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

[srodocker]

im excited to see the results coming in. As I have a m928 shes a big beast and will love any extra go she can get on the low end. Once brakes are done maybe start working on these projects.

 

[R Racing]

I swapped in a #20 button. Stock was a #30!

How is it acting now ?

 

[patracy]

How is it acting now ?

I went for a drive but noted that it was defueling at about 1950rpm. I mean fall on your face defueling. Came back home and noticed a retainer/shim on the ground I must have dropped. While on the road and driving, I saw the same EGTs (perhaps about 10-15F higher, but the gauges resolution isn't that precise). But boost increased about 1-2psi to 12psi. So I've pulled it back down and installed a #17 button and the retainer/shim. I haven't driven it yet, but I'm able to rev it to 2400rpm whacking it, and it'll hum along at 2100rpm no problem now instead of 1950. Honestly I think I could probably drop down to a #15 no problem as well.

 

[Floridianson]

I'm thinking about checking with my local hyd shop to see if they have something and a 1/8" hose to check mine with.

Did you get the gage yet and play with the throttel shaft. When I used the gage on Pete's truck it was defueling at 195lbs from the start. We left it alone and he has not yet changed the button. If you would be real carefull and pay shipping both ways I will send up my gage. I got a nice gage from Napa a 12 foot hose and quick hyd. fitting with the correct threads.

 

[patracy]

Not yet. I've been putting off placing an order. I really need to replace all the hoses in the crane on the SEMTT, but I need to get out there with one other person and a tape measure to figure out exactly what all I need. Surplus center already sells hoses with NPT ends, that was what I planned on using and just going down from a 1/4 to 1/8 to fit the ends with some brass fittings.

 

[Floridianson]

Yea it cost me about 100 bucks for the gage and hose but Napa use a nice swivel fitting on the PT end and a direct line fitting to the oil filled gage.
It would be nice to see where your pressure are running with the decrease in your buttons. You might be good on the buttons just need to dial up the throttel shaft screw some. Just keep it under 200psi. I'm at 190psi on the M816 but no button change. Does your PT have a A/F screw under the tin cover?

 

[patracy]

I plan on ordering one of these:

http://www.surpluscenter.com/Air-Pn...um-Gauges/300-PSI-1-5-BM-LF-GAUGE-21-1700.axd

And this hose:

http://www.surpluscenter.com/Hydrau...TM-X-1-4-NPTM-2750-PSI-HYD-HOSE-916-14144.axd

Then just a simple brass 1/8npt male to 1/4 npt female.

Tin cover?

 

[Floridianson]

Yea thought that the trucks / PT's that came with a turbo had a air / fuel screw under the tin cover on throttel shaft. I have not checked on the M920 yet but guess I should. Thought we could dial them up there too.

 

[patracy]

Yea thought that the trucks / PT's that came with a turbo had a air / fuel screw under the tin cover on throttel shaft. I have not checked on the M920 yet but guess I should. Thought we could dial them up there too.

This thread is about the NHC-250, so none will have the AFC.

 

[Shark Bait]

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[patracy]

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Just FYI, there's a simple way to subscribe to a thread without making a post. Just scroll to the top and click on the thread tools link and you'll see "subscribe to thread".

 

[R Racing]

This thread is about the NHC-250, so none will have the AFC.

Actually my m816 and m813 both have the screw under the center of the throttle shaft. There is like a bb cover that you can drill thru and use a screw to remove. I forgot how to adjust it. (It's on here on 1 of the threads) but it really woke up both of my trucks! !!

 

[R Racing]

I want to say it was 5 turns either in or out. If you find it please add it to this thread as I have 2 more trucks to tweeked lol.

 

[patracy]

That's the throttle screw, an AFC would be on a factory turbo'ed engine.

 

[R Racing]

Here read this starting at post 17.
http://www.steelsoldiers.com/showthread.php?90372-NHC-250-Turbo-Conversion-in-M923/page2

 

[patracy]

Ok?

 

[Floridianson]

Yea just did not know if you changed out the PT.

 

[Floridianson]

I want to say it was 5 turns either in or out. If you find it please add it to this thread as I have 2 more trucks to tweeked lol.

Yea you need the gage too. I took mine 816 from 160 to 195lbs with 2 compleat turns but no button change yet. It can be different so get a gage,drop the buton number and turn the center screw counter clockwise on throttel shaft till you get the pressure up but not over 200lbs. My M920 is the only truck I have with a turbo so if and when I tweek her I will decrease button number. turn the center throttel screw counter clockwise and if I rember right under the tin throttel shaft cover loosen the A/F jam nut and turn it counter clockwise just alittle till she has a good puff of smoke on the blip.

