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Discussion Starter · #1 ·
Many years ago (2006) I had my stock rods cryo treated and they have lived a long and beautiful life in several shortblocks at 900-950whp. In March some buddies and I towed down to Houston from MN for some TX2K16 street action and things got a little crazy. On Saturday, which was our last night there, it was time to turn things up. The hot and humid weather had moved out and some extremely cool, dry air had moved in. My density altitude app calculated the corrected air to -800ft below sea level late that night. Prior to going out for the night, I made several test hits at the desired boost of about 28lbs to make sure the fueling was good and I definitely had to add some fuel. I estimate power at that boost to around 1050-1100whp.

Anyway, about 4am I got into a race with a TT Coyote and I was using my newly set up boost by gear on my eboost and right after I hit the 3rd set point, a MASSIVE fireball ensued and lit up what seemed like the whole sky. I coasted for a short while so the fire would go out and when we got out, there was oil everywhere. Some theorized that it detonated, but all the evidence points to just a failure of a part beyond it's limit. The plugs looked perfect, the piston looked perfect on top and above the oil ring, the combustion chamber looks perfect. Also the bearings look great all things considered as does the crank journal. I will have the crank checked and if it's good, re-use it. The piston twisted slightly in the cylinder after the rod snapped and all the valves smacked so will need to replace those valves. The oil pan and pickup are trashed and I will have to check the oil pump, hopefully it's good.

I figured this could maybe sway guys who thought 4 digit power seemed like a good idea on stock rods. I highly recommend upgrading!

I already have a new teksid block and some billet main caps and I'll be back when I can cobble up the money for a new shortblock. Anyway, time to start assembling the rest of the parts and checking my existing parts........

Race with said Coyote








 

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Dang! That's pretty impressive. Unfortunate, but impressive!
 

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Wow. I've seen them bent on occasion, but never snapped like that. You were definitely making some big power through that motor.

If you don't have any I-Beams yet, Lance (1raresnake) is selling some for a good price.
 

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Took a quick look at the site before my first client this morning, Tony and saw this. I'll share some comments later. It looks, to me like it broke on the down stroke and was a notch/fracture induced tear of the metal in the rod. My guess is it happened at a medium to high rpm with the car accelerating hard — more on that later.

Bad luck, feel for ya. Sorry to see that kind of failure.


Ed
 

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Discussion Starter · #6 ·
The Manley H beam rod is a good piece as long it operates within its design parameters. However, for the ultimate peace of mind in reliability, the Manley 300 M I beam rod with 7/16 bolts is the way to go and this is the rod I will use on my 2001 cobra begin build.

Here is my thread with info on the 300M rod
http://www.modularfords.com/threads/228898-Manley-adds-300M-I-beam-connecting-rods-to-thier-list-of-off-the-shelf-4-6-rods
That very night at the hotel I met a guy from Manley named Tom R. He is a regional sales rep and 25yr employee of Manley. I specifically talked to him about my rods and that I was worried about what could happen and of course what I was worried about did happen just a few hours later. Fortunately I got his contact info and he told me he would hook me up on a set of rods. I have already contacted him and will be getting the 300M 14518 connecting rod set when I am ready.
 

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Discussion Starter · #8 ·
Took a quick look at the site before my first client this morning, Tony and saw this. I'll share some comments later. It looks, to me like it broke on the down stroke and was a notch/fracture induced tear of the metal in the rod. My guess is it happened at a medium to high rpm with the car accelerating hard - more on that later.

Bad luck, feel for ya. Sorry to see that kind of failure.

Ed
Ed, you're correct. Accelerating very hard and I would guess the rod snapped about 6800rpm or so.

I wonder if the cry treatment made the rod flex less which is why it snapped
Your guess is as good as mine!
 

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The H-Beams have some finite limitations in the power production department but 800 or 900 ft/lbs of torque is not one of them. More importantly that rod did not fail on a power stroke it failed on an intake stroke. The beam failure was not a compressive load failure like might be expected from an ignition event. This is how a rod might appear that was subjected to an excessively high compressive load;

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The rod above was subjected to a hydrolock shock when coolant got by a weeping head gasket and accumulated in the open space above the piston. When the engine was started the first cylinder that fired to get the engine running forced this piston up against the coolant filled chamber on the compression stroke and bent the rod in two different directions, once in the direction of rotation until it would not give any more and then once counter to the direction of rotation. Impressively the rod did not break.

