rod length question

[oldschool]

what is the advantage of a 7.100" rod verses a 6.760" rod,in a 572" motor with a 4.5 stroke? any horsepower difference? any reliabilty differnce? im getting ready to freshen up my 572,so now would be the time to change anything.now it has a nice set of childs and albert 6.760 rods ($1200) ,wondering if they are alot better than eagle 7.100 ($700)?
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[aifilaw]

The crank is what detirmines your stroke length, the piston and rod ratio, or I guess I'll get into that later.

The rod length has an effect on your performance based on the rotational mass and torque requirements to move it, to some extent drag. But most importantly, less frictional mass against the cylinder walls...

In other words if I had a 5" stroke engine, and had a 10" tall cylinder I needed to make up then the best possible scenario for power would be a 1" tall piston and a 9" tall connecting rod as far as pure power potential.

Unfortunately you run into some problems with that, piston wall slap, centering, et cetera... the piston has a minimum size based on power and leverage, aside from that you want the maximum heighth possible.

Basically, in consideration of a new build, detirmine your stroke length first of the crankshaft. Then detirmine the smallest piston deck to pin center measurement you can use based on the pistons performance in that application, lastly detirmine your finished piston-to-deck clearance dimension. Once that is complete its time to look for a rod that is that size or slightly shorter.... more than likely you will find one that is slightly shorter, and then make up the difference with either decking the block to achieve your static C.R. or using a different piston.

[firefighter3931]

OS, the 7.1 rod is probably your only choice w/o major surgery to the crank. Eagle specifies that the 7.1 rod be used with their 4.5 crank....most likely due to potential counterweight interference issues. It's possible a shorter rod could be used but the crank would need some surgery and rebalancing. Side loading of the cylinder walls would be increased as well...

I'm going with the 7.1 rod for mine and not trying to re-engineer an existing proven combination. 

When you had the engine built originally did you purchase the rod bolt upgrade ? If not....that would be worth doing while it's being freshened up. 



Ron

[oldschool]

OS, the 7.1 rod is probably your only choice w/o major surgery to the crank. Eagle specifies that the 7.1 rod be used with their 4.5 crank....most likely due to potential counterweight interference issues. It's possible a shorter rod could be used but the crank would need some surgery and rebalancing. Side loading of the cylinder walls would be increased as well...

I'm going with the 7.1 rod for mine and not trying to re-engineer an existing proven combination. 

When you had the engine built originally did you purchase the rod bolt upgrade ? If not....that would be worth doing while it's being freshened up. 



Ron

the motor already has a callies 4.5 crank. it also has a nice set of childs and albert rods w/ arp bolts.i eas trying to see if it would be worth it to change the rods to 7.100.

[firefighter3931]

OS, the 7.1 rod is probably your only choice w/o major surgery to the crank. Eagle specifies that the 7.1 rod be used with their 4.5 crank....most likely due to potential counterweight interference issues. It's possible a shorter rod could be used but the crank would need some surgery and rebalancing. Side loading of the cylinder walls would be increased as well...

I'm going with the 7.1 rod for mine and not trying to re-engineer an existing proven combination. 

When you had the engine built originally did you purchase the rod bolt upgrade ? If not....that would be worth doing while it's being freshened up. 



Ron

the motor already has a callies 4.5 crank. it also has a nice set of childs and albert rods w/ arp bolts.i eas trying to see if it would be worth it to change the rods to 7.100.

OK, i misread your original post.... you can slap  me right now ! 


Obviously the Callies 4.5 and the C&A 6.76 rods fit fine in your engine.  Going to a longer rod will improve the rod ratio and lighten up the rotating assembly (shorter pistons) so theoreticly it should rev faster. Of course you would need new slugs with the rod change. The piston CH would be ~ 1.37in with the 4.5/7.1 combo. Not sure how much meat that leaves up top for spray....something you might want to explore with the piston manufacturer.

If you plan to spray this with a 300 shot it might be wise to look at a better rod....maybe something from Oliver ?

The Eagle rods are rated to 850hp and you're right there now....just on motor. 




Ron

[Challenger340]

When a guy gets into talking various Rod Lengths, I don't think there is any definitive answer, but rather, more questions arise, for given Applications.
Like Ron says, the 4.5/7.1 combo might be "thin" up top, and/or less room for Ringpack considerations(accumulator chamber, heat) between or above Rings, if a guys gonna spray alot,
conversely,
others like the trade-off for Weight & Geometry, less focus on the spray.

Just my opinion,
On Mopars, I tend to always go for the Geometry/Longer Rod deals.
Just personal preference,
but,
based mainly upon invariably ALWAYS fighting the Cylinder Head thing.
Just never seem to have quite enough, and the Valves open on the backside of the Cans. If you can punch up ANY extra Turbulence, it's a "good thing" comparably, to more gifted Headed engines.

That said, if you've got some 572-13 or 600-13 Heads, the shorter Rod just don't matter.

just my 2 cents
Bob out.

[suntech]

I am going with Oliver 7.1 rods, on a billet Callies 4.75. Would do that on a 4.5 also, but Tim Banning (FHO) got some nice 4.75´s made, and i got one. 
There are many benefits with as long rod as possible, and there are downsides, if you go to far, with lack of space for proper ring positioning on piston etc.
My reason for using as long as possible is GEOMETRY!!
When you air/ fuel mix has exploded, and builds up sylinderpressure, the piston is on it´s way back down, and the rod is on an angle to the crank. By using longer rods, you optimize the angle the rod is pushing the crank with, and let it do the job the rod is there for, more efficiant,  and minimize the sideways forces on the piston and cylinder wall.

[oldschool]

thanks everbody for the info.im getting a set of 7.100 oliver rods. ill use my childs and albert 6.760 in another 500" build.

[firefighter3931]

thanks everbody for the info.im getting a set of 7.100 oliver rods.

Excellent choice ! 


Ron