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Old 5th-January-2013, 04:45 PM   #1
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Rod lenth question

Whats the advantages and disavantages in runnin a 6 inch rod vs a 5.7 rod? I found some pretty bad scares on my old piston(sabatage when out of the motor) so i was just gonna buy new rods and pistons while i rebuild my 406. It will b mostly a street duty motor and sprayed at the track. I have no idea what would be better off for my motor.
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Old 5th-January-2013, 04:56 PM   #2
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Also im looking at some forged rods. Scat has some i beam rods that are rated 750hp and eagle has some h beam rods that are rated 750hp. Im unsure really which to go with. I will be spraying this motor at the track prolly close to 400hp and will see alot of street time. What do you guys think? Need some opinions.
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Old 5th-January-2013, 05:21 PM   #3
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The long the rod the better.

400hp from a 406 is a easy task.


The main advantage to longer rod is reliably.

Longer rod moves the piston pin further up the piston, IE: lighter piston, and the rod angle is better so less skirt wear.


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Old 5th-January-2013, 05:57 PM   #4
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Quote:
Originally Posted by TY70SS View Post
Also im looking at some forged rods. Scat has some i beam rods that are rated 750hp and eagle has some h beam rods that are rated 750hp. Im unsure really which to go with. I will be spraying this motor at the track prolly close to 400hp and will see alot of street time. What do you guys think? Need some opinions.
The Scat I beam rod is a great bang for the price. I personally would never run another 3/8" rod bolt rod when you can get these with the 7/16" ARP rod bolts. $300 bucks is a great price, I have them in my 383 although I have the 5.7".

I did run the 6" Eagle H-beam rod in my last 406. Probably should have gone with a 6" rod in this 383, but it's done already and I run the crap out of it. It should easily go north of 500 hp.
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Old 5th-January-2013, 09:36 PM   #5
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Lots of theories about long rods but for street use none relate to reality. It's still debated on engine builder sites all over the internet. If it was that clear there would be no debate. Lighter piston with longer (heavier) rod? Side loads? Ever see a chart showing rod angles vs. length? Miniscule differences. A 454 Chevy has a worse rod/stroke ratio than a 350. (1.53 vs. 1.64 ) Are 454s known for massive bore wear and lack of horsepower? (A 400 with 5.7 rod is 1.52) If the price is the same and no balancing issues arise and you're buying new parts anyway then go ahead and use the long rod combination. Just understand there's almost no power or longer life there.
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Old 5th-January-2013, 09:46 PM   #6
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The down side of the long rod if it's mostly street is the wrist pin intrudes into the oil ring land. This dictates the need for an oil ring support.
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Old 6th-January-2013, 11:23 AM   #7
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The down side of the long rod if it's mostly street is the wrist pin intrudes into the oil ring land. This dictates the need for an oil ring support.
And what about this is a downside?

All pistons with a low compression height will have the oil ring support.

So what negative about it?

Al
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Old 6th-January-2013, 11:31 AM   #8
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Smokey quote:

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Old 6th-January-2013, 11:32 AM   #9
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Dwell time

The longer rod increases piston dwell time at TDC. This increases the force imposed on the piston.
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Old 6th-January-2013, 12:26 PM   #10
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From the article "Expert Engine-Building Tips & Advanced Theories Straight From Darin Morgan of Reher-Morrison"


"Most people tend to overgeneralize this issue. It would be more accurate to compare different rod-to-stroke ratios, and from a mathematical stand-point, a couple thousandths of an inch of rod length doesn't really change things a lot in an engine. We've conducted tests for GM on NASCAR engines where we varied rod ratio from 1.48- to 1.85:1. In the test, mean piston speeds were in the 4,500-4,800 feet-per-second range, and we took painstaking measures to minimize variables. The result was zero difference in average power and a zero difference in the shape of the horse-power curves. However, I'm not going to say there's absolutely nothing to rod ratio, and there are some pitfalls of going above and below a certain point. At anything below a 1.55:1 ratio, rod angularity is such that it will increase the side loading of the piston, increase piston rock, and increase skirt load. So while you can cave in skirts on a high-end engine and shorten its life, it won't change the actual power it makes. Above 1.80- or 1.85:1, you can run into an induction lag situation where there's so little piston movement at TDC that you have to advance the cam or decrease the cross-sectional area of your induction package to increase velocity. Where people get into trouble is when they get a magical rod ratio in their head and screw up the entire engine design trying to achieve it. The rod ratio is pretty simple. Take whatever stroke you have, then put the wrist pin as high as you can on the piston without getting into the oil ring. What-ever connects the two is your rod length."

