Why 2650rpm max?

[m-35tom]

you are trying to compare gas engines to diesels. true that many formulas will work, but the rods are MUCH heavier and the moment of mass as they swing must be pretty high. now let me say a few things that i think many overlook because they want to go fast or they have no experience or they have not read the manual.

this engine was used in many white farm tractors where the max rpm (high idle) was set at 2200 rpm. this should tell you something.

the manual says that you can use max rpm only for breif moments when you need max power. (not for driving down the highway.)

i have seen many of these engines with windows in the block and they all have been broken rod bolts. never have i seen a broken rod.

i like my engine and respect its rpm limitation. to that end i have a suggestion. adjust your high idle setting to 2550 rpm in neutral. simple to do, just remove the cover, turn the screw a couple of turns and then slowly put the pedal to the floor and see how high it goes. of course have enough sense to not go over 2600 if it still wants to, just turn screw some more.

tom

 

[Burgerboy13]

The answer is very simple. Diesels are a lot more rotating mass and really have no need to be high RPM motors. Almost all diesels redline under 3000RPM. Its not really a limiting factor when you take into account that final gear ratios is what gives you your top speed. I think your putting waaaaaaay too much thought into this.

 

[stumps]

This is not entirely true. The internal combustion engine "bible" by Taylor indicates that high compression ratio alone is sufficient to enable multifuel capability. The MAN combustion is probably just icing on the cake.

Not quite. There is a big problem with highly volatile fuels like gasoline. Were it not for the special injectors that squirt a stream of fuel, rather than atomize the fuel, and the deep well combustion chamber in the piston, that keeps the fuel out of the crankcase, the MF would not work very well.

My internal combustion engine "bible" is "Internal Combustion Engines" by Obert. I don't recognize your heathen gospel

(Was that enough smileys to avoid a fight?)

-Chuck

 

[stumps]

Yes, that is my intention. If it is infact the bolt that breaks, It would be easy enough to address. Some top notch ARP bolts or the like would likely take care of the problem. If it isn't the bolt and the rod is the problem then that leaves few if any realistic options to fix it.

We have seen pictures of quite a few failures, and in all cases, there are a pair of rod bolts that have snapped off. It sure looks like they are the source of the failure. In a couple of pictures that showed up on this list, there was clearly a fault crack in the rod bolt that was old... a sign that the bolt had been over torqued at installation, and had failed much later. The crack showed with a light gray center, and a dark gray ring around the center.

Do a search on this site. I am sure you can come up with a few rod remains to test.

-Chuck

 

[doghead]

Let's not forget these engines are 20- 40 years old and obsolete for a reason. Time takes it's toll on all things, including bolts under tension.

 

[stumps]

Let's not forget these engines are 20- 40 years old and obsolete for a reason. Time takes it's toll on all things, including bolts under tension.

Uhmmm, not usually. These bolts, if they were spec'd properly (and I'd bet they were), should be able to stand this service for practically forever. A well designed engine will die from friction affects (read: wear) long before they will die from fractures.

-Chuck

 

[doghead]

All metal fatigues and changes with age.

 

[stumps]

All metal fatigues and changes with age.

Absolutely!

But, when the metal isn't being stressed beyond a certain point, it can take the stress essentially forever. Consider the valve springs in an engine. They are being flexed many hundreds of times per minute, and yet they take it virtually forever. Valve spring failures are pretty rare in modern engines. The rod bolts are also made of a high quality steel like the valve springs, but they are not getting bent nearly as much... actually, they are getting bent so little that you couldn't even see it by eye. They should never fail in a host of lifetimes... and typically they never do. It is only when they are over stressed, by seriously over speeding the engine, or through incorrect installation that they fail.

M35 Tom has said it really well: "the manual says that you can use max rpm only for brief moments when you need max power. (not for driving down the highway.)"