Your failure, Tony, was not a compressive failure, it was a tensile failure. The difference is that the rod was in tension with the piston. My suspicion is the events played out something like this;

Somewhere along the beam of the rod there was the beginings of a notch failure, (more on this later). As the piston was coming up on an exhaust stroke the connecting rod had to slow the piston to change its direction from its upward movement preventing it from going through the cylinder head and then accelerate it down the bore drawing in a fresh change of fuel and air for the next ignition event.

The actual rod failure began as the piston approached TDC on the exhaust stroke. The change in direction and acceleration down the bore for the intake cycle completed the beam fracture. At this point the piston and its rod stub were disconnected from the crank - but still moving down the bore from the energy imparted to both of them by the crank's earlier tug on them prior to the rod fracture.

The piston and its attached rod stub moved down the bore until they contacted the rotating crank and the lower portion of the broken rod still attached to the crank. That contact produced the two munch marks on both sides of the piston skirt that I have highlighted on one side of the skirt with arrows in the pic below;

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The continued rotation of the crank pushing upward against the lower portion of the piston skirt and rod stub drove the piston back up the bore, now rotated slightly clockwise from its impact with the crank, until the open intake valves in that cylinder head arrested its upward motion. In the pic below you can see the rotated orientation of the piston and the intake valve impact just above the one valve relief (highlighted by the red arrow).

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Although the pic does not clearly does not show the impact of the second intake valve, the pic of the chamber does show a contact mark on both intake valves, again highlighted by arrows in the pic below.

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Without its connecting rod to attach it to the crank the piston remained at the top of the bore. In the mean time the remainder of the connecting rod is swinging around on the crank like the nylon string on a weed-wacker, except this time the nylon string is made out of 4340 steel and it is inside your crankcase destroying everything in its path.

So how to stop this level of carnage?

The failure was caused by what a metallurgist would identify as a notch fracture. For the insatiably curious I have attached a pdf on the use of notch fracture in the determination of fracture threshold. Essentially what is happening is equivalent to taking a sheet of paper and cutting a "V" notch in its long side. Once you have the notch then you uniformly pull on the short sides until the paper fractures (tears) at the notch. That is the notch fracture threshold albeit simplified but the essentials are there. The pdf will provide more detail for the curious.

When we operate our engines we subject our connecting rods to several different types of loading. Two that are most often thought of are the compressive and the tensile loading of the rod. Compressive is simply the transmission of th combustion power to the crank for horsepower. This is the loading that most frequently comes to mind when we think about connecting rods. The second loading (not the only other loading) is the tensile loading the rod experiences each time it had to stop the piston from going through the cylinder head, reverse direction and go down the bore. This is tensile loading. The tensile loads a rod experiences are the greatest loads it has to deal with and the greatest tensile load is the tensile load at TDC overlap.

Michael Rauscher at L&M engines has provided a handy piston speed calculator on his website, click here => Piston Speed Calculator. This is a handy tool to look at from time to time as you are going through the build cycle for and engine. Piston speed tends to be a geometry / physics problem (remember those was the classes we didn't like) from high school. Michael's calculator does all the heavy lifting for you.

If we use a standard bore, standard stroke, standard rod length engine for our test calculations we can easily get dats that will help shed light on the connecting rod failure problem. Lets use engine speeds of 1000, 2000, 4000, and 8000 rpm and look for the highest piston speed the engine experiences at each rpm. Michael's calculator will return the following values for us;

RPM............. Piston Velocity (fpm)/Crank Angle
1000........................... 968.26 - 75˚ ATDC
2000........................... 1936.5 - 75˚ ATDC
4000........................... 3873.0 - 75˚ ATDC
8000........................... 7746.1 - 75˚ ATDC

Notice the max piston speed always occurs at the same crank position 75˚ ATDC. This is the geometry part of the problem. The max piston speed and where it occurs is fixed by the crankshaft stroke and the connecting rod length. Change either and you will change the maximum piston speed and the point in the crankshaft rotation that max speed is reached.

