Why 2650rpm max?

[merlot566jka]

A careful and relentless search of this forum and other documents on the LDT-465 have resulted in one thing, do not go faster than 2650 rpm or you'll throw a rod.

I, being the engineer type, ask why? What causes this, and how can we fix it?

To figure it out, I applied some numbers to the question and did some research.
First we start with the research. It is said by race engine builder and author Tom Monroe, that
“Unless your engine will have pneumatic- or solenoid-operated valves, the valvetrain will limit maximum engine rpm, but piston velocity must not be ignored. High rpm operation causes reduced ring sealing , excessive wear and low durability or catastrophic failure from dynamic loads on the piston as it moves up and down. As a crankshaft rotates, each piston travels up and down the bore at an average velocity, or mean velocity, between TDC and BDC. Directly proportional to stroke and engine speed usually expressed in feet per minute, average piston velocity is found by multiplying engine rpm by stroke in inches, then dividing by six. For example, an engine turning at 7500 rpm, with a four inch stroke has an average piston velocity of 7500rpm x 4.000in. ÷ 6 = 5000fpm. For a stroke of 3.50in, average velocity drops to 4375 fpm. The accepted average velocity was 4600fpm, but it is common for race engines to operate at sustained average piston speeds of up to 5700fpm. I am also familiar with instances where average piston velocities have exceeded 7000fpm, although with 200-gram pistons. Advancements in piston design and development of materials used for them, piston rings and connecting rods have made this possible. Although the envelope has been expanded, basics still apply.
You may have wondered how a formula 1 engine stays together at 17000rpm. Notwithstanding pneumatic-operated valves, short stroke is the simple answer. A V-10 wit a 1.7 in stroke has an average piston velocity of 4817 fpm compared to a stock car engine with over 5500 fpm at 9500 rpm! More importantly, when a piston decelerates to a stop at the top or bottom of a stroke, it is accelerated in the reverse direction to maximum velocity. A matter of dynamics, maximum piston velocity far exceeds average velocity.”


Accompanying this was the supportive formula to find the average and instant piston velocity.

“Maximum velocity of a piston occurs when the connecting rod is at 90 degrees to the crank pin. Therefore, the angle of crank rotation from either side of TDC must first be found. To do this you'll need to know connecting-rod length () and crankshaft throw (), or half of the stroke. A calculator or trig tables will be needed for doing these calculations.”
I removed the example, but if you need it, you can pm me or buy the book.



So what does this mean to us? It means that at 2650 rpm, we have an max piston velocity of 3580.7943 feet per minute. Well with in the “accepted average velocity of 4600 FPM”. We shouldn’t be throwing rods like crazy when someone pushes past the 2650rpm limit.

Now this guy is talking race engines, with balanced and blue printed rotating assemblies, and light weight components...right? Nope, this is a basic rule of thumb for any engine building.

But the questions that need to be asked are:
Is the rotating assembly balanced? I sure hope so.

Is the flywheel neutral balanced or specific to the crankshaft? (if someone swapped your flywheel with
a different one that wasn't balanced exactly like yours, something is bound to fail)

Is the crankshaft balancer/harmonic balancer/damper balanced to each engine or are they neutral?

Do the crankshaft balancer/harmonic balancer/damper suffer the same wear and failure that the CUCV 6.2L did?

What is the quality of the LDT con-rods and bolts? The LDS shares the same P/N for the con-rod and bolts as the LDT, so that shouldn't be the limit.

Does the offset rod cap have anything to do with the failures? I don’t know, but I don’t think so.



OK, now why do we still have a limited rpm range? Is it our valvetrain? According to this illustration, we have 216 deg of open intake valve, and 218 degrees of open exhaust valve. Somewhere else I found the number for the lift, and it was based on the valve spring seat pressure tests at closed and open, it was .450in of lift. So we have plenty of cam timing and, as I remember, valve spring to keep this engine alive at higher rpms than 2650. Provided the valves are sufficient in size, I don’t see the valvetrain limiting the LDT to 2650. Especially with a turbo added to the mix, there is enough air coming in and going out to see 2800rpm.
(To add, the LDS has a different intake rocker, the only difference is a hole for oiling, the ratio is the same. )

What about other support systems?

Is the Injection Pump limited by this engine speed? Looking through TM 9-2910-226-34, I see nothing saying there is a limit to the speed of the IP. As a mater of fact the TM has several test procedures for above 2900 rpm.

The alternator? Would an extra 400 rpm kill the factory alternator? Probably not.

Water pump? No way.

The fan? Nope.

Oil pump? Maybe, it could over pressurize, maybe over flow. But looking at the LDS, the use the same oil pump, so I don’t think that’s an issue.

