NHC 250 Turbo Conversion in M923

[PyroJoe]

Anything under
You can run 215 to 220 injector pressure on a small cam

That contradicts every thread I've read (on this forum and others) about the NHC-250's in the past 24 hours. They all say do not exceed 200 psi. One went into detail about the size difference (2" vs 2.5") and how the smaller cam lobes tend to shear off. I'll stick with the majority on this one. This is, after all, about how to safely get more power out of a 250 - not how to live on the razor's edge.

Found a really weird article about the NTC Formula series back when they were new in the 70's. They claimed you are better off driving the engines between 1000-1600 RPM rather than 1300-2000. They did a test and found the lower RPM yielded 7.3mpg while higher RPM gave 6.4mpg. Not sure if that only pertains to the NTC series, or the fact those motors have piston squirters, or maybe because they're turbo'd.

Cummins E290 | 3rd February 1978 | The Commercial Motor Archive

'Cummins E290' first printed in the 3rd February 1978 issue of Commercial Motor

archive.commercialmotor.com

 

[simp5782]

That contradicts every thread I've read (on this forum and others) about the NHC-250's in the past 24 hours. They all say do not exceed 200 psi. One went into detail about the size difference (2" vs 2.5") and how the smaller cam lobes tend to shear off. I'll stick with the majority on this one. This is, after all, about how to safely get more power out of a 250 - not how to live on the razor's edge.

Found a really weird article about the NTC Formula series back when they were new in the 70's. They claimed you are better off driving the engines between 1000-1600 RPM rather than 1300-2000. They did a test and found the lower RPM yielded 7.3mpg while higher RPM gave 6.4mpg. Not sure if that only pertains to the NTC series, or the fact those motors have piston squirters, or maybe because they're turbo'd.

Cummins E290 | 3rd February 1978 | The Commercial Motor Archive

'Cummins E290' first printed in the 3rd February 1978 issue of Commercial Motor

archive.commercialmotor.com

Well I've put more miles towing stuff behind a 923 with an NHC250 than anyone on this site or the military for that matter. With it pushing 215 to 225 on a #16 button and no Pyro cause I know how to not lug the motor and only give it what it needs to maintain what it is pulling. Motor had well over 260k before I had a liner O ring issue.

Some folks read things to make an opinion of what to and not to do. Some of us get it done and know what to do and not to do and move on down the road.

And I did it all without a turbo. Just using air, fuel and knowing how to run a motor and not blow it up.

 

[Bjmca]

So, Bjmca, that was 10 years ago. Where did you end up? I'm guessing the #10 is too hot and #12 is probably borderline where you'd have to drive by your Pyro, while the #15 is probably the safest in case your buddy is going to possibly borrow your truck.

I'll summarize my thoughts on the whole project looking back that far:

The following statement is going to hurt some feelings...

In short summary the NHC-250 is a waste of time and money to attempt to squeeze more power out of.

If you want it reliable and simple leave it alone.

If you actually want to use the truck for more practical things like pulling trailers and equipment on the road, loose the NHC-250 and repower it.

I do enjoy building and modifying things, but the reality is you are never going to make a 250 run worth a damn.

 

[hethead]

I'll summarize my thoughts on the whole project looking back that far:

The following statement is going to hurt some feelings...

In short summary the NHC-250 is a waste of time and money to attempt to squeeze more power out of.

If you want it reliable and simple leave it alone.

If you actually want to use the truck for more practical things like pulling trailers and equipment on the road, loose the NHC-250 and repower it.

I do enjoy building and modifying things, but the reality is you are never going to make a 250 run worth a damn.

Any information whatsoever as to what set up you ran and what problems you had would be greatly appreciated by me. Specifically, boost pressure, rail pressure, EGTs, rpms you loaded it at, etc.

 

[simp5782]

I'll summarize my thoughts on the whole project looking back that far:

The following statement is going to hurt some feelings...

In short summary the NHC-250 is a waste of time and money to attempt to squeeze more power out of.

If you want it reliable and simple leave it alone.

If you actually want to use the truck for more practical things like pulling trailers and equipment on the road, loose the NHC-250 and repower it.

I do enjoy building and modifying things, but the reality is you are never going to make a 250 run worth a damn.

Small cams can become 400 magnums. It's a small cam with big cam iii parts.

 

[PyroJoe]

Still waiting for someone to comment on that 1978 article about RPM....

