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

[poorman4x4]

I also have a cummins 400 big cam III swap coming up in my M923, midnight locate an 80's model kenworth dime a dozen in the south

 

[MidnightDuramax]

Poorman, any tips on where to search or start looking?

 

[poorman4x4]

LQK is a series of salvage yards in the south, they might be in the north I don't know, they have quite a few cummins 400 big cams, also try heavy truck parts . Net there engines are in the 3-5k range but I guarantee a 500,000 mile cummins 400 is worth it, I found mine through a buddy from a good ol boy in Louisiana I'm sure I can find you one if need be, I don't mind at all

 

[73m819]

I find my 36,000 lb, m819 has plenty of power, the trans sucks (3rd to 4th hole), will run 60+ all day, do not see where more power is needed unless 70 mph is wanted which the 900s are not designed for., military trucks are not designed for interstate speed.

 

[poorman4x4]

I cruise at 60 and understand that military trucks weren't designed for speed, some people want to get to 60 a little quicker. My swap is rationed because I use my truck for recovery/ play at the mudholes, hence I need power to turn my tires a little faster, in the mud! I'm gonna install Detroit lockers with the swap to complete my mudhole ready 6x6 MV, I'm not gonna knock the 250 but the truck you have "to each is own" do what suits your interest

 

[Bolkbich]

I find my 36,000 lb, m819 has plenty of power, the trans sucks (3rd to 4th hole), will run 60+ all day, do not see where more power is needed unless 70 mph is wanted which the 900s are not designed for., military trucks are not designed for interstate speed.

I guess youv'e never had a truck on the back of that M819.

 

[Dixmotorpool]

Hasbeen you are THE MAN, really good info.. I am facing the same situation ., times 2... I used to peddle freight with the old 250's back in the 70's. what an improvement the turbo model made, I didn't know about all the other changes... OH well, I can live with these old dogs... I don't need to go fast anyway.

 

[73m819]

What is the STOCK button number in the ip ???, How much is gained going down each number ?????, which is better, changing buttons or tuning up the fuel at the throttle control ?????

 

[Csm Davis]

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.

Guys I am that buddy, and here's my two cents you can turn up a 250 to way more than it will stay together with turbo or not, I like the old 250 but would Not turn it past 300. If you need more than that go big cam, I have three trucks on my yard that are stock with rods in a bad way. I have a 290 that will be in my wrecker one day and I will be building it to 400 specs. If you just want to get a little more fun out of a 939 I would look at the 8.3 in the a2. But if you are going to work it go big cam or other OTR motor.

 

[OPCOM]

The talk about running up hills and the EGT going through the roof..

With the stock PT pump and stock settings, all 'to spec' will this happen or does the PT pump limit the fuel rate, by virtue of its pressure-time RPM-dependent operation? I have not noticed any issues after putting a T1138 transfer case (0.73 OD) in the M818, and it sure is sweet on the road at 60MPH instead of 50MPH. But I guess charging uphill is an extra load. GVW is 23K, it is not ever loaded or towing.

So what is the real exhaust temp headroom in these NHC250's, with everything stock?
Surely they have been used for years pulling 30K trailers in military use.

Should I add a EGT gauge to the stock setup because of the overdrive?
I had both an EGT and boost gauge on the M35, but there was a god reason for that!

 

[73m819]

I an dropping the button from 27 to 22, (waiting on button) I will install a PYRO, just to be safe, also plan to run the top end, no point in screwing with the IP if the top end is out of spec.

 

[OPCOM]

Please post pics how the pyro is installed in the manifold!

 

[73m819]

Please post pics how the pyro is installed in the manifold!

I am going to install the pyro prob in the center section, right at the point where the two ends merge to the center, a couple of inches up from the pipe bolt flange.

I will try to GET my wife to take some pics when I do the install.

 

[RIDDLE1]

I don't about the NHC 250 but on a 5.9 Cummins the front cylinder is the hottest. It is because the manifold doesn't expend gases in the front as well. I would be interested to see how the 250 does

 

[73m819]

I don't about the NHC 250 but on a 5.9 Cummins the front cylinder is the hottest. It is because the manifold doesn't expend gases in the front as well. I would be interested to see how the 250 does

I was under the impression that 5 and 6 were the hottest, because of the lack of air flow and being the furthest from the W/P, but I was told I was wrong and that 3 and 4 are the hottest, because these two have the least surface area to dump the heat. SO that is why I am putting the prob where it's going

 

[patracy]

Just adding in my .02...

I've got a 931 I hope to pick up next weekend. This thread is VERY interesting to me.

I've got a lead on a turbo setup. (HT3B and exhaust manifold) If I can find an intake to clock the inlet towards the passenger side I'll snag it. If not, I might try to install an aftercooler or innercooler setup. Next, I'll do a button change on the PT as well. Last, I'm going to source a 2350rpm gov spring setup.

I'm just aiming for ~300hp and a tad more operating rpm. I'll be swapping out the duals on it to super singles and Goodyear MVTs (395's with the 68mph rating). Running the numbers, the 395's and the gov mod would put it matching the tire rating for top speed. More importantly a 55mph cruise speed would be 1890 rpm.

 

[73m819]

Just adding in my .02...

I've got a 931 I hope to pick up next weekend. This thread is VERY interesting to me.

I've got a lead on a turbo setup. (HT3B and exhaust manifold) If I can find an intake to clock the inlet towards the passenger side I'll snag it. If not, I might try to install an aftercooler or innercooler setup. Next, I'll do a button change on the PT as well. Last, I'm going to source a 2350rpm gov spring setup.

I'm just aiming for ~300hp and a tad more operating rpm. I'll be swapping out the duals on it to super singles and Goodyear MVTs (395's with the 68mph rating). Running the numbers, the 395's and the gov mod would put it matching the tire rating for top speed. More importantly a 55mph cruise speed would be 1890 rpm.

Watch that 2350 rpm, 855s DO NOT like over speed very well, also our 250s have NO PISTON OIL SQUIRTERS to help cool the piston and helping to lube it. the 290s do. your building a 290/300 but it still will be a 250 with a turbo, the difference is the 290 is built as a 290 which includes the piston squirters, ip setup, ect.

 

[patracy]

Correct, I know the oil squirters are a downside to it. It will also have a pyro and boost gauge of course.

About the RPM. Notice I mentioned about the 55mph cruising speed. 60mph would be 2100rpm. Not something I'll subject it to all the time. (Not sure I'd want to go 68mph in a MV that weighs 22K lbs or more)

FWIW, I also have two other NA engines that have been turbo'ed as well. (VW diesel in my samurai, and the turbo'ed 6.2 in my CUCV) I tend to drive by the gauges. (My wife's Jeep Liberty CRD even has em!) My dodge could run up to 1600+ F EGTs when I had it full tilt and pressing it. (Not towing, just racing) Always have to be mindful of what's going on.

 

[patracy]

Another thing to consider with these engines and raising the rail pressure is timing. Raising the rail pressure will increase timing. Timing can be taken out though. Here's a blurb about it.

350 small cam, more power? [Archive] - TheDieselGarage.com

 

[patracy]

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.