Turbo? Yup. That could definitely limit the rpm range. Spinning the turbo too fast causes the intake temps to soar, pushing the compressor out of its efficiency island. That would be a huge reason to limit the extended rpm range. Not to mention raising back pressure on the exhaust valves. Adding a waste gate and matching a better sized turbo would alleviate that problem. But with out exact specs on the turbo we don’t know the range or limits on it. We do know that over running it at high loads results in soaring EGTs. Consistent high EGTs is recipe for disaster.

Coolant system? I could see it, if the heat cant be removed from the coolant efficiently to sustain higher engine temps caused at higher rpm and load, it could cause a failure. But its not rods flying out of the block.

It is worth while to note the difference in the LDT and LDS that may permit higher rpm. The pistons are fairly different. The LDS ones are lighter, lower compression, and are oil cooled. Does this make the 150rpm difference for the two engines? ****, it sure doesn’t hurt, but I don’t see it as the sole factor.
The injectors are different, make a difference in rpm range? Its possible, if the old injectors cant keep up with the flow at 2800 rpm... but if that’s true, then at 2650 the injectors would be really pushing their limits in the first place. Not to mention the IP being the main factor in driving those injectors.
Speaking of the IP, yeah, that’s different too. Whats the difference? Well there are quite a few, too many to outline here, but do those differences really limit the rpm range? I don’t see it.

What to take from this...
Mechanically the engine (LDT) should be well suited to see 2900 rpm.
The turbo is the largest limiting factor on engine speed.
The quality of hard parts and internal/external balancing are equally important.

Anyone have anything else to add?

 

[jwaller]

the main limiting factor I have noticed are the rods. They seem to want to let go on the big end and come out the side with very predictable certainty.

 

[merlot566jka]

While the rods in the LDS and LDT are the same, why the slight 150rpm difference? Does the LDS have the same rod failure rate as the LDT? I dont recall seeing any LDS rod failures.

 

[eric_banks32]

I like your analysis but your missing the part about what the rods and bolts are made out of and what their particular tensile strength is. Also the rods and bolts "stretch" a minut amout with each motion. Also their is the recipricating weight that you touched on with the piston weight that factors in. Therefore at a certain point they will break. If that point is below your 2900rpm est. then they will let go. Their is a formula somewhere I've seen that will give you a breaking point based on rpm vs weight vs rod/bolt strength.

 

[jwaller]

dunno, I wouldn't put a lot of stock in there being such a small difference in that number. prob the same as another vehicle I work on having 5 more hp than the previous year when there is no difference in the vehicle at all. they did it for promo reasons only.

I think the 5 ton multi has oil cooled pistons, dunno, doubt there is any logical reason for the change.

 

[davidkroberts]

to be honest i have no idea why but i have a m35a2 with a rod sticking out of the block to prove the point. If i had to guess I would think it is a design deficency due to the unique design of the Multifuel engines piston. Outside of the FDC the engine is a basic diesel engine. It is the only major change. I never had a chance to research if the MAN multifuel diesel engines have similar failures.

To the original question of the rpm range I dont even go that high anymore. I max at 2400 to 2500 and run about 1/5 to volume of lucas oil to try and cut down cold starting wear from before it builds oil pressure.

 

[merlot566jka]

I've read about the rods and bolts being the failure, but never an answer to WHY they let go. Somewhere, there was a thread that almost had the info, yet no one ever tested the rod bolts or rods to see if they were made from lower quality materials.

My thought proccess was that the rods were made of quality material and the rod bolts aswell, then the fault has to be else where. But the question still remains, what is the quality of the rods and rod bolts?

 

[SCSG-G4]

merlot,

If you want to test the rods (poppet valve push rods), a set of 12 will set you back $25, plus shipping. PM me for details!

 

[eric_banks32]

I've read about the rods and bolts being the failure, but never an answer to WHY they let go. Somewhere, there was a thread that almost had the info, yet no one ever tested the rod bolts or rods to see if they were made from lower quality materials.

My thought proccess was that the rods were made of quality material and the rod bolts aswell, then the fault has to be else where. But the question still remains, what is the quality of the rods and rod bolts?

I think that is the question to answer. But without a major knowledge in metallurgy or a system to test it, it will remain unanswered. Maybe some one that has had a rod failure can post some pictures and maybe we can see where it is breaking and determine whether its the bolt or the rod itself. That would eliminate some concern. If it is the bolts that fail I see no reason why it couldn't be fixed with some ARP bolts or the like.

 

[73m819]

Still WHY does not the 5t motor take a dump like the deuce if the RODS and BOLTS are the same, about the only difference that you have found is the heavy pistons in the deuce, could at a high rpm the heavier pistons put to load on the lower rod end and bolts, above what the design is, where the lighter pistons keep the load just under the break threshold

 

[eric_banks32]

I wonder what the results would be to put a rod in a press using some THICK wall pipe to simulate the wrist pin and the crank and spread her till she lets go.

 

[JasonS]

Comments in red.