So the next question coming to my mind is MPG. Assuming 14R20 tires on everything to compare apples-to-apples, what is everyone getting? On the same trip back from Kansas, my 8.3 got 12mpg (923). The 14L's ranged from 8-11 (923, 925, 934). I've heard of people getting 5-6 and I wonder how they drive.

I bought/sold many of the 900 5tons and kept 3 for myself. My plan for the 934 is to make a toter to haul the crawler to Moab/KotH. Can't decide if I want to turbo the 250 in it, swap in a p-pump 8.3, or DT466. The difference in mileage and reliability tells me the latter 2 options are worth it. My 8.3 923 doesn't have any of the power struggles that all of these 14L's have had (the 14L's all struggled with a specific hill in Kansas whereas the 8.3 chugged right up it). We currently use the 8.3 923 to haul 10,800 lbs of water to the cattle. It frequently gets fired up cold and taken right off. 10k miles later and it's still going strong. Do that with a 14L and I'd expect something to come flying out of the block. But since I'm not going to drive this van other than long distances and allow it the proper warm-up/cool-down times, I'm not going to worry about that part here.

Edit: And I'll add that now, I've got the 8.3's pump maxed out, and I really couldn't ask more of this truck. And I know it's nowhere near the 300/950 specs the tranny is rated for - so I don't fear anything breaking. But to be fair to the 14L, I should adjust it to have a chance to compete with the 8.3. And if I can't adjust it without destroying it, then that decides the battle right there for me. It's just so funny to me how heated of a battle there is with the 855 vs 6CT.

 

[hethead]

I just took a quick look so maybe I’m wrong but I think the ‘78 article was about a formula 290 big cam engine, which was specifically designed to run at a lower rpm (to increase efficiency), so doesn’t really compare to the 250. Or a small cam 290.

I don’t know all the changes made, but I assume valve timing and injection timing as well as duration was different. Probably the fuel curve of the pump as well. I don’t run my 250 under 1600 rpm under load because it starts to lug. I’ll usually down shift at that point.

I specifically wanted an 855 because it’s a huge semi engine, not a medium sized school bus engine, so maybe I’m just a child But I’m also hoping to get more miles/years out of it with a 50,000 lb set up.
(truck and trailer gross weight)

 

[PyroJoe]

I just took a quick look so maybe I’m wrong but I think the ‘78 article was about a formula 290 big cam engine, which was specifically designed to run at a lower rpm (to increase efficiency), so doesn’t really compare to the 250. Or a small cam 290.

Oh, good catch! Now I'm just plumb confused. The engine I bought says it's a "small cam". "Formula 290" is on the intake manifold. So maybe it had a manifold swap, or maybe they've mislabeled it as a small cam. I guess I'll find out next week when I get it in the shop.

 

[WillWagner]

Found a really weird article about the NTC Formula series back when they were new in the 70's. They claimed you are better off driving the engines between 1000-1600 RPM rather than 1300-2000. They did a test and found the lower RPM yielded 7.3mpg while higher RPM gave 6.4mpg. Not sure if that only pertains to the NTC series, or the fact those motors have piston squirters, or maybe because they're turbo'd.

Cummins E290 | 3rd February 1978 | The Commercial Motor Archive

'Cummins E290' first printed in the 3rd February 1978 issue of Commercial Motor

archive.commercialmotor.com

That is because an engine turning at a lower RPM burns less fuel, back then, they knew that but hadn't figured out how to move the torque curve to 12-1600 rpm, they made the best at 2000 +. Also, back then you couldn't have used the "gear fast, run slow" concept of today because there was no such thing as road speed governing invented yet. Gear fast run slow means gear the vehicle so that when at speed have it so the engine is at it's sweet spot for torque and fuel mileage and then limit the physical road speed of the vehicle. Road speed governing can only be done by running out of rpm. There are only 2 ways to do that.

"Formula" engines were developed for fleet use. Governed at 1900 rpm rather than 2100 for fuel economy. With a formula engine, the trucks were able to be geared a bit faster, but run a slower RPM.

Piston cooling nozzles were introduced for the turbocharged engines. Turbocharging increases cylinder pressures which in turn makes heat, throw a bunch of oil on the under side of a hot piston, and the heat is dissipated. When the blocks were machined, the process was automated. everything got it done and there were block off plugs installed in the engines that didn't need PC nozzles.

