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Connecting Rod Decision


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Given the choice to pick between to equal length connecting rods, both of sufficient strength for your application, which of these do you choose:

1) Pressed pin I beam 616 grams

2) Floating pin I beam 657 grams

Assume pistons weigh the same.

Application is a road course endurance racing engine which will live under 6500 rpm. Piston speed under 5750 fpm. Stroke ~3.27" if it matters.

If you care to share... What would you do and why?

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Use the stock rod, because it is in the rules? How much did the original piston and rod combo weigh? I find that when a customer shows up with all the fancy rotating/reciprocating bits, the parts are usually heavier than the originals. What has been the failure mode you are experiencing, that is indicating a change in con rod design in the first place?  And if the only real difference is the weight, why would you even consider the heavier rod? Just adding a bush to the end can not contribute "41g" to the weight.  So is the real question "Is it worth 41g to be able to tell everyone I have full floating pins?" Absolutely, and it opens up the potential to cite cir clip reliability in your next engine failure!

 

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Material or process change for either? 

 

Assembled correctly either would work, but you are buying a different rod for better material (forged vs powdered metal), rebuildability (cracked rod vs dowel pin) and cross section\shape where the beam meets the ends. 

 

And cost....if you aren't "1 x" the cost of new oem rods from the dealer\network, you are thinking too hard.

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2 hours ago, Hanz and Franz said:

Use the stock rod, because it is in the rules? How much did the original piston and rod combo weigh? I find that when a customer shows up with all the fancy rotating/reciprocating bits, the parts are usually heavier than the originals. What has been the failure mode you are experiencing, that is indicating a change in con rod design in the first place?  And if the only real difference is the weight, why would you even consider the heavier rod? Just adding a bush to the end can not contribute "41g" to the weight.  So is the real question "Is it worth 41g to be able to tell everyone I have full floating pins?" Absolutely, and it opens up the potential to cite cir clip reliability in your next engine failure!

 

Both of these are OEM rods, just different year models. The question in my mind is it worth 41 grams to have full floating pins. I don't care what anyone thinks once the engine is built, I want to make the choice that gives the engine more longevity at high rpm use.  I have not experienced a failure mode, just building new engine to use an aluminum block and trying to pick the best of the OEM options to go in the new build. I'll count this as a vote for pressed + lighter, thanks.

 

1 hour ago, Black Magic said:

Material or process change for either? 

 

Assembled correctly either would work, but you are buying a different rod for better material (forged vs powdered metal), rebuildability (cracked rod vs dowel pin) and cross section\shape where the beam meets the ends. 

 

And cost....if you aren't "1 x" the cost of new oem rods from the dealer\network, you are thinking too hard.

Both powdered metal cracked rods, and both OEM from different years. Thanks

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  • Technical Advisory Committee

Are there other changes other than just the con rods between the different years?  If so, then every component needs to be used from that year which makes that engine as a whole for that year stock; no mixing and matching parts.  You can consider it a 'platform swap' if that year engine differs from your car's year, as long as all the components are stock for that year.  

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Engine configuration (inline 4, V8)? Pin weight? Piston material/type (hypereutectic, forged)?

 

When you're getting down to the fine points, there are a few details to consider. I've used quite a few different combos over the years and more depends on the assembler than the assembly.

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5 minutes ago, NigelStu said:

Are there other changes other than just the con rods between the different years?  If so, then every component needs to be used from that year which makes that engine as a whole for that year stock; no mixing and matching parts.  You can consider it a 'platform swap' if that year engine differs from your car's year, as long as all the components are stock for that year.  

 

Sorry I should have clarified, the car is 100% EC (literally over 15,000 points, I am not kidding) so the valuation rules are not part of the decision, only finding the best technical option.

 

Thanks for reply

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2 minutes ago, CBraden said:

 

Sorry I should have clarified, the car is 100% EC (literally over 15,000 points, I am not kidding) so the valuation rules are not part of the decision, only finding the best technical option.

 

Thanks for reply

Starting to sound interesting ...

 

If you like you can PM me and we can hash out the specs.

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2 minutes ago, mender said:

Engine configuration (inline 4, V8)? Pin weight? Piston material/type (hypereutectic, forged)?

 

When you're getting down to the fine points, there are a few details to consider. I've used quite a few different combos over the years and more depends on the assembler than the assembly.

V8, hypereu piston - pin+piston weight is same for OEM pistons (pressed and floating), but I will likely buy replacements and getting pin weights for them before buying isn't easy on all options under consideration (which seems wrong). Crank will get a spin balance after parts are selected (and I will check part to part variance and grind as necessary before that)

 

Thanks.

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I really hate most powdered metal rods. Although the ultimate strength can be reasonable, powdered metal rods do not have the fatigue strength or notch sensitivity of forged parts. This just drives down the lifespan of the parts and requires earlier replacement when running at high revs for long.

 

I would take nearly any forged rod instead, with dowel pin location and reasonable bolts. Cracked rods can slip or slightly mismatch when torqued, which will pinch the bearing. They aren't really meant to be rebuilt, and once you are investing to rebuild a motor you plan to service at any regular interval i would let them go.

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19 minutes ago, Black Magic said:

I really hate most powdered metal rods. Although the ultimate strength can be reasonable, powdered metal rods do not have the fatigue strength or notch sensitivity of forged parts. This just drives down the lifespan of the parts and requires earlier replacement when running at high revs for long.

 

I would take nearly any forged rod instead, with dowel pin location and reasonable bolts. Cracked rods can slip or slightly mismatch when torqued, which will pinch the bearing. They aren't really meant to be rebuilt, and once you are investing to rebuild a motor you plan to service at any regular interval i would let them go.

