M818/ Cummins NA250

[BBuckRacing]

I have a 71 M818 with the cummins NA 250 in it the truck is bobbed and has 53's on it. seems as though it doesnt run as well as it should, or is it just the tires? i was also wondering since its mechanical if there is a way to crank up the fuel in the injection pump similar to the 12 valve dodges?? thanks for the help

 

[Wildchild467]

I am not an expert with anything on the 5 tons, but i have seen threads on turning up the fuel for the Cummins 250. Do a search (botton at the top of this page) and it will come up. Welcome to SS by the way!

 

[wreckerman893]

Does it lag mechanically or is it just top speed?

5 tons are gearbound by tranny and rears.....on a good day a stock 5 ton would not make 60 MPH.

You can turn a 250 up to 2300 RPM but you do so at your own peril.

You will have to take the pump to a pump shop to have it reset unless you have that skill.

The 250 is an obsolete engine and parts are getting difficult to find.

I know this to be true due to the prob I had with my 816 wrecker.

You can swap in a 350 or 400 but you are still going to be gearbound.

Hope this helps.

 

[BBuckRacing]

I wouldnt say it lags, it just doesnt perform like i had expected. with the tires i have on (53 inch goodyears) [which may also be a big part of the problem] it does 65-70 no problem on the highway but as large as the engine is i expected it to pull hills more easily. turning the pump up was just a preliminary solution, i have a turbo that i may condsider putting on as well

 

[BKubu]

The NHC250, while a reliable motor, is not a powerhouse...and you just killed whatever power it did have but adding the larger tires. The larger tires will give you more road speed, as you probably know, but you will regret them if you have to climb hills frequently. Going to 1400X20s would have been better, but that is water under the bridge. I have a friend with 1600s on his M923 and he HATES them! They look cool, though. My $.02...

 

[BBuckRacing]

thanks, yea i know those things are retarded heavy which also gave me a stopping problem. it does make the truck look much better but definatly robbed us of what little power it had. we will most likely be turboing the motor or doing a swap in that case

 

[jasonjc]

Just some thing to think about, the michelins atleast, have a max speed of 55mph only. I do not have the spec for the goodyears. I have no idea what will happen with running 70mph for a long time. But I would not want to be there when they blew!! I had a 12.50R20 blow out a pice of the side wall the size of a phone book, and that was at 55mph.

 

[BBuckRacing]

Yea the goodyears are only rated for 55 as well we wanted to see if the engine had a problem or not so we cranked it up and my brother in his truck tailed us to 70. that was the first and last time ive had it over 50

 

[BKubu]

Just some thing to think about, the michelins atleast, have a max speed of 55mph only. I do not have the spec for the goodyears. I have no idea what will happen with running 70mph for a long time. But I would not want to be there when they blew!! I had a 12.50R20 blow out a pice of the side wall the size of a phone book, and that was at 55mph.

Jason, that is an important point. Thanks!

 

[Floridianson]

I have a 71 M818 with the cummins NA 250 in it the truck is bobbed and has 53's on it. seems as though it doesnt run as well as it should, or is it just the tires? i was also wondering since its mechanical if there is a way to crank up the fuel in the injection pump similar to the 12 valve dodges?? thanks for the help

Had my pump done here in Gville and he took rail pressure from 172 to 180 and what ever else he tweeked and it run much better. Seems not to lag any more and runs better through the whole rpm range. $400.00 is what I paid and that was with new gasket, spyder gear and OD paint.
So yes they can be tweeked but you should make shure the slector and it's rubber hoses are in good shape as well as the whole fuel line/ filter system.

 

[wreckerman893]

A turbo is not necessarily going to give you a huge increase in power......the deuces were turbo'd to make them smoke less under load.
It will cram more air into the cylinders and help you burn the extra fuel being fed to it.
A 5 ton (or deuce for that matter) is never going to give you the performance a civy truck will due to gearing, weight, HP of the 250 and a few other factors.
A bobbed 5 ton is a cool vehicle and you can lord it over the civy four wheel drives....get used to the lathargic performance and enjoy.

 

[cyamaha2007]

I drove a m818 that had some pretty big tires with a Cummins 6CTA 8.3 swapped in it and it pulled really good. I was really suprised it was not a turd at all. If i had the time i would do the swap. Also there are a lot of aftermarket parts for it.

 

[papabear]

Loose the jumbo tires unless the cool factor prohibits that.

I have 4 250 cummins in cargos an 818 and an M816....you will bog down on hills etc cause they are made and geared to pull heavy loads off road etc. at low speeds.

IMO the jumbo tires are most of the problem...the truck/engine is prolly performing as designed.

 

[WillWagner]

Another fact to consider, the NHC is non piston cooled. You jack the fuel and you'll be looking for cylinder kits. They are getting harder to find than hens teeth.

 

[BKubu]

I drove a m818 that had some pretty big tires with a Cummins 6CTA 8.3 swapped in it and it pulled really good. I was really suprised it was not a turd at all. If i had the time i would do the swap. Also there are a lot of aftermarket parts for it.

There is one problem here. He is talking about the older NHC250 and you are referring to the newer 6CTA 8.3 Cummins. Without a doubt, the newer motor is better. Guys who think the older motor is better are guys who have never owned a 6CTA motor...or they have driven a dog in the military and decided that a sample of one was sufficient to make a judgement. I have put more miles on M939A2 series trucks than a lot of guys in the military. I have owned the following (not bragging...just pointing out that I am not BSing when I say I have seat time): two M925A2s, two M936A2s, and an M932A2. I have driven them thousands of miles. I have also owned NHC250-powered trucks (both M809 series and M939 series)...no comparison. Nothing wrong with the NHC250. It is a great motor. In fact, I own one (an M929 dump), but there is no comparison. One is a modern (relatively) motor...the other is a motor that was new when the LDT465 was recent technology.

 

[halftrack]

I would have to totally agree with Bruce's post.

When I only owned M809 series style trucks, m816 being one of them, I thought the NHC250 was the best thing invented. Now that I own a M936A2, it blows the NHC250 away. Comparing a truck with a NHC to a CTA motor is like comparing a MB jeep with a M998 Humwvee. Even with the wrecker body on it the truck gets up and goes. I never felt that with the older M816.

 

[cyamaha2007]

yeah id consider a motor swap if the lack of performance bothers you.

 

[BKubu]

I would consider putting the 1100s back on there. I think they would get as much performance out of smaller tires as they would a larger or more efficient motor. The tires are killing it. They were not designed for it. I have said this before. This is a reason people call them HEMTT rims...because a truck with 450 HP and over 1,200 LB-FT use those rims and 1600 tires.

 

[BKubu]

I will add this point of clarification. If your truck is used mostly for show, or is used almost exclusively on level ground (i.e., you live in a flat state), you will be happy with 1600s. If, however, you drive mostly in hilly areas, or pull heavy loads, you won't like 1600s as much. I would love to hear someone say they have them on an M818, for example, and haul fully loaded M127 trailers through western PA (for example).

 

[Blythewoodjoe]

OK, I hate to hi-jack the thread, but yet I do.
I have a M813 that has what I assume to be the 14.00 xl's it was surplused with from the Marines. I also assume it has the older engine. It seems to run well and will hold in the upper 50s (mph) on most of the interstates around here. I towed a M818 with it the other day and it stayed above 45, so it was legal at least.

So here's the question. If my truck was configured like this by the Marines, has it had other modifications to the engine or anything? It seems to perform well with the 14.00 xl's on it.

Joe