ALL Cummins engines liked to eat a cam even when left in stock form. Duty cycle has a big influence on this. The 250 will last a long time in stock form. When HP was added via adding more fuel pressure, like when the SC started being offered in 270, 290, 300, 350, and adding a turbo, that was when cam failures got more frequent. The injection pressures broke cam follower pins, which in turn. took out the lobes. Big cam made the ramping of the pressure less violent and enabled the torque to start being made at a lower RPM. All the engines across the platform, v8 504, 555, 470, 903, K models, N series, the early 10L and even the early N14 ate injector lobes especially as HP/fuel increased. It is an inherent issue with a mechanically activated fuel injector system. And, IIRC the v8 504 and 555 also had a small and big cam variant and both the N series and K models were made in V style engines. Big V's were fun to work on, small ones, not so much.

Too much shiat floating around in my noggin....

 

[PyroJoe]

That is because an engine turning at a lower RPM burns less fuel, back then, they knew that but hadn't figured out how to move the torque curve to 12-1600 rpm, they made the best at 2000 +. Also, back then you couldn't have used the "gear fast, run slow" concept of today because there was no such thing as road speed governing invented yet. Gear fast run slow means gear the vehicle so that when at speed have it so the engine is at it's sweet spot for torque and fuel mileage and then limit the physical road speed of the vehicle. Road speed governing can only be done by running out of rpm. There are only 2 ways to do that.

"Formula" engines were developed for fleet use. Governed at 1900 rpm rather than 2100 for fuel economy. With a formula engine, the trucks were able to be geared a bit faster, but run a slower RPM.

Piston cooling nozzles were introduced for the turbocharged engines. Turbocharging increases cylinder pressures which in turn makes heat, throw a bunch of oil on the under side of a hot piston, and the heat is dissipated. When the blocks were machined, the process was automated. everything got it done and there were block off plugs installed in the engines that didn't need PC nozzles.

ALL Cummins engines liked to eat a cam even when left in stock form. Duty cycle has a big influence on this. The 250 will last a long time in stock form. When HP was added via adding more fuel pressure, like when the SC started being offered in 270, 290, 300, 350, and adding a turbo, that was when cam failures got more frequent. The injection pressures broke cam follower pins, which in turn. took out the lobes. Big cam made the ramping of the pressure less violent and enabled the torque to start being made at a lower RPM. All the engines across the platform, v8 504, 555, 470, 903, K models, N series, the early 10L and even the early N14 ate injector lobes especially as HP/fuel increased. It is an inherent issue with a mechanically activated fuel injector system. And, IIRC the v8 504 and 555 also had a small and big cam variant and both the N series and K models were made in V style engines. Big V's were fun to work on, small ones, not so much.

Too much shiat floating around in my noggin....

I love it! Thank you for sharing all of this! But don't you mean they made the best HP at 2000 RPM? Torque was rated at 1300 RPM.

 

[simp5782]

I love it! Thank you for sharing all of this! But don't you mean they made the best HP at 2000 RPM? Torque was rated at 1300 RPM.

2100

 

[WillWagner]

Old stuff ran hardest at higher rpm. That is why the stinger came about on the PT pump. The plug in the gov hsg is removed and a rod welded/brazed to the end of the plug. Defeats the governor and you have almost unlimited RPM. As RPM comes up, so does the fuel pressure. Yes, the torque was a bit lower than rated RPM, but not but much on SC. Big cam lowered the torque RPM due to the approach ramp and a kind of pre injection injection cycle. On the lift ramp profile there is a small "start of injection" lift, then it flattens and then the main start of injection starts.

Yes torque was at a lower RPM, but nobody ran at that RPM because it took away road speed, road speed got you to point B quicker. Not until the introduction of the "Command" power in 1991 did people start running at lower RPM. When the mechanical N14 came out, I can't tell you how many people came in with poor mileage complaint. It was because they were running it like an old engine, foot to the wood at rated RPM and not using the upper gears. After a test drive with them, showing them that short shifting and running a gear or 2 higher at lower RPM got you to the same place in as good or better time did they start driving them like that. They would return a month or so later and have better results to report. The electric versions of the N14 did that by itself. It FORCED the driver to short shift and run in higher gears but to the old school guys, seeing lower RPM gave them the impression that the engine had low power....just because the RSG or gear down settings would shut the RPM off hen it saw road speed reach the programmed stop point. Settings in the programming shut RPM off very low in order to force an up shift. This was another perception of low power.

