Why 2650rpm max?

[73m819]

Would putting in the 5t lighter pistons thus lowering rotating weight mass , give you the higher rpm, this would only work up to a certain rpm, after that the bolts would have the same stress as a lower rpm with the heavier pistons

 

[wbdodgeiv]

Maybe we need a new rod design, or copy an old one.... designed to take the stretching load off off the rod bolts and convert it to shearing force. Compliments of the Cummins crew...

Tensile loads are better than shearing loads. For steel a good rule of thumb in shear strength is half of the tensile strength. In shear you never what the bolt to actually touch the sides of the hole or you will get fretting which will break the bolt in short order. In shear the clamp load has to be high enough that the friction forces between the parts keep them from moving.

You want as high a clamp load as possible on the fastener. To get infinite life in fatigue you need the alternating load to be as small a percentage of the clamp load as possible, and under no circumstances have the bolt load exceed the clamp load. Generally you want the max load the bolt will ever see to be 75% of the proof load on the fastener.

I would bet that we have stronger bolts now than they did back in the 50's when the engine was designed. You would have to have the spec on the existing bolts to know for sure.

 

[patracy]

I sent an email to ARP to see if they had any or could make them. Might be cheap insurance...

 

[Srjeeper]

I have nothing better to do while out to sea for half a year at a time

Some say i am over thinking this,

guys, i just want to fix a problem.

i dont want to go over 55,

have any reason not to devote some time to finding a solution. this is a discusion, humor me!

Son, after reading all this, couple things come to mind.

So I'll ask you to please humor this 'Old Man' for just a minute or two here.

Yep, a half a year at sea gives ya "Way too Much Time" to think things over.

You've already decided you have a problem, when in fact you probably don't. Now to top that all off, your settin about fixin this problem you don't have while yer in the middle of the dam ocean.

If ya don't want to go over 55 in your deuce (assuming you have one) then you'll never need to exceed 2600 rpm's, so that means all this 'flabber gassin' about RPM's is for not!

Think about this.....these trucks were never intended to last into 2010. They were designed to go into battle, run off road, if they didn't get blown all to **** they'd run em till they were in pieces, leave em and move on.

Then along comes GL, starts selling them to every body that could get his hands on one and here we are. Drivin trucks that have sat for month after month on some base. Jump in'em and go sailing down the 'Interstate', mile after mile at 2500+ - rpm's for hours when all of a sudden...BANG!!!!!.

The 45 year old rod or rod bolt has finally had enough and lets go.

'Holy Wow' we got ourselves a "PROBLEM".

No ya don't, a 45 year old bolt broke and that kinda stuff happens all the time, day in and day out in cars, trucks, planes, tractors, bicycles, motorcycles, etc..

But....all us guy's here on SS are only hearin about the broken bolts that punch a big 'Ol Hole' in some guy's block.

Think about all the 'THOUSANDS' upon 'THOUSANDS' of deuces runnin around out there right now that never have and most likely never will toss a rod.

So relax out there on that vast big blue ocean, enjoy the ride and we'll see ya when ya make port.

Ya probably outta be more worried about the bolt on the ships rod letting go, cause that will most probably turn into a much bigger problem for ya then the one on your deuce.

 

[jimk]

All metal fatigues and changes with age.

I don't think anyone needs to worry about rod/rod bolts getting weak due to age. Tightening a bolt is a stressful but not repetitive, hopefully, lol. No fatigue there. Since bolt is designed to fall short of its 'yield point' , even when running, it will remain stable. Flex under normal use is small too. Fatigue is never an issue. Most automotive rods can handle many billions of cycles (say 100+ years of constant use). Big rigs can run million + miles before rebuild and rods are re-used.

Valve spring are very different. They have tension/compression stresses that bend the metal. That can cause fatigue. That creates heat and may increase creep. It is common for high mileage auto spring to get very weak (not so on springs that just sit).

The only thing I can think of that might change in time is hydrogen embrittlement. There might be proton decay but we won't live long enough to see that happening (current estimation of proton's half life is more than 6.6×10 to 33power yr. a very long time). Other metals are even more stable that steel, a gold coin under no stress would be immune to metal fatique and age. Mercury is a metal that cannot fatigue at room temperature. Chemically hydrogen is an akili metal though we know it as a gas on earth it is assumed to be a liquid metal inside Jupiter. Ok I'll shut up


Wiki-
Steel generally exhibits a very linear stress–strain relationship up to a well defined yield point (Fig.2). The linear portion of the curve is the elastic region and the slope is the modulus of elasticity or Young's Modulus. After the yield point, the curve typically decreases slightly because of dislocations escaping from Cottrell atmospheres. As deformation continues, the stress increases on account of strain hardening until it reaches the ultimate strength. Until this point, the cross-sectional area decreases uniformly because of Poisson contractions. The actual rupture point is in the same vertical line as the visual rupture point.