Read more: http://www.chevyhiperformance.com/te...#ixzz2HDHia4kN
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Old 6th-January-2013, 12:34 PM   #11
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Cool

This is gettin good. RICK
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Old 6th-January-2013, 12:39 PM   #12
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And what about this is a downside?

All pistons with a low compression height will have the oil ring support.

So what negative about it?

Al
Not sure if it is really a negative or not. Some people say it is and some say it isn't. I have ran two sets myself with an SRP piston and I have a set here for a Ford with SRP pistons. For my street car I prefer to stay out of the oil ring land.
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Old 6th-January-2013, 12:40 PM   #13
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So with the longer rod, does the piston stop moving while the engine is running?
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Old 6th-January-2013, 12:42 PM   #14
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Quote:
Originally Posted by MarkW View Post
From the article "Expert Engine-Building Tips & Advanced Theories Straight From Darin Morgan of Reher-Morrison"


"Most people tend to overgeneralize this issue. It would be more accurate to compare different rod-to-stroke ratios, and from a mathematical stand-point, a couple thousandths of an inch of rod length doesn't really change things a lot in an engine. We've conducted tests for GM on NASCAR engines where we varied rod ratio from 1.48- to 1.85:1. In the test, mean piston speeds were in the 4,500-4,800 feet-per-second range, and we took painstaking measures to minimize variables. The result was zero difference in average power and a zero difference in the shape of the horse-power curves. However, I'm not going to say there's absolutely nothing to rod ratio, and there are some pitfalls of going above and below a certain point. At anything below a 1.55:1 ratio, rod angularity is such that it will increase the side loading of the piston, increase piston rock, and increase skirt load. So while you can cave in skirts on a high-end engine and shorten its life, it won't change the actual power it makes. Above 1.80- or 1.85:1, you can run into an induction lag situation where there's so little piston movement at TDC that you have to advance the cam or decrease the cross-sectional area of your induction package to increase velocity. Where people get into trouble is when they get a magical rod ratio in their head and screw up the entire engine design trying to achieve it. The rod ratio is pretty simple. Take whatever stroke you have, then put the wrist pin as high as you can on the piston without getting into the oil ring. What-ever connects the two is your rod length."

Read more: http://www.chevyhiperformance.com/te...#ixzz2HDHia4kN
Now you're talking, sounds like you got some facts going on there.
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Old 6th-January-2013, 01:18 PM   #15
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Ok!

Lets do some math!

SBC 400 3.750 stroke 5.565 rod = 1.484 rod ratio

SBC 350 3.480 stroke 5.700 rod = 1.637 rod ratio

SBC 302 3.00 stroke 5.700 rod = 1.900 rod Ratio

===================================

3.750 stroke with 5.700 rod = 1.52 rod ratio

3.750 stroke with 6.000 rod = 1.60 rod ratio

3.48 stroke with 6.00 rod = 1.724 rod ratio


================================================== ==
At anything below a 1.55:1 ratio, rod angularity is such that it will increase the side loading of the piston, increase piston rock, and increase skirt load. So while you can cave in skirts on a high-end engine and shorten its life, it won't change the actual power it makes. Above 1.80- or 1.85:1, you can run into an induction lag situation where there's so little piston movement at TDC that you have to advance the cam or decrease the cross-sectional area of your induction package to increase velocity. Where people get into trouble is when they get a magical rod ratio in their head and screw up the entire engine design trying to achieve it. The rod ratio is pretty simple. Take whatever stroke you have, then put the wrist pin as high as you can on the piston without getting into the oil ring. What-ever connects the two is your rod length."
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