Thread: 5.7" VS 6" Rod

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Does anybody have any really dyno sheets to prove the advantages or disadvantages of using the longer 6" Rods in the SBC.
The industry has gone to longer rod applications, but were is the gain.
Interested in building a 383 with 6" rods for Great low end torque, say 2500-3500 RPM.
Already have the 5.7" 383, makes tones of power on the bottom end, but we all know there is always room for improvement.
By the way, this is for a 95 Chevy TBI engine.

I have heard that longer rods mean more torque, something to do with acting as a longer lever. But I have also heard that longer rods are a must in high rpm (high hp) engines because they cause the piston to spend more time at TDC which allows more time for pressure to build up and act on the crown of the piston (time is very limited at high rpms). I have heard NASCAR teams talk about rod lengths in the 6.2"+ range, but most wont say the exact length. Increasing rod length does add to reciprocating mass which reduces an engine's rpm capabilities.

I think we all missed the boat a little on the first question.
I know all about the longer dwell time and I have read all the books there are on rod to stroke ratios.
I wanted actual dyno sheets that prove or disprove the longer length rods are better in a lower rpm situation.
I am in the process of rebuilding my 383 for a second time.
We build a lot of circle track and street/strip engines, most of them will get longer rods.
What I am after is if anybody has built a 383 with 6" rods for pulling - tow rig, and has done a dyno to see if the added .300" was a help or not.
I know at 5000 rpm it will help, but lets face it, I only have a 25 gallon tank, and the track is 90 miles away.

Just trying to compare other builds to my original one.

Originally posted by CNC-Motorsports

Just trying to compare other builds to my original one.

For any dyno comparison on rod lengths to be meaningful, the cam timing would have to be optimized for each combination. If the rod length is not significantly different, I doubt that there would be any difference. But, when you have a difference of rod length where you can measure piston position differences in degrees at a specific crank angle, i.e. piston velocity differences, then cam timing has to be adjusted accordingly to have any meaningful results. Otherwise, you're not getting a true comparison. Hope this helps.

I am retired from a career in "theory" and I am certainly a learning novice in SBC technology. Further the talking is over for me since I have gone ahead with brand new 5.7 rods in my 350 rebuild, but I would like to learn more with respect for the experience of engine builders, especially of the SBC design. First a partial answer to the thread question may be found in the article on a long rod in a 400 block to make a 350 as found in:

http://www.airflowresearch.com/artic...le03/A3-P1.htm

There the advantage seemed to go to the longer rod and there is dyno data. Of more interest to me as a real puzzle is how the dwell time can be different for TDC and BDC as shown in the Isky site above. How can this be? The only way I can see this is through a small amount of slop in the piston pin, but in an ideal circle conversion of linear motion to circular motion I can't visualize why there should be a difference in dwell time between TDC and BDC. In another site this is said to be equal:

http://www.chevymania.com/tech/rod.htm

The point overlooked in the discussion above is that of fuel octane and detonation. The long rod in the first site above ran on 87 octane at 11:1 C.R. and I briefly considered purchase of a 383 based on 5.7 rods with only 9.5:1 C.R. but that engine required 93 octane fuel. So can someone explain to me how the dwell time can be different for TDC and BDC? In addition is it not better to opt for a more powerful engine on more common (poor) low octane fuel. This is just a theoretical discussion for me but I hope to learn more about "my" engine, the SBC 350.

Don Shillady
Retired Scientist/teen rodder

Originally posted by Don Shillady

There the advantage seemed to go to the longer rod and there is dyno data. Of more interest to me as a real puzzle is how the dwell time can be different for TDC and BDC as shown in the Isky site above. How can this be?
Don Shillady
Retired Scientist/teen rodder

Hi Don,

Piston motion is what changes with rod length and stroke variations. Peak piston velocity is the easiest way to explain what you want to know. To accurately calculate piston velocity you need to meld together a little trig and a little calculus and you can generate these neat graphs that let you see the difference in "dwell time" for TDC and BDC.

Without going through all that I will try to give you a visualization of what happens.
The piston will reach peak velocity when the connecting rod centerline is approximately at a 90-degree angle to the rod throw. In other words, a line that runs from the center of the piston pin to the center of the rod journal is at right angles to a line that runs from the centerline of the main to the centerline of the rod journal. In most engines this happens around 74 crankshaft degrees after TDC. From that point, the piston is slowing down until it stops at BDC. So, 74 degrees to peak velocity and 106 degrees till it finally stops at BDC is: 74 +106 = 180. You can imagine what a graphed curve of those numbers would look like.

Now, all you have to do is mirror that curve and you will see what it looks like going from BDC to TDC. Coming off of BDC it take 106 degrees for the piston to reach its peak velocity and only 74 degrees to finally stop at TDC. There you have it, the piston moves much slower across BDC that it does across TDC.

Clear as mud, right?

Hope this helps.

OK, I think I get it. It is due to the difference in piston speed as well as geometry. Maybe someone should put a flywheel on that crank and maybe this is more of an effect with an automatic transmission than with a heavier flywheel? Anyway I see how it can happen due to piston speed, interesting.

Don Shillady
Retired Scientist/teen rodder

Originally posted by Don Shillady
OK, I think I get it. It is due to the difference in piston speed as well as geometry.

Don Shillady
Retired Scientist/teen rodder

Great.

When designing engine components I find that understanding the stroke/rodlength relationship to be the single most important factor in developing useful parts. It is the heart and soul of the whole gas exchange process.

Don,
Here's a link to a graph that allows you to put different rod to stroke ratios and compare piston volicity, position and acceleration. It doesn't change that much so you can put some really exagerated numbers to get a better idea how the different rod lengths affect the variables listed above. The graph is at the bottom of the page.
http://andre.stechert.org/VTEC.net/a...nPosition.html