Connecting rod design?

Machineman

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What are people thoughts on con rod styles i have been working on modeling rod for my 3.0 tdi build and have been informed that h beams are inferior apposed to a i beam design i have modeled both styles and weight seems to be similar although depending on cross section

 

Fix_Until_Broke

Top Post Dawg
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I believe the H beams are much stronger than stock and easiest to manufacture, however the I beam is ideal from a strength to weight ratio and most efficient use of the material due to the moment of inertia.

TDIMiester had a great explanation on this a few years ago - if I find it, I'll post a link
 

mrchill

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I usually use H beams. I find them lighter than I beams....and more than strong enough.
 

[486]

Top Post Dawg
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02 golf ALH
H-beam's easier to hack outta a generic rod blank
just gotta use a horizontal mill cutter to get the sides hollowed out, instead of a small cutter in a vertical to profile the windows in the big end

I beam is preferred for reasons that elude me, but it takes either a lot of time with smaller cutters, or dedicated forging dies to make the blank for only the one rod length
 

Machineman

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Well this topic has me stumped I'm starting to wonder if it's worth it I have the 4340 material to make the rods already but I am wondering if I will never achieve the breaking point of stock ones I have ran 75 lb of boost on bhw rods without any failure and the 3 liter TDI rods are even more robust they have 30mm pins and a larger cross section in the beam they already been confirmed to 500 awhp with bobbys car I just don't want to be the one that pushes them to 650 Hp and they break
 

andy2

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I'd say it depends on how much torque that you put through the engine.If you plan on running a big or large single then the factory rods might be ok.

If running compounds then mabye consider a rod upgrade.

My old ALH based engine years ago broke 3 out of 4 H-beam rods.
I now use aftermarket I-beam rods with a 3/8" rod bolt.

The funny part was that I had the BEW rods in my old ALH and decided to upgrade to the H-beam rods as I was worried about the BEW rods failing.

It might be possible that the PD rod would take all the power/rpm I could make with only mabye upgrading the rod bolts,who knows ?
 

adamss24

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If you intend to drag the engine then by all means- upgrade the rods, if you are thinking to drive the car sensibly and let it warm properly then stock rods will last... I have been in bobby's car and it sounded fine on a full throttle and he was driving it properly! Them 3.0 tdis are strong engines!
 

Machineman

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I'd say it depends on how much torque that you put through the engine.If you plan on running a big or large single then the factory rods might be ok.

If running compounds then mabye consider a rod upgrade.

My old ALH based engine years ago broke 3 out of 4 H-beam rods.
I now use aftermarket I-beam rods with a 3/8" rod bolt.

The funny part was that I had the BEW rods in my old ALH and decided to upgrade to the H-beam rods as I was worried about the BEW rods failing.

It might be possible that the PD rod would take all the power/rpm I could make with only mabye upgrading the rod bolts,who knows ?
You broke a set of rostens right ? So you're running I beams ?? Are those the cuttingham ones
 

andy2

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Yeah,Cunningham I-beam.They are a heavy rod in comparison to the H beam but that doesn't seem to matter at all.
 

[486]

Top Post Dawg
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02 golf ALH
Still on the smaller journals to fit the larger bolts, I assume?

ETA: on stock rod failures, every ALH with supposedly "weak" rods I take apart already has #2 and 3 bent from getting slugged with oil
No rod has any strength if it's already bent.

ETA2: take this post as a vote for stock rods, possibly with better bolts
 
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Machineman

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Yeah,Cunningham I-beam.They are a heavy rod in comparison to the H beam but that doesn't seem to matter at all.
Were your rods from Cunningham forged machined or billet machined ? I know they do either or
 

Pat Dolan

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The whole purpose of a rod is to remain in column to carry compressive load, and from the lateral reciprocating forces, and I beam has cap strips to give greater bending strength for same given mass.
 

Machineman

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If you are interested in them I might sell them cuz I might upgrade to M9 they have never even been torqued
 

diffas

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Actually I might. I ordered 2.7biturbo bolts but they are bit beefier. I think they are m9 even etka says m8.5. Wanna change? Haha. To me they are useless anyways.
If you are interested in them I might sell them cuz I might upgrade to M9 they have never even been torqued
 

TDIMeister

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I have never, ever seen separated rod caps and pistons through the head due due to failed big-end bolts in a TDI. More like a subtle but noticeable squiggly bend along the length of the main rod column to something more dramatic like Andy's pic above but in any case, the bolts have not been the weak spot.

Edit: @Machineman, since you've already modeled both I- and H-beam rods in SolidWorks, you can easily query the software to report the moments of inertia Ix and iy of both designs. It would be very illuminating if you would post the results. And if your SolidWorks has the FEA module, you could do a quick and dirty stress analysis at different rod inclinations. I can give you some representative cylinder pressure vs. crank angle data for that analysis if you like.
 
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diffas

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Oe bolts most likely holds just fine but rather safe than sorry. High reving engines with heavy pistons are most likely cause of rod caps to seperate.
 
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