The tensile loading on the rod is essentially the force required to bring the kinetic energy in the moving piston rod assembly from it's high point to zero. We know where each of these events occurs. The high velocity point is 75˚*ATDC (and BDC) and the zero velocity point is TDC (and BDC). The kinetic energy is (MV[SUP]2[/SUP])/2. the dangerous part here is the velocity squared component. It causes the stress on the rod to go up by the square of the piston velocity which means that the rod stress at 2000 rpm is 4 times the stress at 1000 rpm and the stress at 4000 rpm is 16 times the stress at 1000 rpm. You guessed it, the stress at 8000 rpm is 64 times the stress at 1000 rpm.

Remember our piece of paper with the "V" notch in one long side. When we start raising the tensile loading on our connecting rod by these sorts of increments we rapidly get to the point where small surface imperfections become the test notch that initiates the fracture condition which severs the rod beam. Remember when you dropped the socket that bounced off the connecting rod beam? That introduced a notch fracture site in that connecting rod at the point of impact. Depending on the size and depth of that notch you can predict the failure rpm for the damaged rod.

Decades ago before we had the H-Beam rods that Fred Carrillo invented and everybody copied after his patents expired we would use 1/8 inch steel plates that we would cut to mimic the shape of the rod beam and then weld them to both sides of the beam. When we were done fabricating we would sand away the welding splatter and then polish the rod beams until they looked like the bumper on a car. The reason for the polishing was to remove any surface imperfections that could initiate a crack. The rods still broke partly because of the diminutive dimensions and the power levels they had to handle but also because on the inside of the weldment there were - you guessed it rough surfaces, cracks and bubbles that were the starting points for the rods ultimate demise.

Today with the vacuum degassed 4340 steel CNC'd products from firms like Manley we just have to be sure the surfaces have no imperfections that would allow a notch fracture to propagate through the beam. The higher we spin our engines the more important the condition of the rod beam is no matter if it is an H-Beam design or an I-Beam design. The location of the notch will be the origin of the notch fracture that will kill the rod -*and our engine.

This post is already too long but there re two other areas (among many others) that are important considerations for high rpm 7500/8000 rpm or higher engines. The first is the rod bolts. The only thing keeping the entire rod and piston from going through the cylinder head at TDC is two tiny 3.8" rod bolts. Through 7000 rpm those are pretty much OK if they are 8740 bolts. From 7500 RPM and up the bolts ought to be ARP 2000 steel. ARP spec's their 8740 steel at a 200,000 psi tensile strength. They spec their 2000 steel at a 220,000 tensile strength. Doesn't seem like a lot more but ask anyone who has used bothe and they will tell you the difference is impressive.

There is a third rod bolt steel that ARP offers for our rods. ARP calls it Custom Age 625+ and it is spec'd at 260 to 280,000 psi tensile strength. Remember how much more robust the ARP 2000 fasteners were than the 8740 fasteners. 625+ is like 2000 steel on a double overdose of steroids. The bolts are available through distribution (Jegs & Summit) and predictably come at a premium. A complete set of 16 for eight rods costs (as of this writing) right around $572. If you need these fasteners you already knew about them because your engine builder told you about them. This is a situation where if a little is good more than enough is definitely not good and it is expensive to buy.

The Manley I-Beam rods are absolute jewels but are every bit as susceptible to notch type beam failures as any other rod design. There are good reasons to use the I-Beam Manleys but notch failure avoidance is not one of them. Theist thing to do for an engine that sees 7500 rpm and higher operation is take special efforts to protect the rod beams from any damage no matter how incidental and it doesn't hurt to polish the surface so any potential damage is more readily visible and the surface is more resistant to notch type failures.

There is some more I want to share about your upper rod bearing shell but I will do that in another posting, this one is already too long.

