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

[Beerslayer]

The flood of M939 series trucks has started, and we have a new toy to play with. After having read ~all of the related threads on M939 series trucks, motors, transmissions, there seems to be some room for a separate discussion of how to get more power out of the NHC 250 when it is installed in front of the Allison Automatic.

The M923 I have has the Allison MT 654 transmission, rated for 300 HP and 950 ft lbs torque. The Allison dealer did not want to up the HP on my motor in front of that transmission beyond what the transmission is rated for. I understand why.

From what i have been able to glean, the NHC 250 is 855 cubic inches, and is the same motor as the Cummins 350 and 400 but with smaller injectors and less fuel to them.

The driveability of the truck would be greatly enhanced with another 100 HP or so, and I think that as an owner operator I am grown up enough to be able to safely operate it with increased HP without tearing up the transmission. Especially considering how difficult it is to even get a full weight load on the truck.

Here is what I would like to hear from you about:

1. What are your thoughts on the ability of this transmission to handle the increased HP?

2. I have been told that all one needs to do to get more power is to install, say 350 HP injectors and turn up the pump, that a turbo is not needed. Maybe just a pyrometer. Is this true?

3. What models of Allison transmissions have you encountered that were installed in M939 series trucks?

This has already been thoroughly discussed, but seems pretty specific to the M800 series trucks, so here we go!

 

[wreckerman893]

I would say that you need a turbo if for no other reason than to keep down the smoke.

For what it would cost to do all those mods you can prob buy a M915 and take the engine out of it. They just cut four of them up at my buddys place at Camp Shelby.....engines and trans (Allison) together went for a less than 4 grand.

 

[wsucougarx]

Subscribed.

 

[quickfarms]

subscribed

 

[M35A2-AZ]

I was told to add a turbo.

subscribed

 

[Josh]

I'd add a Turbo. The more air you cram into a diesel the more power you can get. I'm sure ya knowing this but, a diesel engine is just a massive air pump. The easier it is to get the air in and out again the more power/mpgs its going to have. There is a maximum the engine can handle, but I've know of people getting 350hp out of an old 250s simply by adding a turbo, an intercooler, and ajusting the fuelrate and timing. These are on civy trucks, but the engine should be the same.::

 

[Beerslayer]

I would say that you need a turbo if for no other reason than to keep down the smoke.

For what it would cost to do all those mods you can prob buy a M915 and take the engine out of it. They just cut four of them up at my buddys place at Camp Shelby.....engines and trans (Allison) together went for a less than 4 grand.

Ouch. What a shame chopping them up like that. I had been watching the auctions for 400s in the crate, but better to get one out of a working truck with a matching transmission. I like the idea as I could also get a front spring back and shackles at the same time. Please don't ask what sort of truck defilement I have planned for those.

I'll PM you for his contact info.


Not giving up on this 250 though yet! It runs like a top and is already installed.

So, Cummins Experts, are there naturally aspirated 350 and 400 engines? Or is a turbo a necessary part of the package to get that fuel burned?

 

[Monkeyboyarmy]

The turbo will actually help keep the engine cool. If you are pulling a hill and you let the engine lug, the egt's will go right up.Drop a gear or two and let the turbo spool up and the egt's will drop 300 degrees.

For what it would cost to do all those mods you can prob buy a M915 and take the engine out of it. They just cut four of them up at my buddys place at Camp Shelby.....engines and trans (Allison) together went for a less than 4 grand.

NO NO NO! Don't these people know that there are poor souls like me trying to find parts for these trucks?

 

[Hasbeen]

The 350 and400 engines are "NTC" family of 855 Cummins engines. They were configured as "Power Torque", "Formula", and "Fleet" designations, in a variety of horsepower, rpm ranges, and torque curves depending on performance, driveability, or fuel economy.
Horsepower ranges were,250,370,290,300,315,350 and 400.

The biggest difference in these engines over the NH250 is they are "Big Cam" engines vs. the NH family (nh250) are "Small Cam"

virtually none of the components are interchangeable between SC and BC Cummins engines, even though they are all 855 displacement.

As mentioned above, you CAN make more horsepower out of your existing engine, but in my opinion, the h.p. gain will shorten the lifespan simply because the rotating assembly, valvetrain,(lower cam followers and upper rocker boxes and camshaft) weren`t designed for it.
The Big Cam improvements of "lower press fit" cylinder liners, full flow cooling, bypass lube oil system, Aneroid Fuel Control , Turbocharged, aftercooled, and pulse style exhaust manifolds made it possible for them to have both a long dependable life cycle, and extra horsepower.
If you have access to the later Big Cam style 855, and you plan on putting a lot of miles on the truck, the Big Cam is the better investment.

If you don`t plan on putting a lot of miles on it, you can cannibalize intake/aftercooler,turbo,exhaust,fuel pump and injectors from a Big Cam engine and improve yours.

If you retrofit yours with fuel/turbo upgrades I would make two suggestions.

