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Discussion Starter #1
if you had the choice to save weight on the piston or connecting rod which would be more important my logic tells me in the piston the reason I ask is that I haven't ordered pistons yet and I might go with a longer rod to save on piston weight

Its for my 396 that im building I bought some scat 6.135 rods and can still go to 6.385 if it would benefit me at all
im shooting for 1.5hper cube and I think im on the right track so far but to get the 14:1 compression I want the domes are going to be very large and im trying to save some weight anywhere I can

thanks
Tyler
 

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if you had the choice to save weight on the piston or connecting rod which would be more important my logic tells me in the piston the reason I ask is that I haven't ordered pistons yet and I might go with a longer rod to save on piston weight

Its for my 396 that im building I bought some scat 6.135 rods and can still go to 6.385 if it would benefit me at all
im shooting for 1.5hper cube and I think im on the right track so far but to get the 14:1 compression I want the domes are going to be very large and im trying to save some weight anywhere I can

thanks
Tyler
Go with the longer rod. You'll have to order custom piston because I don't know of anybody making pistons for that rod.
Better yet, go with a Eagle 6.635 (+.500) rod and a 1.27 compression height.
Both the 6.385 and 6.635 rod are a walk in the park for the 427. Pistons are sitting on the shelf for both, they're just labeled for use with a 4.25 crank.



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Discussion Starter #3
is the oil ring going to be on the pin with the +.5 this isn't a drag only deal.... guess the piston would be the same compression height as a 4" stroke with 6.385 but I have never looked at how much room they had on there ring pack
 

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is the oil ring going to be on the pin with the +.5 this isn't a drag only deal.... guess the piston would be the same compression height as a 4" stroke with 6.385 but I have never looked at how much room they had on there ring pack
Its the same compression height as a 4.25 crank with a 6.385 rod. Yes, the pin is in the oil ring. If that bothers you, go with the 6.385 rod.



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Didn't want to muddy up the thread earlier, but seeing as it seems resolved...

I've been curious, why aluminum rods aren't more widely used, especially in a marine application. And why aluminum rods have a fatigue life stigma, yet aluminum pistons do not?

How much of the stigma is just urban lore? I admit I have reservations based off no real reason, even though I know of at least one engine (4.25 stroke, 7200 rev limited) with Bill Miller rods that's been trouble free for over 10 years (10,000+ miles and a lot of time on the limiter).

What's the real deal?
 

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Discussion Starter #6
Didn't want to muddy up the thread earlier, but seeing as it seems resolved...

I've been curious, why aluminum rods aren't more widely used, especially in a marine application. And why aluminum rods have a fatigue life stigma, yet aluminum pistons do not?

How much of the stigma is just urban lore? I admit I have reservations based off no real reason, even though I know of at least one engine (4.25 stroke, 7200 rev limited) with Bill Miller rods that's been trouble free for over 10 years (10,000+ miles and a lot of time on the limiter).

What's the real deal?
think everyone has steered away from them for at least a few reasons, wives tales and material cost, the piston prolly doesn't fail due to it having twice the bearing surface as the rod holding it (two contact points instead of one)
 

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Didn't want to muddy up the thread earlier, but seeing as it seems resolved...

I've been curious, why aluminum rods aren't more widely used, especially in a marine application. And why aluminum rods have a fatigue life stigma, yet aluminum pistons do not?

How much of the stigma is just urban lore? I admit I have reservations based off no real reason, even though I know of at least one engine (4.25 stroke, 7200 rev limited) with Bill Miller rods that's been trouble free for over 10 years (10,000+ miles and a lot of time on the limiter).

What's the real deal?
Bill Miller are the best aluminum. Rod on the market. I like them allot.
 

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Didn't want to muddy up the thread earlier, but seeing as it seems resolved...

I've been curious, why aluminum rods aren't more widely used, especially in a marine application. And why aluminum rods have a fatigue life stigma, yet aluminum pistons do not?

How much of the stigma is just urban lore? I admit I have reservations based off no real reason, even though I know of at least one engine (4.25 stroke, 7200 rev limited) with Bill Miller rods that's been trouble free for over 10 years (10,000+ miles and a lot of time on the limiter).