-Chuck

 

[jimk]

>cut>also, i did touch on balance in my first post, it is a fairly important issue, but by the looks of things through discussion, the balance doesn't appear to be the problem...

Page 54 or the LDS-465-1 Troubleshooting Manual states there are two types of pins, hollow and solid, an that they can be used in various engines as long as they are a matched set. As they are most likely made of them same material they should have different weights. Apparently balance isn't very critical??

I'd think Carrillo or Crower would make a high strength custom rod.

Page(53-54) also discussed the weak early machined retainer.



A bit of topic but since many here are into hot rods so I'd like to share something. The 73-74 Pontiac TransAm's had an optional engine, the Superduty 455. In '74 it was the most powerful of all Detroit auto engines. Pontiac actually had planned to use it for the 71 GTO but it was delayed 2 years while they developed a stronger connecting rod. During these 2 years the HO455 with sold. It used with cast rods. Since any Pontiac V8 engine an use any rod (except the 301/303/366 racing engines) it is a great way to hot rod the weakest link. It can extend the operating range of any engine by over 1000rpm, with no loss in safety factor. A 326 with it is safe to 9500rpm.

Features include-

Forged SAE8640 (Chrome nickle molybdenum alloy- a bigger deal in 74)
Heat treated to 35 Rockwell C
Shot peened
7/16 boron steel rod bolts
Bolt ends counter sunk for micrometer stretch tightening(.006")
Windows near bearing ends as bolts are .001" away from shell (so area is not super thin)
Lightened small end (the area that reciprocates)
Massive big end ring (to prevent distortion)
Max material near bolt head (where a big hole is)

 

[phil2968]

All metal fatigues and changes with age.

So that's whats wrong with you, the plate in your head has cracked from old age!

















Just had to. Sorry.

 

[patracy]

The answer is very simple. Diesels are a lot more rotating mass and really have no need to be high RPM motors. Almost all diesels redline under 3000RPM. Its not really a limiting factor when you take into account that final gear ratios is what gives you your top speed. I think your putting waaaaaaay too much thought into this.

Really? My little 1.6 VW turbo diesel spins up to 5500rpm. (80's tech) My dodge currently revs to 3250, but will hit 4k shortly... The wife's 2.8 VM diesel in her liberty revs to 4K something. Heck even my yanmar 1cyl diesel is setup to rev to 3600rpm continuous.

That said, has anyone upgraded their rod bolts?

 

[merlot566jka]

I think we are straying a bit off topic.

Tom,
This isnt about going faster, this is about why it doesnt rev higher...if i wanted a faster car, I'd drive my Corvette.
My initial discussion is why is there a failure, and how can we fix it.
It seems that the engine would be capable of higher rpms. So what is causing it to fail?

Rotating mass weight is important. But as a general rule, the beefier the bottom end, the more stress it can handle. The reason the diesel is built the way it is, is to be able to withstand the massive loads put on the rotating assembly.
When this engine was designed, I imagine 2200rpm was the safe limit. An engine run at 3/4 its power output will always last longer than an engine run at 100%. This is well understood. With advancements in technology, materials and understanding of dynamic loads, I feel that a solution to this problem is achievable.

Comparing gasoline rotating assemblies and diesel rotating assemblies isnt an apples to oranges comparison. The stresses and loads are realitive, and the geometry is the same. The difference is the diesel is overbuilt for higher combustion pressures and loads, and the gasoline is built to rev higher. Natuarlly the diesel, being over built, has serious problems at higher rpms due to the mass of weight being slung around, not to mention the longer stroke increases the max piston velocity, increasing loads further. BUT, the same solutions that solve rpm limitations for gasonline engines apply to diesel engines.

Of course I have read the manuals, Ive downloaded them and read each one, almost word for word. Looking for more information about failures, revisions, and solutions. I have nothing better to do while out to sea for half a year at a time

burgerboy: yup, I think your right!