First I would consult a Fuel Injection rebuilder for the possibilities and combinations that are reasonable as far the the size (injector flow code /pump clibration) to work with your camshaft and piston set up.

Second, If you turbocharge it, be advised that there is a tiny drilled passage in each cylinder head that will need to be plugged with a 1/8 npt pipe plug. These hole are directly above the intake port, and function as part of the crankcase vent system of the naturally aspirated NH250. If you put "boost" pressure from a turbo into the intake, they whistle like crazy and pressurize the crankcase......oooops.

Sorry for the long winded reply, and if you need contact info for a good "old" fuel injection shop, p.m. me. There are are not a lot of shops still around that are familiar with the older Cummins "PT" fuel systems.

Hope this helps.

Hasbeen

 

[Beerslayer]

Really. Thanks for the long ^^ winded reply, great information.

My use would be occasional, not daily, so turbo and bigger injectors would probably be fine. I won't be working it hard and I drive my rigs easy.


So any thoughts on the Allison MT 654 and 350-400 HP? I know they put the MT 750 behind the NTC 400. Will the MT 654 stand more HP?

 

[jstier]

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

 

[jwaller]

turbo it. with the 939's I bet you could even get the intercooler to fit under the hood.

 

[Hasbeen]

The "business" end of the fuel pump is the small "cone shaped" cover on the lower rear with four attaching bolts.
Under the cover is a cylindrical tube that houses the orfice or "button" that controls the fuel "rail pressure", and the shims that govern the r.p.m.

the buttons are marked with a number that is a size reference. It`s been way too long since my job was rebuilding them, the injectors, and turbos, but I believe you remove the cover with a 7/16 socket, remove an internal snap ring from the tube, and replace button and/or shims while on the vehicle.....pull yours and call a fuel shop and buy a smaller button (increase rail pressure) and make more power even with the injectors you currently have. But like someone else stated....more fuel....more heat.....get you a long coat hanger to reach up and roast your hot dogs on the "blue flame" coming out of the stack

kidding (sort of) the fuel guys can give you a reasonable cause/effect scenario. In the old days guys used to restrict the return fuel line coming from the injection pump in order to produce the same increased rail pressure. Guys would use vice grips to "crimp" the copper return line slightly, or I`ve hear of drivers that kept a dime with a small hole drilled in the center, that they would insert under the flared fitting in the return line to make more rail pressure by restricting the return flow.....they would install and remove on each trip while driving a "company" truck.

Man I`m showing my age....

Whatever you decide....good luck to you.

Hasbeen

 

[goodguyzy]

great thread so far, I looked at brads turbo project on his 923. I plan on installing a turbo and increase fuel with one of these methods. Since I'm not an expert on diesel engines I plan on consulting and or working with a cummins shop.

 

[Beerslayer]

So how about that Allison MT 654? Will it stand up to 350/400 HP? Allison says it is rated for 300 HP and 960 ft lbs torque? Thoughts on this?

I don't mean for heavy use, but as a hobby truck, occasional hauling driven reasonably. How tough are those Allisons?

 

[gunner01]

your allison tranny is under rated by about 25% . You should be able to safely put 350-375 HP to the tranny. remember that driving conditions and driver conditions vary. that being said if you are the sole driver and you dont try to impress people with this truck by doing stupid things ie: mudbogs etc your truck should hold together just fine with occasional use and simple to moderate offroading. If you ge to the xtreme offroading the HP wont kill the tranny but the increased torque will

 

[MilitaryRestoration]

Subscribe and if in time will chime in with my own project... bought a ntc-400 that will be replacing the nhc-250 currently in the 900 series truck. The truck stuck a piston and after evaluating costs to rebuild the old 250, the 400 was a much better cost effectiveness decision. The motor is currently turned up say to 425 but prior to putting it in i may detune it down to 350-375. From what I have heard and talked to people... the transmission will take it, just dont put your foot in it, aka drive respectably... good luck and when i get mine done, expect a post

 

[M813A1]

To Beerslayer yes Cummins made Naturally Aspirated 350's and 400's engines they are torque monsters and sound strong !! I think I am going to just add the fuel button and the fuel line mod to the front of the block. and maybe try the dime in the return line to incease tthe fuel pressure !! And I will add a pyrometer to watch the EGT temps .

 

[Bolkbich]

Pittsburgh power set me up with a bunch of turbo's over the years. They sold me a turbo and a pulse manifold for a small cam 335 that made 36 lbs of boost at 200 psi fuel pressure...every 2 lbs increase of boost drops your egt 50 degrees.. stock boost was 16 psi ...with no intercooler i was making 480-500 hp at 800-900 degree's on the pyro. I have 335 turbo, injectors , fuel pump( that was set up by pittsburgh with a # 5 button) on my m817 for 15 years now. It still runs like a top making 375 +- hp. Dont cheap out and use a stock cummins turbo, the aftermarket turbos make much more boost at much lower rpms....and boost is your friend.

 

[MidnightDuramax]

@ MilitaryRestoration: Where did you get your 400 from? I am looking to do the same thing with my M923.