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Old 11-12-2004, 09:40 PM
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6" rod small block chevy

I have searched the site and cannot find any info pertaining to the advantages of a 6" rod engine. I have been told that there is a significant increase in torque and fuel efficency due to the increased time the piston stays at TDC. This makes sence to me. And I would assume the displacement would stay the same if the wristpin was moved up to allow for the longer rod. Anyone with info or a link to such info please reply.
Thank you
s22479a
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Old 11-12-2004, 10:02 PM
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Yes, the displacement stays the same. I am runnng a 6" rod in my 388ci SBC. While there are "supposed" to be advantages, the difference is really tiny. I will try to find the study I read on this issue. They compared the difference between a 5.565, 5.7 and 6" rod, while there is a "slight" diffeence it is not something to write home about. The magazines have blown this subject out of proportion (IMO).

Yes, with a 6" rod the piston "dwells" at TDC for a little longer period (remember we aren't talking seconds here we are talking fractions of a second). This same thing means that the pistond speed has to be increased in order to make up for the extra time spent at TDC. I will not try to fully explain it all here (not sure I really can).

There are benefits and there are drawbacks. All in all it all comes out in the wash. 6" rods decrease the amount of cylinder side loading (over the shorter rods), due to the shorter piston used with a 6" rod you get more piston rock which can hurt ring sealing, you also typically have the wrist pin bore in the oil ring groove which means you need to run an oil support ring. (this bothers some people).

There are more pros and cons as well, my point is depending on the use of the engine it might not be worth the extra effort to go one way or the other. I used the 6" rods because I already had them. If I would of had the 5.7's I would of used them. I have also used 5.565" rods with great success.

I will look for the study and post a link to it.

Hope that helps some.

Royce
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Old 11-12-2004, 10:10 PM
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Thank you

Thank you for your reply Royce. All info is a help. It will give me a direction to investigate as i break it down into each pro and con.
Thanks again.
s22479a
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Old 11-12-2004, 10:41 PM
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This site seems to have a pretty good article

http://www.grapeaperacing.com/GrapeA...ectingrods.pdf
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Old 11-12-2004, 11:09 PM
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I can not find the study I read before it was strictly on the differences between different rod lengths. It broke it down into very good details.

The link above has some good info but, I do not agree with some of it.

http://www.aros.net/~rbuck/rick/rodstudy.htm

I am pretty sure this is the link I used before but, it looks like tha page is not longer up. I will keep looking I printed it off and I will see if I can find it.

Royce
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Old 11-13-2004, 07:15 AM
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I agree with Royce on the hype. From what I have read, the advantages are much less than some of the articles will lead you to believe. I have run both and have no issues with either. I think for higher HP applications the 6 inch rods are good to prevent side loading and help with rod angularity which helps with longevity in high RPM/HP setups. Seems to me the advantages are more of a mechanical longevity than power output.

Chris
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Old 11-13-2004, 12:22 PM
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.Even though this topic has been beat to death I will comment my thoughts and experience.

First of all, I would like to dispel the theory of long rods increasing piston speed. Stroke is the only determining factor of increasing piston speed. The rod is nothing more than a mechanical attaching device that changes a linear motion into a rotating motion. It wouldn't matter if the rod was 15 feet long the stroke, therefore the total distance the piston movement, stays the same. The piston still moves no further back and forth than if a 15 foot rod rather than a 5.56 rod. It’s only further away from the moment center of the crank.

Second, it is true the piston will sit at TDC longer with long rods. The crank will rotate further before all the clearances are taken up and pulling the rod down on the intake stroke. Allowing cam lobe changes that allow a certain amount of ramming affect with the right head port shape and valve angle. The compression stroke providing the expanding gasses to build more pressure before the piston moves down. This is a two fold instance here. More top end power CAPABILITY comes from a longer rod because of the increased pressure being exerted on the piston in the top 1/3 of the bore. The torque building VE comes from the top 1/3 of the stroke. With the middle third producing very little power and the bottom third doing near nothing but preparing the cylinder for blow down.

A long rod with a short piston CH allows 2/300 more RPM with the rods pulling from the piston pin bores. The short piston is lighter as well as the center axis of rotation has been moved closer to the crown. This makes the piston more stable in its bore the same as increasing the skirt length an Appropriate length. But the trade off with increased skirt length is added weight. That's fine for a low RPM engine, but no good for a power producing engines designed to run 7500 or more.