Jimk, I agree...balance wasnt very critical. No where in any of the TM's did i see a single thing about crank balancing, keeping flywheels with cranks or making sure dampers matched. I saw different pistons recomened for different motors, different pins for rods, and not a word about thier weight importance.
And that small blurb about those rods increasing the rev range of the pontiacs is just proof that solutions can be had for rotating assemblies! We dont have to accept 2200, 2600, 2900 rpms if we dont want to.

Some say i am over thinking this, others are suggesting a different vehicle or engine. guys, i just want to fix a problem. i dont want to go over 55, i dont want to pay $3000 for a cummins, and i dont have any reason not to devote some time to finding a solution. this is a discusion, humor me!

 

[Burgerboy13]

Really? My little 1.6 VW turbo diesel spins up to 5500rpm. (80's tech) My dodge currently revs to 3250, but will hit 4k shortly... The wife's 2.8 VM diesel in her liberty revs to 4K something. Heck even my yanmar 1cyl diesel is setup to rev to 3600rpm continuous.

That said, has anyone upgraded their rod bolts?

Sorry I was talking about large diesels. A smaller engine with less rotating mass can rev higher but due to its smaller displacement doesn't produce the as near the torque of larger diesels. I wouldn't like to see a 8.3 Big Cam Cummins spinning over 4K while pulling 80,000lbs. Though I am a novice when it comes to diesels so I could be wrong.

 

[SCSG-G4]

a 1.6 l is a small diesel, so is the 2.9 VM and the yanmar! Small engine, low mass, hig engine, large mass. Does your 5.9 Cummins have pistons that weigh over seven pounds each? He did say "ALMOST ALL" redline under 3000RPM, there are exceptions. I've seen diesel pistons that were 36 inches in diameter (in a tug boat), I don't think you will turn them at 3000RPM.

 

[stumps]

Really? My little 1.6 VW turbo diesel spins up to 5500rpm. (80's tech) My dodge currently revs to 3250, but will hit 4k shortly... The wife's 2.8 VM diesel in her liberty revs to 4K something. Heck even my yanmar 1cyl diesel is setup to rev to 3600rpm continuous.

That said, has anyone upgraded their rod bolts?

Yeah but, your entire set of pistons weighs less than one MF piston.

-Chuck

 

[91W350]

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...

 

[stumps]

One thing I have always wished we could get access to is the reasoning behind the various decisions made in specifying this truck. There is a reason why 56MPH, and 2600RPM were selected as the maximum, and the tach's and speedometers were labeled thus. I suspect that it is like the WEP (War Emergency Power) setting on fighter aircraft... letting you have a little more in case of emergency, at the expense of engine life. I suspect that 2600RPM was (is) very close to 100% for this engine. I suspect that the US Government didn't care all that much about these trucks making a million miles before the engine needed servicing. It is pretty clear from some letters that David Doyle found that they were concerned about the engines only making 10,000 miles, though.

I think that if you solve the engine's tendency to throw rods, you will start to see its tendency to stretch caps. And if you solve its tendency to stretch caps, you will start to see its tendency to bust cranks. And if you solve its tendency to bust cranks, you will start to see its tendency to burn holes in the pistons... It is close to its practical limits. There isn't going to be a revelation that saves the day... not at such high speeds on such a long stroke engine with such a high compression ratio and such a heavy piston.

-Chuck

 

[stumps]

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...

That's pretty close to what we have in the MF engine right now.

-Chuck

 

[JasonS]

i like my engine and respect its rpm limitation. to that end i have a suggestion. adjust your high idle setting to 2550 rpm in neutral. simple to do, just remove the cover, turn the screw a couple of turns and then slowly put the pedal to the floor and see how high it goes. of course have enough sense to not go over 2600 if it still wants to, just turn screw some more.

tom

Will this affect fueling rate?

 

[stumps]

Will this affect fueling rate?

Nope. It is purely a maximum RPM thing. Your adjustment on fueling rate affects HP across the whole RPM range.

-Chuck