(To add, the LDS has a different intake rocker, the only difference is a hole for oiling, the ratio is the same. )
LDS-465-2 has a different rocker; not the plain jane LDS465-1



The pistons are fairly different. The LDS ones are lighter, lower compression, and are oil cooled.
The documention that I have read indicates that the LD, LDT, LDS have the same compression ratio. All of the LDT and LD multifuel engines that I have seen have the piston cooling oil regulator; the military manuals do not indicate a difference. Why do you think that the LDS pistons are lighter?

What to take from this...
Mechanically the engine (LDT) should be well suited to see 2900 rpm.
The turbo is the largest limiting factor on engine speed.
My observation of engine failures on the website suggest that the rods are the limiting factor.

 

[eric_banks32]

Still WHY does not the 5t motor take a dump like the deuce if the RODS and BOLTS are the same, about the only difference that you have found is the heavy pistons in the deuce, could at a high rpm the heavier pistons put to load on the lower rod end and bolts, above what the design is, where the lighter pistons keep the load just under the break threshold

I think thats the answer. More recipricating weight do to the larger piston.

 

[mktopside]

I wonder what the results would be to put a rod in a press using some THICK wall pipe to simulate the wrist pin and the crank and spread her till she lets go.

Rods in general normally break on the exhaust stroke, even the ones that look like they were bent or crushed on the power stroke.

Like a baseball pitcher throwing out his elbow, a heavier piston would lower the threshold at which the rod can reverse the direction of the piston.

 

[steelandcanvas]

Rods in general normally break on the exhaust stroke, even the ones that look like they were bent or crushed on the compression stroke.

Just curious, what is your source or experience for this statement

 

[mktopside]

Just curious, what is your source or experience for this statement

ASE master tech with L1. Been turning wrenches professionally for quite some time. Though I will say I am not a diesel mechanic.

Think about it for a second, all things being equal if you are making enough power to break a rod on the power stroke, you would have already smashed the aluminum piston to pieces.

 

[eric_banks32]

ASE master tech with L1. Been turning wrenches professionally for quite some time. Though I will say I am not a diesel mechanic.

Think about it for a second, all things being equal if you are making enough power to break a rod on the power stroke, you would have already smashed the aluminum piston to pieces.

That statement is missleading. Rods will bend on the compression/exhaust strokes. They will pull apart/break on the intake/power strokes. It all depends on the stresses in the motor and the condition of the material. Most don't fail all at once. It is from abuse over a period of time.

I've seen far more rods fail than pistons and I've tore down and built a lot of motors. Piston failure can be attributed most to detonation and foreign material. Rod failure accures mostly do to exceeding stresses on the motor such as over revving, improper bearing clearance, improperly torqued bolts, and not enough oil to the rotating parts and on and on......

And you still left no real proof of your statement that they break on the exhaust stroke. The most stress is put on the rotating assembly on the during the compression and power stroke. It is only logical that this is the most likely time for the bottom end parts to fail.

 

[ODdave]

dumb question but could it be that your vehical maximum speed (55mph) is exceded if the engine where spun 150 rpm more on the deuce than it where on the 5ton and that this is the actual reason for the max rpm to be lower?

 

[mktopside]

That statement is missleading. Rods will bend on the compression/exhaust strokes. They will pull apart/break on the intake/power strokes. It all depends on the stresses in the motor and the condition of the material. Most don't fail all at once. It is from abuse over a period of time.

I've seen far more rods fail than pistons and I've tore down and built a lot of motors. Piston failure can be attributed most to detonation and foreign material. Rod failure accures mostly do to exceeding stresses on the motor such as over revving, improper bearing clearance, improperly torqued bolts, and not enough oil to the rotating parts and on and on......

And you still left no real proof of your statement that they break on the exhaust stroke. The most stress is put on the rotating assembly on the during the compression and power stroke. It is only logical that this is the most likely time for the bottom end parts to fail.

I don't have any "proof", just what I've seen over the years. Yes, they can certainly crack or bend on the power stroke, but how many have you seen in your experience in any engine that broke on the power stroke?

What I was getting at, is that in this situation if you have two motors that are almost the same except one has heavier pistons, and that one is the one that throws rods; I would say that is the reason why. Piston size and weight wouldnt have any effect on the rod on the power stroke...... But it would be huge on the exhaust stroke.

Mike

 

[eric_banks32]

Piston size and weight wouldnt have any effect on the rod on the power stroke...... But it would be huge on the exhaust stroke.

Mike

I think the exact opposite there is much less stress on the whole assembly on the exhaust stroke. The piston is traveling up and the valve is open relieving most of the resistence. Where on the power stroke the the cylinder has just fired causing extreme preasure to be put on the rotating assembly forcing the piston back down with no relief other than the volume of the cylinder inlarging. I agree that piston size has a huge affect on the rods failing just trying to understand your exhaust stroke logic.