SBC 400. 6" Rods vs. 5.7" Rods. - Team Camaro Tech
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  #1  
Old May 14th, 06, 09:17 PM
~JM~ ~JM~ is offline
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Default SBC 400. 6" Rods vs. 5.7" Rods.

Looking for advise on the pro's & cons of 6" & 5.7" rods.

I've been thinking I'd like to go with a Scat cast internal balance crank & 6" rods.

I've seen some mention of piston selection & long term durability problems with the 6" rod.

Thank you
~JM~
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  #2  
Old May 15th, 06, 04:02 AM
67SSragtop 67SSragtop is offline
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Default Re: SBC 400. 6" Rods vs. 5.7" Rods.

I'm not sure what the cons of 6" rods would be, but from what I've read they give a better 1.6:1 rod:stroke ratio and make internal balance easier on the machine shop. I went with the scat 9000 internally balanceable rotating assembly. The shop did have to take a decent amount off the crank, but everything worked out well. With a standard base cam, had to grind a small amount off the rods on 1,2,5,6.
Hope that helps,
Scott
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  #3  
Old May 15th, 06, 04:55 AM
CNC BLOCKS N/E CNC BLOCKS N/E is offline
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Default Re: SBC 400. 6" Rods vs. 5.7" Rods.

We have always used the 6 inch rods so we can internally balance and it also frees up some friction as there is less thrust on the cylinder walls using the longer rod.
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  #4  
Old May 15th, 06, 06:02 AM
JimM JimM is offline
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Default Re: SBC 400. 6" Rods vs. 5.7" Rods.

I did a LOT of studying on this when I built my 383, note I'm by no means an expert but I listen well.

The big advantage of a 3.75" stroke / 6" rod combo is reduced reciprocating wieght. My pistons wieghed in at 404 grams, compared to a stock ht piston over 600 grams. That's HUGE! That's why they are easier to internally balance, less counterweight is required.
It's also my belief that a lighter wieght reciprocating assembly will result in much lower loads on the crank and rods, and result in a faster, hagher reving engine that will last a long time.

I can't prove it, bit I also believe the long rod motor is less prone to detonation due to the longer "dwell time" at tdc.

I think with currently available pistons and rods, it should be considered a no-brainer. If you are going to replace the entire rotating assy anyway, go 6" / internal balance.
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  #5  
Old May 15th, 06, 07:31 AM
onovakind67 onovakind67 is offline
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Default Re: SBC 400. 6" Rods vs. 5.7" Rods.

Quote:
Originally Posted by CNC BLOCKS N/E
We have always used the 6 inch rods so we can internally balance and it also frees up some friction as there is less thrust on the cylinder walls using the longer rod.
More thrust on the cylinder walls would equal more thrust on the crank, resulting in more torque. I guess the trade-off would be the wall deflection and the ring sealing lost with that, as well as the ability of the combustion energy to maintain pressure on the piston at higher piston speeds.
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  #6  
Old May 15th, 06, 07:48 AM
JimM JimM is offline
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Default Re: SBC 400. 6" Rods vs. 5.7" Rods.

Quote:
Originally Posted by onovakind67
More thrust on the cylinder walls would equal more thrust on the crank, resulting in more torque. I guess the trade-off would be the wall deflection and the ring sealing lost with that, as well as the ability of the combustion energy to maintain pressure on the piston at higher piston speeds.
There seems at first glance to be something wrong with this math.
Assuming the "total thrust" produced by combustion remained the same, it would seem that less thrust against the side of the cylinder wall would result in more downward thrust applied to the crank, not less.

Really don't think the difference is enough to matter tho.
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  #7  
Old May 15th, 06, 08:37 AM
Eric68 Eric68 is offline
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Default Re: SBC 400. 6" Rods vs. 5.7" Rods.

There is often a small weight advantage to the longer rod combo. Not always though and it is certainly not HUGE. You have to compare apples to apples -- with the same brand and type pistons, same brand and type rods.

Here's an example that will have you scratch your head

SRP flat top 383 piston for 5.7" rod = 472 gm
SRP flat top 383 piston for 6.0" rod = 419 gm

Eagle SIR 5.7" rod = 535 gm
Eagle SIR 6.0" rod = 590 gm

Piston + rod weight for 5.7" combo = 1007 gm
Piston + rod weight for 6.0" combo = 1009 gm

I just built a 383 combo with 5.7" rods (to allow for more compression height and more room for the ring package on a blower motor) and to my surprise the 5.7" combo balanced internally just fine. I think aftermarket cranks often have extra weight in the counterweights and will often internally balance with shorter rod combos -- but you never know until you check.
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  #8  
Old May 15th, 06, 08:59 AM
JimM JimM is offline
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Default Re: SBC 400. 6" Rods vs. 5.7" Rods.