Ed

p.s. The PDF doc on notch fracture was too big to attach to the post so here is the link to it if you are interested in more detail, click here => http://www.hindawi.com/journals/isrn/2012/689386/
 

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This post is already too long but there re two other areas (among many others) that are important considerations for high rpm 7500/8000 rpm or higher engines. The first is the rod bolts. The only thing keeping the entire rod and piston from going through the cylinder head at TDC is two tiny 3.8" rod bolts. Through 7000 rpm those are pretty much OK if they are 8740 bolts. From 7500 RPM and up the bolts ought to be ARP 2000 steel. ARP spec's their 8740 steel at a 200,000 psi tensile strength. They spec their 2000 steel at a 220,000 tensile strength. Doesn't seem like a lot more but ask anyone who has used bothe and they will tell you the difference is impressive.

Ed
I agree with Ed in that the 3/8 Terminator rod bolts are quite small. Thus, one of many advantages of going with the Manley I beams is a larger 7/16 ARP 2000 bolt and this is the same size bolt that Manley specs out for the power stroke diesel motor, so that should clue you in that the 7/16 bolt will handle a lot more abuse than the 3/8 bolt. Furthermore the 300M steel is also designed to take quite a bit of abuse as well since it is a material that is used on aircraft landing gear. 300M ALLOY STEEL (4340M)

To quote one material supplier:

"300M is a vacuum melted low alloy steel with the inclusion of vanadium and a higher silicon composition. It has a very good fatigue strength and resilience. Where fracture toughness and impact strength are crucial, 300M is a great choice.

Applications:
Aircraft Landing Gear
Airframe Parts
Missile Components
Motorsport Applications "

http://www.twmetals.com/300m-bar-rod-wire.html

Here is a 5.0 mustang article that shows how tough the 300m rod really is as it's an indestructible part.


"This is the first thing we think of when putting high compression and 52 pounds of boost in the same sentence. But what's really important here is the impressive connecting rod--a Manley tool steel, A-beam, Pro Series 300M Lightweight measuring 5.933 inches and weighing a muscular 650 grams--and the bearing, which is still in ready-to-run condition. Fred attributes the lack of bearing damage to the 6.0-liter oiling mod. John had Manley develop this tough-as-nails rod for him; its main claim is its extremely hard material. Fred says it's so tough that it tears up the production tooling, but it's proven bulletproof, even after abuse such as this. In fact, although the piston pin is frozen, the rod and bearing are otherwise usable."

http://www.mustangandfords.com/how-to/engine/m5lp-0906-mercury-cougar-cobra-hybrid/

In other words, use a 300M rod and be done with it.
Jan
 

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There are several types of high strength steel that all share similar nomenclature and some dissimilar nomenclature. For example there are minimally the following types of 4340 steel,

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In addition to these the following naming conventions are also commonly used;

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If you find a steel named 4340M or 4340-7, or ASTM A646 or 300M-8 they are all 4340 steels with similar but sometimes slightly different recipe and material properties. All these steels can be called 300M. With a little inspection it becomes obvious that 4340 and 300M are near but not identical twins and depending on the chemistry, heat treat and purpose the were manufactured for they can each be more or less robust is various physical property attributes.

Metallurgy in this instance is best left to the metallurgists. That said for our typical use, 300M is normally a tougher material than 4340 all things being equal - which they usually are not.

The connecting rod bolt story is a very interesting challenge between weight and strength. Frequently for components in low speed high stress applications like diesel engines, bigger tends to be better which is why we see such monstrous components on over the road diesel trucks or for that matter the consumer diesels that appear in large pickups and smaller personal transportation vehicles.

When we get to high rpm race engines there is a tug of war between strength and size because of the impact weight has on the longevity of rotating components. F-1 is the pinnacle of these sorts of balancing acts. For mere mortals like ourselves the rule of thumb is smaller and stronger for high speed versions of our engines. When the beam strength of something like a connecting rod falls short of the mark for our application we need to upgrade the component to sustain the loading we are applying to it.

More frequently than not the H-Beam design is more than adequate for the job at hand. If we are producing 1500 HP at 7500 rpm that means we have 1050 ft/lbs of torque at 7500 rpm. Once you go over about a thousand ft/lbs of torque, rod beam strength becomes a growing problem for the engine. Contrast that to the engine that makes 1500 HP but at 9000 rpm. The torque required at 9000 rpm for that power level is only 875 ft/lbs of torque - definitely a lot but manageable.