What's the real deal?
Aside from having 2 load bearing points, the piston only has to deal with its own weight on the intake stroke, and it doesn't include the pin. The rod as to deal with it all from the fork up. Most aluminum rounds don't fail at the small end, they fail mid beam, or where the rod transitions into the fork, or as Rocky Childs called it, "the hinge"



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steelcomp was here
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The further away form the center of the crank, the more effect the weight will have.
 

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Scott,

So you are thinking a alum rod would live in a river/enduro application? With advances in metals and bearing these days I dont see why it wouldnt if you had the budget. On the flip side I would think if that was the case then NASCAR or some of the car enduro motors would have already gone that route.
 

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Scott,

So you are thinking a alum rod would live in a river/enduro application? With advances in metals and bearing these days I dont see why it wouldnt if you had the budget. On the flip side I would think if that was the case then NASCAR or some of the car enduro motors would have already gone that route.
The only application I see for aluminum rods these days is where the bottom end needs a little cushion from extreme cyl pressures. Other than that I don't see the need or benefit from aluminum rods. The biggest problem with them is they're a ticking time bomb. it's not a matter of if, only when...unless you're using them in a completely unrelated application like a low hp/low rpm street engine where you're not using them even remotely close to their intended use but that's not a fair comparison. Not sure of anyone mentioned windage issues, either. Big, fat, bulky alum rods don't help in that department. ONe last thing to consider; weight in a recip. assembly does not contribute or detract from overall power, it only helps in acceleration of the masses. If the application is endurance or any other app where the rpm is more continuous then IMO the weight savings is really of no benefit over the reliability concerns. Steel rods with the lightest possible piston will usually be a better combination.
 

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Re: Alumi rods... I've heard of a couple people running them at Bonneville with success.

I couldn't find the video of the truck that ran them, but I know of a mini-truck that ran them in an Esslinger 2.9ltr Pinto engine in this series
Ricky Johnson wins the TRAXXAS Off Road Championships 2010 - YouTube

You really can't get much more abusive than that type of racing. If I recall correctly they were GPR's, but I'll check. The engine builder called them " poor man's Ti rods" he claims the engine rev'ed quicker.
 

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Scott,

So you are thinking a alum rod would live in a river/enduro application? With advances in metals and bearing these days I dont see why it wouldnt if you had the budget. On the flipside I would think if that was the case then NASCAR or some of the car enduro motors would have already gone that route.
Adding to what Scott posted, all the lower cost aluminum rods like Manley, Super Rod, Howards are history. GRP, MGP, Miller, and Alan Johnson/C&A are not cheap. If you have to replace them once, you could have to replace them just once, you could have bought a set of best steel rods with the very best bolts.

Comparing a street driven car with even a recreation lake boat is a bad comparison. Stock GM dimple rods do quite well in 750 HP street engines. See too many 750HP plus boats running stock rods??

Nascar isn't allowed to run aluminum rods, and even if they did, they would replace them every race. They can run them in Indy and F1, but they don't.



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Re: Alumi rods... I've heard of a couple people running them at Bonneville with success.

I couldn't find the video of the truck that ran them, but I know of a mini-truck that ran them in an Esslinger 2.9ltr Pinto engine in this series
Ricky Johnson wins the TRAXXAS Off Road Championships 2010 - YouTube

You really can't get much more abusive than that type of racing. If I recall correctly they were GPR's, but I'll check. The engine builder called them " poor man's Ti rods" he claims the engine rev'ed quicker.
Abusive is a understatement when it comes to short course or desesrt racing. But the short course guys only have to run 4 short races a weekend then the motor comes out to get checked.

GN7, Side question. Do you have to run a roots style blower per GN rules? or can it be centrifugal?
 

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Aside from having 2 load bearing points, the piston only has to deal with its own weight on the intake stroke, and it doesn't include the pin. The rod as to deal with it all from the fork up. Most aluminum rounds don't fail at the small end, they fail mid beam, or where the rod transitions into the fork, or as Rocky Childs called it, "the hinge"
Makes sense. Also makes sense that the piston only has to deal with compressive and tensile loads, where the rod is also wagging side to side. I'm guessing the flexation/flexure in the side to side motion is what kills most any rod. Rev it to the moon and exitus blockus. Seems there should be a window or limit that aluminum rods should work well within. I wonder, at what point that occurs. Certain piston speed I presume...
 