That is the second way to limit or govern road speed, making the added electronics think and limit the road speed yet still deliver the proper fueling to make power at the lower engine speed

 

[US6x4]

And for anyone wanting video (because this isn't a thread without video!), here are some of the best links I've found:

The truck in the second video is mine and for the sake of compiling numbers it's running a #17 button and the most I've seen is 14+ psi & 900° F. I don't know what rail pressures it's running at but the new button has caused new fuel leaks around the manual shutoff valve...

 

[PyroJoe]

The truck in the second video is mine and for the sake of compiling numbers it's running a #17 button and the most I've seen is 14+ psi & 900° F. I don't know what rail pressures it's running at but the new button has caused new fuel leaks around the manual shutoff valve...

Awesome! Glad you stumbled up on this! So for my question about exhaust placement - does your stack being in front of the window vs. behind like on a 900 series make it unbearable for long trips? I definitely had to have my Deuce's RH window rolled up if I was driving it more than 10 miles, or ear plugs in.

 

[US6x4]

Awesome! Glad you stumbled up on this! So for my question about exhaust placement - does your stack being in front of the window vs. behind like on a 900 series make it unbearable for long trips? I definitely had to have my Deuce's RH window rolled up if I was driving it more than 10 miles, or ear plugs in.

It's loud for sure and I love it; 1000-1600 rpm sounds like a jet spooling up for take off when the turbo whine is louder than the exhaust note. The jakes sound super sexy though. They let out a sharp, staccato hammering at high rims and a mellow burble under 1700 or so.

A long drive for me would be 40 miles and I wear VIC-1 headsets while driving so that cuts most of the noise down

 

[hethead]

For my 2 cents, I drove my M809 from PA. to Florida, to Seattle and the exhaust was not bad. The rest of the mechanical noise with zero insulation was much worse but I didn't find it that bad at all, really.

I have Goodyear 395's and went around 60 mph on gps at 2100 rpm.

I straight piped it (muffler rusted out) and increased the fuel pressure and now it's too loud. I ride an old Norton and Ironhead with drag pipes and I usually like loud, but it's almost painful. Hell of a bark.

 

[PyroJoe]

I straight piped it (muffler rusted out) and increased the fuel pressure and now it's too loud. I ride an old Norton and Ironhead with drag pipes and I usually like loud, but it's almost painful. Hell of a bark.

I feel the exact same way! I've always taken the sawzall to the muffler before the new car even comes off the trailer, but the 855 was just too much for me when I tried it on my 934. I now realize that there was something funny about the acoustics of the 934 with the pipe being between the cab and box, though. I thought I was going to shatter glass driving by peoples' houses. I put the 90° cap back on, but still unbearable.

On the 6CT, I just ran a vertical 4" tube exiting towards the sky, above the cab, with a rain cap. It sounds great! So, I thought I'd try this on a 14L 925 since, like you mentioned, the muffler was crumbling. Straight-up-exit was too loud. I put a Dynomax 4" muffle on it today, exiting straight up, and it was still too loud. I then put the 90° elbow back on, exiting to the side and a bit to the rear, and I can once again drive the truck without ear plugs. The race muffler also gives it a pretty throaty tone. I figure once I get to the stage of adding the turbo, I'll make a video comparing NA and turbo, both with and w/o the muffler and 90° pipe. As much of a difference as a 90° to the side made, I'm sure a rear-exit will be the way to go for comfort, and to still keep other drivers awake

 

[Ajax MD]

For my 2 cents, I drove my M809 from PA. to Florida, to Seattle and the exhaust was not bad. The rest of the mechanical noise with zero insulation was much worse but I didn't find it that bad at all, really.

Wow, you're nuts.
I wear hearing protection in my M813 and I haven't even taken any long trips.

 

[hethead]

Wow, you're nuts.
I wear hearing protection in my M813 and I haven't even taken any long trips.

What! I can‘t hear you. Haha. I did wear ear plugs earlier this week. Picking up a log trailer put a little load on it and it really barked (with the temporarily straight piped stack). I had to pull it up a steep hill down cobblestone main street to avoid the highway (bad tires on the trailer) and I thought I was gonna shatter the windows of all the little antique shops. Definitely vibrated some china on the shelves.

 

[hethead]

Just came across this wily bastard. He’s nothing if not entertaining and he sure likes turning up 5 ton 855s.