is on 11's, what fuel button does the group recommend? The 923 is strong and can blow by the Deuce once rolling, he just needs a little low end help. The 923 has never blown so much as a puff of black smoke so I know he's got more power in him. The Deuce blows so much it's like rolling behind a locomotive but in his defense, he has alot of filtered waste oil in the tank.

 

[Jayco36REQS]

I turned up the pump about 3/8 of a turn and now I don't loose speed going up hills. It has just a wisp of black smoke going up the same hills.
Subscriped

Do you have any documentation on how to do this? I have tons of experience modifying all sorts of gas motors, but have zero experience playing with big diesel motors... Definitely a newb with these, but always willing to learn!

 

[Jayco36REQS]

Ok, so I haven't gotten my M931 home yet, but I've already started spending money on it LOL.

Picked up a HT3B and manifold this AM for a great deal.

I know I'll have to get creative with the air filter pipe and the charge air pipe from the turbo.

VERY curious to see how your install goes... Keep up posted with progress pics!!!

 

[Hasbeen]

Do you have any documentation on how to do this? I have tons of experience modifying all sorts of gas motors, but have zero experience playing with big diesel motors... Definitely a newb with these, but always willing to learn!

https://quickserve.cummins.com/qs2/pubsys2/xml/en/procedures/18/18-005-024.html?q=+category:service language:en^2

[h=1]005-024 Fuel Pump Fuel Rate with Throttle Shaft Plunger[/h][h=3]Table of Contents[/h]

Adjust

[HR][/HR]

[h=2]Adjust[/h]

TOC

To raise or lower the fuel rate, the ball must be removed from the end of the throttle shaft. Mark the ball with a center punch. Drill a 4 mm [0.187 in] hole in the ball. SMALL | MEDIUM | LARGE Next Use an easy out extractor or a slide hammer to remove the ball. SMALL | MEDIUM | LARGE Previous Next Adjust the throttle shaft plunger screw to raise or lower the fuel rate. Turn the screw clockwise to lower the fuel rate. Adjust the fuel rate with the screw. SMALL | MEDIUM | LARGE Previous Next CAUTION Do not turn the plunger out closer than 6 mm [0.250 in] from the end of the shaft. The fuel pressure can push the plunger out of the shaft. Spilled fuel is a hazard. SMALL | MEDIUM | LARGE Previous Next If the maximum flow (1) the pump can be adjusted to is below the correct value, replace the fuel pump, refer to Procedure 005-016 SMALL | MEDIUM | LARGE Previous Next After the fuel rate is adjusted to the correct value, install a new ball in the end of the throttle shaft. This will help prevent tampering. SMALL | MEDIUM | LARGE Previous

Last Modified: 29-Oct-2004

Feedback / Help
Copyright © 2000-2010 Cummins Inc. All rights reserved.


As I`m trying to post this "copy and paste" web page from Cummins` website, I`m frustrated because I`m pretty sure the link or the illustrations will not show up or cooperate.
I`m not sure if it is because the Cummins website require a login ( I have one) or I`m just a technically challenged idiot (nowhere on this computer will a 9/16 wrench fit so I`m lost)

Jayco36reqs you asked so I tried.......

Those fuel systems were designated by Cummins as "PT" meaning pressure/timed. The Fuel injection pump itself was designate "PTG" pressure / timmed / governed....the key here is pressure. As I recall there are basically 4 adjustments that all work in concert within the injection pump, but I can`t stress enough, the need for any or all of the adjustments to be in small increments or checked on a calibration "test stand" by a fuel injection shop. Typically a reputable fuel shop will make adjustments up to 10% above the rating, or spec, that the engine spec call for, with no argument (but you have to request it).


The 4 adjustments are the orfice or "button" in the rear of the pump.
The r.p.m. shims (next to the orfice)
The throttle shaft / leak off adjustment (detailed above in the cummins literatue) under the tin side plate on the pump.
The "torque" shims inside the front cover with the drive gear/flyweights.

You can crank them all off the chart, but the guys in Columbus Ohio were pretty smart when they came up with the PT system. Everything has a price( cause and effect), but it seems horsepower really gets down to the way we manage it.

Most of us never really "load" our vehicles......so extra fuel adjustments (other than max. rpm adjustments) aren`t really a big deal. What I mean is.....the diffence between a nh250 fuel delivery rate with the engine banging the governor (high idle) sitting in the driveway, vs. the same rpm at "full load" no matter what speed you`re running is drastically different.

Cummins rated that engine (without piston cooling nozzles, aftercooler, turbo etc) to live a long life........

If we crank em up AND load them up.....we need to pay more attention to the temps.

I have forgotten most of what I knew about these fuel systems, when I was fresh out of Diesel Mechanics school and workin in a fuel injection shop in the 80`s....but the guy I worked for still has a shop, was the best "small cam" Cummins guy I`ve ever seen, and actually like helping people.

If you need better info or recalibration work on your pump or turbo...pm me and i`ll give you his contact info in Jackson Mississippi.

Good Luck with your project.

Hasbeen




​

 

[Jayco36REQS]

WOW... thanks Hasbeen... Much appreciated!

I just got my 923A1, and am finding it a bit gutless to say the least. I've driven big trucks most of my life, even up to running 10,000 gal Jetfuel trailers for the Navy. I've also worked on all kinds of gas engines. Just never had the need to crack into the big diesel engines. So not new to engines, or injection, ro forced induction systems.. just these big motors.

Long term I plan to turbo it.. but I'd like to get a little more power out of it for hills or in the woods for now. Like you said.. cause and effect is always there. I'm eager to learn form those who have played with these to see where the envelope is.

Thanks again!
Gary

 

[Clayshootr]

I have a M931 that came out of Camp Shelby. I was disappointed with the performance of the truck. I had it almost a year before I finally decided to take it in to the shop. It would fall flat on any kind of a hill and when in neutral the rpm's were very slow to increase when the throttle was opened.
The shop pulled the pump and found it was very rusty inside. They put it on the stand and rebuilt it. They changed the button, increased the rail pressure, and did something with the governor. He said it would shut the fuel off at like 1900rpm when high idle no load rpm was 2350. All I know is it is a different truck now, a lot more responsive.
It is still a under powered 250 but it is much better after the work. I know all the adjustments that are talked about above work!
I hope to turbo this thing, just dont know how much the tranny will take.

TJ

 

[Jayco36REQS]

I know a few guys in the pulling world that are into the Super Stock tractors running 1,500HP twin and tripple turbo diesel tractors.. crazy stuff. I talked to one this evening and he got a little too excited to play with my truck.. I told him I wanted to keep the auto trans, and not looking to do 6-wheel burnouts! lol

So I'm looking between turbo or blown... I've got way more experience with blower motors.. but never on a diesel. Seems no one has tried to blow one of these 250's. Could be a wicked combo if they could handle the bottom end boost.