Consider for a moment an engine that makes 2500 HP at 10,000 rpm. It needs 1300 ft/lbs of torque (at 10,000 rpm) to produce that kind of power. Rod beam strength becomes a very big issue with this type of engine as does big end concentricity because of the higher bending force of the 1300 ft/lbs of torque and also the tug on the rod at TDC overlap by the crank decelerating the piston at TDC overlap.

These two different forces acting upon the connecting rod each ask for a different and in some ways competing solution. The easy fix for the bending force and also for the tensile loading at TDC overlap is more beam section area i.e. steel mass. The problem with increasing the beam section of the rod is that it is counterproductive to producing reliable high rpm performance and in practice places a ceiling on the ultimate engine speed that is limited by the tensile strength of the connecting rod bolts.

Connecting rod bolts suffer a similar performance dyslexia. Increased strength is easily achieved by increased fastener cross sectional area - a bigger bolt. The problems start to arise with increased engine speed. The larger fastener creates a bulkier big end on the connecting rod which both contributes to the low speed structural integrity of the big end of the connecting rod while simultaneously limiting the maximum engine speed the connecting rod can operate at without loosing big end concentricity at high engine speed. It seems like you can't win for loosing.

The F-1 guys have been battling this sort of problem for decades ever since pneumatic valve springs became available and the engines easily blew by the old 12,000 rpm engine speed ceilings that used to dog them. Once into the higher engine speeds the connecting rod and rotating assembly components took on special significance because of the inertial loading (and premature failure) poorly designed or chosen components would have.

In general it is difficult to get any significant detail about F-1 engines until years after they are no longer used. That is particularly true when you begin sniffing around anything that would enhance engine performance like higher operating engine speeds. The pic below is of a piston / rod assembly from a 1999 Ferrari F-399 F-1 engine.

Automotive design Material property Font Auto part Silver


The immediately apparent attempts to remove anything that would add weight is stunningly apparent. Equally interesting was their use of a hybrid rod design that was part I-Beam and part H-Beam. The effort was an obvious attempt to gather the best performance attributes of each design and integrate them into a single hybrid design. Notice the use of a sub millimeter top ring (not good for blown engines), no second ring (OK for racing but not for street), a vanishingly small, most likely low tension oil ring (also not good for blown engines) and amazing lightening of the rotating end of the connecting rod.

This is a purpose built piston rod assembly for a race engine that operated at 18,000 rpm, certainly way above where we are but, all the same principles apply and same problems present themselves - the only difference is the scale.

In general you want to use the smallest, strongest componentry you can to produce the power you are looking for. You also have to strike a balance between how you build the engine to produce that power and importantly the componentry you select to provide not just the power but also the longevity to both race and survive to win. That smallest / strongest design model is the whole reason ARP provides not just 8740, but 2000, L-19, Inconel 718, Custom Age 625+, ARP3.5[SUP]®[/SUP] and AerMet[SUP]®[/SUP] steel connecting rod bolts. Some of those fasteners exceed 300,000 psi tensile strength and a bunch of them come very close.

Of course this type of technology comes at a price. There is certainly another price that the owner, sometimes engine builder (when they are the same) will pay for choosing poorly. As the Knight Templar warned Indy in the movie Indiana Jones and The Last Crusade, 'Choose wisely...' (I think I see Tony coming so I gotta end this quick :))

As the builder user you need to decide how you intend to make your power. If you choose high torque and lower engine speeds that says something about your prats selection. If you choose lower torque and high engine speeds that is yet a different components BOM. If the engines built one way and are operated the other way it is a virtual certainty that an unhappy event will visit and it will be sooner rather than later.

Ed
 

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If you target 1400 hp @ 8500 rpm, Scott you will need 865 ft/lbs of torque at 8500 to do the job. That is still within the H-Beam space and would require ARP 2000 bolts. You are getting close to what I consider the tipping point which is 1000 ft/lbs of torque. 1000 ft/lbs of torque at 8500 would be right at 1600 HP. An engine like that, if it were mine would have the I-Beam rods.


Ed
 
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