Stress?strain curve - Wikipedia, the free encyclopedia
Proton decay - Wikipedia, the free encyclopedia
www.wikipedia.org/wiki/Hydrogen_embrittlement

 

[doghead]

I never meant to imply that the rod bolts were getting weaker with age(as the root cause of the failures).

I was only making a generalized statement.


At this point in the thread, I am wondering if the OP realizes that the trucks have a governor.

 

[patracy]

ARP rod bolts range in price from $50-150 a set. The oil pan gasket is ~$25. So it might work out to be less than $200 for a bolt upgrade. I've been researching all the rod failures and I've only seen the cap separating from the rod. It probably would be worthwhile to upgrade the main bolts as well while you're in there. Again, if someone had this hardware laying around, I'll see what I can come up with.

 

[stumps]

...Valve spring are very different. They have tension/compression stresses that bend the metal. That can cause fatigue. That creates heat and may increase creep. It is common for high mileage auto spring to get very weak (not so on springs that just sit).

I think you missed my point.

Even though a valve spring is flexed a visible amount, the steel is still capable of handling the bending billions of times without breaking. I have taken apart many engines with multi hundreds of thousands of miles on them, and the valve springs are still the same height and strength as when they were new. It is rare to find one that has changed enough to matter.

My point was that the rod bolts are indeed seeing small changes in tension due to operation, but not enough to be visible, and not enough to cause fractures over time. If they break, they were either seriously overstressed (somehow), or improperly made (or specified) in the first place.

-Chuck

 

[jimk]

I think you missed my point.

It is rare to find one that has changed enough to matter.
-Chuck

Maybe for a deuce. I don't have much experience with springs. I remember when I first rebuilt my 70 GTO 400 (30y ago), those 130K mile springs all measured 50-80lbs seat pressure. The factory spec was 100 or 110. The lesson learned was always replace them. I don't even test old springs now, just toss them then use ~130lb Crane springs. New springs also benefit from 40years of metallurgic improvements.

My pal Scott had a V spring break on his 03 Corvette ZO6. Mileage was about 30K. RPM was 1800 when it let go. Destroyed the engine. It was out of warranty due to time. The dealer ignored him. The factory rep took a look and gave him a crate engine. Told him he would have pay for install. Later Scott found out that it was a problem others had also had. Seem Chevy knew and upgraded the later parts but never bothered to tell the old customers. Apparently they were hoping people would make it past warranty so the manufacturer didn't have to retro fit saving $. They really wanted to stick it to the owners, who probably would liked to know, even if they had to upgrade at their own cost, to protect their 50K investment.

Just 2 examples of many

 

[merlot566jka]

91W350, That looks very similar to LDT rods!

Stumps, yup, snowball effect. I expect that.

SRJeeper: Perfect! Its like a speach from my fathers mouth. Made me smile.
Nope, dont have a deuce yet. And I agree with you on the "problem", while this wasnt a problem for the truck in service, its a problem for people who buy the trucks afterwords. I want to go 55mph, and I dont want a rod to let go at 55mph.
I'm watching out for our nuts and bolts out here in the gulf, but my job is to work on FLiR (foward looking infrared radar).

Jimk, good to see you hit on some of the meatalurgy (sp?). I think its a combination of both poor quality metal and improper installation.

Doghead, Yeah, I know theres a governer. I know it can be set to prevent overspeed, but I dont want to prevent the engine from going over 2650rpm...I want it to thrive at 2650rpm.

Patracy, Thats the attitude! I guess I wanted to find a possible solution and a motivation to try it. Looks like you have the same mindset.

All, I think I am satisfied with the discussion and results here. I feel that the rod bolts apear to be the weakest link in the 2650rpm question. An improvement would be new ARP rod bolts, provided they manufacture such an item. Thank you for the technical inputs and analysis.

 

[doghead]

Now to solve the problem why my Cummins NHC-250 only revs to 2100 rpm .

 

[Flyingvan911]

Now to solve the problem why my Cummins NHC-250 only revs to 2100 rpm .

It wants to be a Multifuel!

 

[Srjeeper]

Now to solve the problem why my Cummins NHC-250 only revs to 2100 rpm .

Yer bolts are weak, yer nuts came loose and yer rods bent..

pay attention.......

 

[doghead]

My urologist said the same exact thing...

 

[m16ty]

Now to solve the problem why my Cummins NHC-250 only revs to 2100 rpm .

The old Cummins will throw a rod also if you run it too fast.