 

[Major Asstyrd]

Here is some basic diesel info for those coming from the gas world or just not familiar with diesels. They are the opposite of a gas engine in that more fuel = more power and heat. You turn up the fuel to get power, and it will run hotter and smoke more. With a gas engine, more fuel makes it slower. That's called "running rich." Less fuel is "running lean" and that raises power and heat. With a gas engine you HAVE to increase the air flow into the engine to make more power. This usually increases rpm, which usually makes more power by itself, and it allows you to turn up the fuel to match so it doesn't run too hot. That’s expensive in a naturally aspirated engine because you have to change camshafts and maybe go bigger on the valves and port the head. With a diesel just crank up the fuel and maybe injection timing, and if it runs too hot or smokes more than you like, add a turbo. Or replace a small turbo with a bigger one. More air (oxygen) going into a diesel = cooler running.

Nearly all diesels are turbocharged now. The large diesel hydrocarbon molecule has a lot of power within it but when the engine is naturally aspirated, it doesn't suck enough air in to match up well to that molecule. Turbocharging force-feeds air into the cylinder and the oxygen within the air really wakes up the burn. More oxygen allows more fuel duration (a longer squirt) while keeping heat OK.

Anyone wanting to jack up their diesel's power should start with an exhaust gas temperature gauge (pyrometer) with sender probe in the manifold). Know your baseline first. Then as you increase the fuel duration you will see the average temp rise. Experts on these engines will know more about max EGT than me. More power is generally not good when towing heavy because the EGT goes up when your foot is into it more. One way to drive around this is to downshift to raise RPM. That brings in more air, and air cools EGT. Of course doing that decreases road speed, and RPMs may go up too high for the engine design as posters have said.

So to get the jacked up engine working right, more mass air is needed, and that is usually done with a turbo. It is not exactly about high turbo boost pressure. A larger compressor can supply the same mass of air at relatively low pressure as a smaller compressor at high pressure. The main downside to high boost is the air is heated up more. In the performance diesel pickup world where we see over 40 psi boost pressure, we see air temp over 400*F coming out of the compressor. This is why charge-air coolers (misnamed intercoolers) are so important. They cool the boosted air so it’s denser going into the engine. That means more oxygen molecules in a given volume. A CAC is good on any turbocharged engine, especially a diesel. It’s so important on a high boost engine it is usually mounted in front of the radiator. On some vehicles it is below the radiator.

Turbo sizing is complicated and I won’t get much into it except to say if the turbine wheel and housing are on the small side, there is more exhaust back pressure which gets the wheel spinning faster, meaning the shaft and compressor wheel too. That is called “quick spooling” = air out of the compressor faster. But EGT will go up due to that back pressure, and too much boost at the compressor is not good. Some turbos have wastegates that vent the turbine housing to slow the turbine wheel down when a certain max boost in the compressor is reached. This allows fast spooling and more air early, without overspinning the turbo. On the other hand, a large turbine, usually found on a large turbo, tends to offer lower EGT but the big wheel in a large capacity turbine housing is slow to get moving. So it can be “laggy” as engine rpm starts increasing, smoking out the intersection and the engine not making power until the turbo spools up enough so the compressor wheel is pumping some volume.

Most people wanting to put a turbo on a NA engine, or go to a larger turbo, usually get an expert’s advice or they select a turbo that is designed to work on approximately the same engine size and power rating that they want to achieve. For example I know of a turbo that came on a 14L CAT that made 500 hp and if you put a certain smaller turbine housing on it, it turns into the turbo that came on a 600+hp CAT. This is because the wheels spin faster and so there is enough air to match the increased fueling on the higher-rated version of the engine.

I know of several excellent diesel injection companies in the midwest that are also big turbo distributors. They have drive-in service. They are not so much into hot-rodding big trucks but they are a good place to go when you know what original equipment turbo or injector you want. One is big in ractor pull race parts. One does extrude-honing of injector nozzles if you want them larger. I can share the info privately.

Hope this helps.

 

[jarhead1086]

I have 4 months of salty roads so she is in the garage getting worked over. Easy part is done. I have an intercooler like Bjmca (sp?)used in his conversion which simplified the hook up. Its getting powder coated (junk yard dog) and will be back in a couple weeks. Rebuild kit on order. I still plan on rear 5" exhaust. Does anyone have pictures on how you plug the holes for the natural aspiration? My PT pump came stock with a 20 button. I had the pump benched at a shop but the EGT's told me to turn it way back down (could hit 1050 on a flat no load while still climbing temp so I pulled out). Sure ran and sounded good though. I will experiment back up once I have this all together. I am also wondering the best places to tap in the in-out on the intercooler. I haven't a clue what I am doing but I know its fun.