I wonder what those numbers would look like with just the "small end" wieght of those rods?

It is kinda startling tho.
I wonder exactly how much a more common trw forging weighs?
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  #9  
Old May 15th, 06, 08:59 AM
onovakind67 onovakind67 is offline
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Default Re: SBC 400. 6" Rods vs. 5.7" Rods.

Quote:
Originally Posted by JimM
There seems at first glance to be something wrong with this math.
Assuming the "total thrust" produced by combustion remained the same, it would seem that less thrust against the side of the cylinder wall would result in more downward thrust applied to the crank, not less.

Really don't think the difference is enough to matter tho.
In order to load the cylinder wall, there has to be force exerted on it. Newtons law applies here, you can't push on one end of the connecting rod without an opposite reaction on the other. If the angle of the rod causes it to exert more force on the wall, it also exerts more force on the crank.
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  #10  
Old May 15th, 06, 10:24 AM
Eric68 Eric68 is offline
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Default Re: SBC 400. 6" Rods vs. 5.7" Rods.

Quote:
Originally Posted by onovakind67
In order to load the cylinder wall, there has to be force exerted on it. Newtons law applies here, you can't push on one end of the connecting rod without an opposite reaction on the other. If the angle of the rod causes it to exert more force on the wall, it also exerts more force on the crank.
That is true -- and as a result the shorter rod supposedly makes a hair more TQ than the long rod motor.

There was a good thread here several years ago that went into the engineering calculations like side loading, acceleration rate, dwell time at TDC, etc. on the various rod lengths in an SBC. Wish I could find it.

The thing that sticks in my mind was that the differences were very small (like 2-3%) and many of the benefits were offset by something else (like onovakind is saying about side loading and friction vs. increased rod angularity making more TQ) that it did not make a significant difference.

and Jim, good point about small end vs big end weights. I would think that when measuring big end / small end weights for a 5.7 vs. a 6.0" long rods you would find that big end weight is pretty similar between the two rod lengths . . . maybe Carl can chime in on that one.
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  #11  
Old May 15th, 06, 10:29 AM
onovakind67 onovakind67 is offline
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Default Re: SBC 400. 6" Rods vs. 5.7" Rods.

This is probably what you are referring to, the connecting rod study done by Rick Draganowski:

http://www.rustpuppy.org/rodstudy.htm

There is very little difference in the geometry of the different length rods.
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  #12  
Old May 15th, 06, 10:29 AM
JimM JimM is offline
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Default Re: SBC 400. 6" Rods vs. 5.7" Rods.

I don't have my balance sheet with me here at work. The numbers I remember are 404 gram piston wt, and 755 reciprocating wt, including piston, pins, locks, rings, and small end.
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  #13  
Old May 15th, 06, 10:39 AM
Eric68 Eric68 is offline
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Default Re: SBC 400. 6" Rods vs. 5.7" Rods.

That's the one
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  #14  
Old May 15th, 06, 01:53 PM
Larger Dave Larger Dave is offline
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Cool Re: SBC 400. 6" Rods vs. 5.7" Rods.

Whether you favor short rods or long is kind of like the hair styles women wear in High school (what ever is envouge in your teen years seems to stick with you for life). I grew up listening principally to what Smokey Yunick, Bill Jenkins and John Lingenfelter had to say on the subject.

Smokey summed it up thusly: "Put in the longest damned rod that will fit in the motor, period! End of subject."

I have 6" rods in my SBC 400.


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  #15  
Old May 15th, 06, 02:22 PM
TexasPerfProd TexasPerfProd is offline
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Default Re: SBC 400. 6" Rods vs. 5.7" Rods.

LD I agree with that quote.
You dont have to do alot of math you dont even have to know how to do the math. Millions of dollars (figuratively speaking I didnt actually count it all )of research has been done by professional race teams on this issue. All you have to do is be able to see whats in these guys engines. There are many older engines from these teams trickeling down all the time. I would say they would agree with the same quote almost 100% of the time from what I have seen. Now that doesnt mean you want a full blown race rod/piston combination for your street/strip ride. It does tell you longer rods have a advantage even if it is only 2% power increase and a ever so slight reduction in bore wear. Now if your buying all new anyway and its basically = $ I say why not take some of their research and use it to your advantage. Just my opinion but Ill take every stinking hp I can get as long as it is suitable for the engine being built.
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