Short stroke gas engines will usually just float the valves if ran past redline. Almost all long stroke diesels will reward you with a thrown rod if you push her too hard. It's not just a problem with multifuels. As with anything, you push it too hard and parts are going to fly.

 

[stumps]

The old Cummins will throw a rod also if you run it too fast.

Short stroke gas engines will usually just float the valves if ran past redline. Almost all long stroke diesels will reward you with a thrown rod if you push her too hard. It's not just a problem with multifuels. As with anything, you push it too hard and parts are going to fly.

Yep!

The only difference between the MultiFuel, and an OTR diesel (in the rod throwing tendency) is the Military turned the governor up well past the speed that would usually be the limit for a good long operating life.

Look at what they did to the gasser to make it a MF. They kept all of the axle, transfer case and transmission ratios basically the same and added an overdrive 5th where 4th used to be. If they hadn't cranked the governor up the MF would have been seriously slow with those axles. They had to make a serious compromise to get any kind of speed out of the truck at all. They clearly weren't looking for a million mile engine.

The same MF engine in an Oliver or White tractor would have the governor maxed out at 2200 RPM, and would never throw a rod... unless you ran it out of oil.

-Chuck

 

[jimk]

I drove a commercial tractor trailer for 25+ years. 90% Macks. The early years w/U models, the bulk with CH/Vision's using strangled EM7-250 or 315s. In all those years, with many dozens of co-workers, I only -heard- of one internal engine failure. Some new guy managed to grossly overspeed his CH's engine (pretty hard to do- I'd guess going down a hill). 3 or 4 pushrods failed. I was told the ends fell out. I saw my pal have one catastrophic turbo failure in 86' then in the mid 2000 there were many turbo failures with the new variable nozzle units. None of these hurt the engine. I had one myself around 06. After the Co turned my 315 down I had compressor surge. Told 2 different shop dudes, neither knew what I was talking about. The senior mechanic said 'they don't have variable nozzle turbos'. I remember walking over to one someone was working on (his?) and showed him the servo and linkage. These engines were designed to have it all in at 1100rpm (called high torque rise, 1000 was where it started to happen), and be all done at 1800. No load was 2000. Warning on the visors said 2100. They way the company had the computer /gearing arranged 1500-1600 was all you would get in top gear. 1200 was torque peak. These low speed was great for ring life and fuel economy. Turbo and aftercooling made all this happen. Long stroke helped too. Longer than the MF. BTW output was supposed to be 1000-1400 lb-ft, fuel economy was about 9mpg highway. It was designed this way and worked well (well, until Renault appeared, lol). The MF would seems, to be designed differently, for different usage, and different life span, as Chuck points out.

Guys that wrench know and see it all, a few of whom are around here. They have been quiet. I'll bet they are too busy, either working, or laughing.

 

[jaxsof]

It isnt about beefy bottom ends, its about rotating mass. How fast can you swing an empty bucket? Now try it full. there isnt any bottom end beefier than this one,
http://gcaptain.com/maritime/blog/emma-maersk-engine/the-worlds-largest-marine-diesel
and look at its R-limit! (102 rpm for those not wanting too read that far) Metal does age, but the intended stress for those bolts should far outway any age related fatigue. And by comparison, the afore mentioned VW is 109-122 cid, Dodge/Cummins is only 360 inches, and the LD is what, 465? The 250 Cummins is 855. The Cat 3116 is 402 and the 3126 is 440. Diesels make torque, their peak is usually very low in the power band.

Ive seen others here who have converted their dueces to 6bt Cummins and baby Cats. Both of these have a much lower recprocating mass, and the Cat is still limited to 2400rpm and the Cummins to 3000 (at least from the factory)

Oh, by comparison, the GE FDL7(7 liters per cylinder) in the v-16 varient, makes 4400hp all day long, and the computer can compensate for 3 dead cylinders. It is limited to 1040rpm.

Perhaps there is something to this technology stuff!

Another thought(seem to have a lot today, and I appologize for that, we seem to have a lot of these engines(the LD's) here giving up at around 800 hrs. If one were to have a fuel pump that wasn't 100% new, and was gradually worsening, it could be dripping enough that there was too much fuel on top of the piston causing hydro-lock, or just as bad, a little fuel too early causing a pre-ignition. Wouldnt have to do it more than a few revolutions. Has anyone looked at the TOP of the piston of the cylinder that let go to inspect for overheating/blast damage?

What I would like to know is why the 2-strokes can rev so high?

 

[Gamagoat1]

Now to solve the problem why my Cummins NHC-250 only revs to 2100 rpm .

I'll bet if you checked the tach with a known rev indicator you'll find it's wrong. The spring looses it's strength over the years.

 

[jasonjc]

ONLY a 102 rpm's?? It's broke we HAVE to fix it!!!!!!!