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The further away form the center of the crank, the more effect the weight will have.
Makes a lot of sense. Closer to the crank is more of a rotating mass, closer to the piston more of a reciprocating mass.
 

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The only application I see for aluminum rods these days is where the bottom end needs a little cushion from extreme cyl pressures. Other than that I don't see the need or benefit from aluminum rods. The biggest problem with them is they're a ticking time bomb. it's not a matter of if, only when...unless you're using them in a completely unrelated application like a low hp/low rpm street engine where you're not using them even remotely close to their intended use but that's not a fair comparison. Not sure of anyone mentioned windage issues, either. Big, fat, bulky alum rods don't help in that department. ONe last thing to consider; weight in a recip. assembly does not contribute or detract from overall power, it only helps in acceleration of the masses. If the application is endurance or any other app where the rpm is more continuous then IMO the weight savings is really of no benefit over the reliability concerns. Steel rods with the lightest possible piston will usually be a better combination.
This just seems too general to me. 4.5" stroke 7500rpm vs. 4" at 6500 could mean the difference between ticking time bomb and hundreds of hours on the water, couldn't it? Is it really a ticking time bomb? May it just as easily fail at 10 hours as it may at 15, 20, 100, 150? Should the owner of an engine with aluminum rods have no expected reliable service time frame?
 

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This just seems too general to me. 4.5" stroke 7500rpm vs. 4" at 6500 could mean the difference between ticking time bomb and hundreds of hours on the water, couldn't it? Is it really a ticking time bomb? May it just as easily fail at 10 hours as it may at 15, 20, 100, 150? Should the owner of an engine with aluminum rods have no expected reliable service time frame?
Nope. Even under Xray they can't tell when one's "time is up".
Again, if you're using a set of BME rods in a 450hp street hot rod that never sees 6000rpm, then I'd say they could last a lifetime but in any performance app where the rod is seeing moderate loading, it's going to have a cycle life. Too many variables to say what that would be.
 

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This just seems too general to me. 4.5" stroke 7500rpm vs. 4" at 6500 could mean the difference between ticking time bomb and hundreds of hours on the water, couldn't it? Is it really a ticking time bomb? May it just as easily fail at 10 hours as it may at 15, 20, 100, 150? Should the owner of an engine with aluminum rods have no expected reliable service time frame?
There isn't enough data to determine how long an aluminum rod will survive in a non race situation. Too many variables. Piston and pin weight play a huge part of besides stroke and RPM. Companies like Miller have a good idea how long they will survive in a fueler or blown alky deal, but then so do the racers.
I imagine the pro stock guys do as well. How long it last in a street car, or recreational boat is hard to nail down. How many times did it get buzzed to 7000, and for how long? An hour meter or rev counter is useless. No telling how long the rods were near yield.
Then you have to take into account the rod itself. GRP, or MGP machined from 7075 mill plate, or a Miller, or Alan Johnson/C&A forging? There is a HUGE difference.

Unlike steel, unless you keep the rod well below its yield point, every cycle is held in memory, and the yield is way lower than steel. Forces at 6000 that may never fail a steel rod, will in time fail a aluminum rod. Question is, when? I guess that depends on how many times you ran it 8000. Where the steel rod may only "remember' the 8000 blast, the aluminum rod remembers all the time at 6000 as well.

Because its impossible to make aluminum with the same tensile strength of 4340 or 300M, forces that are well below the yield of the steel rods have no real effect on it life span. But those forces can very well be very close to the aluminum's yield, and it does shorten the rods life.

The big thing too many over look as well is, that temperature will shorten an aluminum rods life as fast as stress loads. The life span difference of a aluminum rod with 300 degree oil is huge compared to 250. The tensile strength of aluminum plummets at temperatures steel laughs at. I wouldn't even consider a performance boat with aluminum rods and no oil cooler.
Rods forged from 2024 take the heat a little better, but all those rod manufactures are gone.

If aluminum rod at 6000 rpm will eventually fail, and it will, the cost of having to replace them just exceeded a set of Carrillos with Carr bolts if you had bought them in the first place. And the Carrillo and the bolt will out live you at that RPM



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