NHC 250 Turbo Conversion in M923

[red]

I just picked up a big cam 400 that is complete. (Which I got for free) Only downside is I was told it has a rod knock. I was planning on using it as a donor for the nhc250 in my m925a1 for a turbo swap and probably add jakes to it as well. What I was wondering. Should i swap the jakes, injectors, and injection pump over as well? Or should i just swap the turbo and cooler? Another option I was thinking of doing was swapping my internals into the bigcam (as long as the block is still good and didn't spin a main bearing).... that way I would have cooling nozzles. Anyways, I have quite a few options and would love to get some input on what I should do.

Many of the internals won't transfer over from the small cam NHC250 to the big cam NTC400. Swapping the turbo, intake/exhaust manifolds, and jakes are a good idea to the nhc250. Make sure to install a EGT gauge and keep the temps below 1200F pre turbo, preferably below 1100F.

If that's not enough power then rebuild the NTC400.

 

[ryanaubra]

I pulled all the good stuff (manifold, turbo, aftercooler, etc.) off a big cam 350 about a year and a half ago and put it on my NHC250. I love it. If you are not planning on pulling a 40k# trailer up the Rockies, then all of that talk about piston squirters is a bunch of pointless nonsense. I made mine into a bobber, put 53's on it, and run it like a scalded dog all over the place. I added a boost gauge and pyro. I have never had the exhaust temp get over 700. Make sure it is intercooled or aftercooled. The one piece of advice i would give is replace your main seals. The increase in pressure blew both of mine after a little while.

I had planned on putting a 290 big cam in there, but soon realized that the front motor mount is completely different and would have to be custom fabbed. One day i may do it. Still have the 290 sitting here. If you want to get the fuel to it, do the dual fuel line mod and add an injector pump from a big cam motor, maybe injectors too.

 

[74M35A2]

Turbo big block is pretty fly. Should be the best of both worlds. Those that do it seem to have EGT's in the basement, I'm guessing because they have not upped the fueling rate? One would presume you can go a lot more fuel than where you are at now (lower button #?)?

Nice job, regardless. 855 at idle sounds like a war machine, not a school bus or Fed-Ex truck.

 

[simp5782]

I know where a good many NTC 335 and 400 Magnums are listed for sale. running for under $1200. Granted they are old but still well capable engines.

 

[Farmallmilitary]

Not surprising most the stuff wouldn't transfer over from a nhc to an ntc. But do you think it would be beneficial to swap injectors, and pump as well from the ntc? I already have everything, the only added cost is going to be injector o-rings. I also plan on running a pyro and boost gauge as well to make sure I don't blow the old dinosaur up. I also read somewhere that one can only use 2 jake housings due to hood clearance. Is this true?

 

[simp5782]

You can use Multifuel engine mounts and raise the cab a few inches then space your hood hinges up a little bit to get more clearance.

 

[red]

Not surprising most the stuff wouldn't transfer over from a nhc to an ntc. But do you think it would be beneficial to swap injectors, and pump as well from the ntc? I already have everything, the only added cost is going to be injector o-rings. I also plan on running a pyro and boost gauge as well to make sure I don't blow the old dinosaur up. I also read somewhere that one can only use 2 jake housings due to hood clearance. Is this true?

Physically the pump will swap over and I'm pretty sure the injectors will as well. Boost and EGT gauges are good. Also when playing with the fuel system make sure the fuel psi after the pump is at or below 200psi. Higher than that and the lobes on the cam for the fuel injectors will fail (big cam's are able to handle higher pressure).

The small cam will never reliably make the power of the big cam 400 but can be reliably turned up to 290-300hp, maybe higher if you tweak the fuel/air right and watch the EGT's like a hawk.

 

[Farmallmilitary]

Sounds like I got some work cut out for me. Hopefully it turns out well. Should be alot more fun to drive with a turbo and jakes to mess around with!

 

[wrenchturner6238]

It does make it more enjoyable to not have to fight so hard to keep speed up. Cruising down the road the foot throttle is up off the floor. I can keep speed up a hill bobtail even pass my buddies when we are on a road trip

 

[grendel]

I have a very nice pump from an NTC 350 if anyone's looking.

 

[wcuhillbilly]

Sounds like I got some work cut out for me. Hopefully it turns out well. Should be alot more fun to drive with a turbo and jakes to mess around with!