Long rods have forever been discussed in street rods because it a term that can be used to impress those who are unaware. Please don't be offended by the statement. When hotrodders sit around discussing theory's it's fun to say, "short stroke long rod engine". Or "short stroke, big bore engines". Magazines always try to sell the product they are writing a tech article on because that's how they make money and build engines for you to read about. Iron Eagle For instance will send them a rod package for free that whoever magazine will promise to say "Iron Eagle" as many times as possible and so therefore the rods show great promise. It's all marketing.

Not to say advantages aren't there with a long rod engine in a street machine. That street machine had better be built to the hilt though. A long rod engine will see 2-300 more RPM before piston failure with 20 or so extra horse power being seen at 7000 or higher as long as the carb, cam, and head design are properly designed to take full advantage. If there not, well, all you’re really left with is an engine that doesn't respond well with low RPM throttle input and extra ring life. Hardly worth the extra cost IMO
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Old 11-13-2004, 01:46 PM
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AMEN on that post

Nice Description on the subject there johnsongrass1....
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Old 11-13-2004, 02:17 PM
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Uh.....yeah!!! What he said........
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Old 11-13-2004, 05:02 PM
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johnsongrass1,
Like I said above, I don't think it is really worth a ton of effort in most cases. The differences are TINY as you mentioned.

I do have one question. How can piston speed be the same in a "6 inch" rod engine as it is in a "5.565 inch" engine. If the piston is going to stay (dwell) at TDC longer with the longer rod at some point it has to catch back up to the "shorter rod" Either that or the shorter rod engine has to have a "dwell" spot somewhere else in the stroke, correct? This is assuming a given RPM lets say 5000.

There are also arguments that the pistond speeds of X vs. Y is increased on the down stroke. My opinion is that it all has to even out if you average it. I am not sure it's worth worrying about, because the differences are so small in most cases you will not see a true performance difference. Most engines to not see extended periods of time at 7500RPM (I am talking street engines).

Anyway very good input.


Royce
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Old 11-13-2004, 10:02 PM
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Royce,

I have to agree with you. If there is more dwell at TDC(and BDC as well), piston speed will be increased at some point in the stroke. I believe this is actually seen at the acceleration point when the piston begins to move from TDC/BDC and deceleration as it approaches TDC/BDC. So piston speed is higher.....at some point in the stroke.

Chris
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Old 11-13-2004, 10:20 PM
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i've read in two or three different places that longer rods actually slow down piston speed. by slowing down the piston speed it takes away centrifugal weight helping parts last longer. if i can find it i'll post a link.
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Old 11-14-2004, 04:27 AM
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Piston acceleration rates increase/decrease from the point halfway before and after TDC and BDC. The speed, or more appropriately, Terminal Velocity, remains the same with the maximum velocity being reached at exactly halfway up or down the stroke.

When looking at a circle, It's plain to see there isn't any one moment to "dwell" on. Piston dwell time is somewhat of a loose descriptive term used to often to describe the instant center when the piston changes direction. If you place your finger on a circle and trace around it in the same fashion as a connecting rod is chasing a crank throw, you can see at no point is there a place that doesn't pivot around the roll center. Dwell time is really just a place that the crank is still moving but the rods are not because the clearances have to be taken up on the opposite side of the bearing. There is a place along the stroke that the piston is moving so little it's not productive to consider it to be movement. We can call this Dwell. IMO about any less than .001 of an inch doesn't affect our study's. Increasing rod length gives the crank more measurable movement of the crank throw before a change of .001 is seen at the piston.

Accel. the piston slower with longer rods means there's less centrifugal force required to change it's direction on the intake stroke. It's also the stroke most engines will blow up on. Compression will help the rod slow down by counteracting the forces and the compressing of the rod under the power stroke will help the same way. It's a good bet if the engine blows off the throttle it's the bottom end, but if it blows on the throttle it's probably a valve.


Cam and head choices are given here to take full advantage of the slower accelerations to improve power. Holding a valve open longer and utilizing the scavenging affect from the cam overlap can also produce worthwhile improvements.

It's all in the combination.
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Old 11-14-2004, 09:58 AM
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May be i'm wrong , but using 6" rod reduce the rotating weight that a bid advantage ?

i'm wrong ? Piston are a lot smaller.. ?

I'm building a 410sbc whit 6" rod right now
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Old 11-14-2004, 02:34 PM
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Yes, with better throttle response to boot. A smaller piston will reduce weight but you need to make sure the rods you are using don't counteract the reduced piston weight.
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