When powdered rods fail they come apart. Spontaneous disassembly. Hypereutectics break/crack/shatter. Very good stuff as long as you keep it below the failure point but if you go beyond ...:o

 

I keep examples of exploded engine parts to show people what to expect from the various rod and piston types. Forged rods and pistons take a tremendous beating before finally failing.

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21 minutes ago, mender said:

When powdered rods fail they come apart. Spontaneous disassembly. Hypereutectics break/crack/shatter. Very good stuff as long as you keep it below the failure point but if you go beyond ...:o

 

I keep examples of exploded engine parts to show people what to expect from the various rod and piston types. Forged rods and pistons take a tremendous beating before finally failing.

 

Great write up. Only one thing i wanted to add....

 

Cracked rods offer no method of repair for bore out of round\spun bearing. You can skim the end of a forged rod or rod with caps and then re machine the rod round to size it. 

 

So if you spin a rod bearing or just have old stuff on cracked rods you need to buy new rods one way or another.

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I understand the limitations of powdered metal and hyper-eutectic pistons, and feel good about the safety margins designed into these OE parts in this application/power/rpm level.  

 

I think I am hearing a consistent theme of less weight is the more important factor, and that pressed pins are absolutely fine.

 

Thanks

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11 hours ago, CBraden said:

 

Sorry I should have clarified, the car is 100% EC (literally over 15,000 points, I am not kidding) so the valuation rules are not part of the decision, only finding the best technical option.

 

Thanks for reply

 

15000 pts! Wow, is it a rocket ship?

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11 hours ago, mender said:

Starting to sound interesting ...

 

If you like you can PM me and we can hash out the specs.

 

I know there are a multitude of things to consider, but generally speaking would you lean towards the rod assembly that is net lighter at the piston end of the assembly? Would total pin end of the rod weight be more critical than net rod weight? 

 

Interesting thread & discussion... I really know nothing, just curious  :)  usually what I think is 'right' ends up being 180° off lol. 

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1 hour ago, pintodave said:

 

I know there are a multitude of things to consider, but generally speaking would you lean towards the rod assembly that is net lighter at the piston end of the assembly? Would total pin end of the rod weight be more critical than net rod weight? 

 

Interesting thread & discussion... I really know nothing, just curious  :)  usually what I think is 'right' ends up being 180° off lol. 

Reciprocating weight (piston, rings, pin and small end of the rod) is what pulls a rod apart and also eats up some power, so lighter is better - generally speaking. :)

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10 hours ago, turbogrill said:

 

15000 pts! Wow, is it a rocket ship?

 

No, it is really just a front running car in this series. The formula in the swap calculator for an Opel GT with over 200 hp is ludicrous, try it for a laugh. I tried to argue that real weight (in this case >2500 lbs) is all that matters, but my arguments did not prevail...  the car was designed under the old rules, if I built starting today I would pick a different car that would be faster and have more fuel, and be a 500 point car. 

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30 minutes ago, CBraden said:

 

No, it is really just a front running car in this series. The formula in the swap calculator for an Opel GT with over 200 hp is ludicrous, try it for a laugh. I tried to argue that real weight (in this case >2500 lbs) is all that matters, but my arguments did not prevail...  the car was designed under the old rules, if I built starting today I would pick a different car that would be faster and have more fuel, and be a 500 point car. 

 

Isn't the swap based on real weight? Or what? Confused.

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3 minutes ago, turbogrill said:

 

Isn't the swap based on real weight? Or what? Confused.

 

No because weighing racecars is a mystical dark art that was deemed not feasible for this race series. The calculator assumes that your car weighs less than stock by some % (even though clearly this is a bad assumption for some swapped cars, more so if they choose to swap to a beefier rear end and run wheels and tires 3x the 155x13 OE wheel+tire weight).

 

Since there is already a million page thread on this, let's please not open this quagmire again on this thread - PM me if you care to get into details, it is what it is.

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49 minutes ago, CBraden said:

 

No because weighing racecars is a mystical dark art that was deemed not feasible for this race series. The calculator assumes that your car weighs less than stock by some % (even though clearly this is a bad assumption for some swapped cars, more so if they choose to swap to a beefier rear end and run wheels and tires 3x the 155x13 OE wheel+tire weight).

 

Since there is already a million page thread on this, let's please not open this quagmire again on this thread - PM me if you care to get into details, it is what it is.

 

aha, didn't know that. 

 

If you press you might not be able to reuse that piston, not sure if that matters.

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1 hour ago, CBraden said:

 

No because weighing racecars is a mystical dark art that was deemed not feasible for this race series. The calculator assumes that your car weighs less than stock by some % (even though clearly this is a bad assumption for some swapped cars, more so if they choose to swap to a beefier rear end and run wheels and tires 3x the 155x13 OE wheel+tire weight).

 

Since there is already a million page thread on this, let's please not open this quagmire again on this thread - PM me if you care to get into details, it is what it is.

Any discussion other than the narrow focus of the stated topic runs the risk of getting the thread closed.

 

I couldn't tell you what the exact criteria are for deciding to close a thread or who does the deciding but best not to stray too far.

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1 hour ago, turbogrill said:

 

aha, didn't know that. 

 

If you press you might not be able to reuse that piston, not sure if that matters.

Thanks, will not be re-using pistons (block needs cylinders bored/honed).

 

50 minutes ago, mender said:

Any discussion other than the narrow focus of the stated topic runs the risk of getting the thread closed.

 

I couldn't tell you what the exact criteria are for deciding to close a thread or who does the deciding but best not to stray too far.

Got it! I will not respond to any more rules type questions here for that reason.

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