Turbo and Jakes are a whole another world when it comes to the old 855's. Absolutely LOVE my Jakes... the Jake to look for is the 425, Models you will find are 25a and 25b and 425. 425 was the last, most updated. I have the 25a's and they are good, but a pain to adjust. with the jake ontop of the rocker box, adjust your valves right the first time and then set the jake on. you will play hob trying to get to the valve adjusters after the jake is on. Pay attention to the oil plugs in your exitstin rocker boxes. Make sure you get the pass through plugs out of the donor engine rocker boxes, this allows the jake a supply of oil in order to activate the jake valves. Also, problem I ran into was the middle head was a couple thousandths lower than the others. had to run a stacked gasket in order to provide just enough clearance that the jake push cylinder would not hold the exhaust valve open.. Check out "816 Jakes and Turbo" in the 5 ton hotrodding section.

I have a very nice pump from an NTC 350 if anyone's looking.

What is the story on the pump?.. think mine is dying or needs a rebuild. my donor stuff (turbo, manifolds etc) is all off a small cam NTC 350, just couldn't use the pump as it was in pieces.... Either way I could use a good core to swap on, or send out for rebuild....

 

[armyrino]

Hey guys I have an 84 m923 with the nhc 250. She runs great. I am going to do the dual fuel line mod. What is the best way to turn the injection pump on these motors. I don't want to make it to much just want it to go over bridges a little faster lol. Any thoughts? Thanks

 

[red]

Hey guys I have an 84 m923 with the nhc 250. She runs great. I am going to do the dual fuel line mod. What is the best way to turn the injection pump on these motors. I don't want to make it to much just want it to go over bridges a little faster lol. Any thoughts? Thanks

Need a EGT (exhaust gas temperature) gauge first before adjusting the fuel. 1200F is max temps, preferably 1100F for sustained.

Once you have that and want to adjust the fuel flow correctly:
Dual fuel line mod
Need a pressure gauge able to read 200psi+
Lower numbered fuel button

Good video on replacing the button https://www.youtube.com/watch?v=mvQsUXV8bPE

(for fine tune adjustment) Adjust the throttle shaft screw.

Do not exceed 200psi of fuel pressure if you want the engine to live. The cam lobs get sheared off above that with these small cam engines.

 

[simp5782]

Hey guys I have an 84 m923 with the nhc 250. She runs great. I am going to do the dual fuel line mod. What is the best way to turn the injection pump on these motors. I don't want to make it to much just want it to go over bridges a little faster lol. Any thoughts? Thanks

Or take it off. Have the button lowered and the fuel metering adjusted properly so it wont just dump too much fuel.

Sent from my SM-G935P using Tapatalk

 

[grendel]

What is the story on the pump?.. think mine is dying or needs a rebuild. my donor stuff (turbo, manifolds etc) is all off a small cam NTC 350, just couldn't use the pump as it was in pieces.... Either way I could use a good core to swap on, or send out for rebuild....

Came on an old Pete we scrapped out. Fairly new, motor ran well, but no one wanted it. So, we took it apart.

 

[tim410]

Need a EGT (exhaust gas temperature) gauge first before adjusting the fuel. 1200F is max temps, preferably 1100F for sustained.

Once you have that and want to adjust the fuel flow correctly:
Dual fuel line mod
Need a pressure gauge able to read 200psi+
Lower numbered fuel button

Good video on replacing the button https://www.youtube.com/watch?v=mvQsUXV8bPE

(for fine tune adjustment) Adjust the throttle shaft screw.

Do not exceed 200psi of fuel pressure if you want the engine to live. The cam lobs get sheared off above that with these small cam engines.

What type of gauge do you need and where the best place to install? Is it somethin you have to monitor all the time?

 

[Floridianson]

I like oil filled gage and you should have a plug in a port right under the emergency shut off valve. Only need the gage to tweek the PT then remove.

 

[tim410]

I like oil filled gage and you should have a plug in a port right under the emergency shut off valve. Only need the gage to tweek the PT then remove.

Ok so i guess its idle pressure or at full throttle? Like 2000 rpm
i did dual line while ago and have had button in tool box for a while. Just wired pyro so will have button installed soon

 

[simp5782]

Ok so i guess its idle pressure or at full throttle? Like 2000 rpm
i did dual line while ago and have had button in tool box for a while. Just wired pyro so will have button installed soon

You need to drill the ball out on the throttle linkage where it goes into the pump and turn it counter clock wise to adjust for the new button. it helps get more fuel but to not turn it too much.

 

[Floridianson]

Ok so i guess its idle pressure or at full throttle? Like 2000 rpm
i did dual line while ago and have had button in tool box for a while. Just wired pyro so will have button installed soon

Went a little over board on the hose but I can hook it up and watch while driving then unhook it. You should be able to blip the throttle and see what the rail pressure is. It will go to set pressure then drop right off. The last 939 series truck 250 I checked already had the pressure turned up to 200 but no button change and came that way from GL. The older 809 series I have checked have had around 160 to 170 lbs.