advantage of 6" rods?
03-23-2009, 11:21 AM
#3
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so if i was gonna build a 355 i should go with 6" rods?
i was told by someone that it slows the piston down at TDC for more torque... don't know if thats true or not.
i was told by someone that it slows the piston down at TDC for more torque... don't know if thats true or not.
03-23-2009, 11:26 AM
#4
With 6" rods, the piston remains stationary longer ( and slightly slows the piston) at TDC AKA Dwell time. This allows the pressure to build more before forcing the piston back down on the compression stroke. Should be good for a few extra HP but nothing noticable on the but dyno. FTR I have 6" rods in my 355.
Only possible downside may be a tad more piston slap in the cold due to the shorter profile piston. Mine is overall very quiet even in the extreme cold.
Only possible downside may be a tad more piston slap in the cold due to the shorter profile piston. Mine is overall very quiet even in the extreme cold.
03-23-2009, 11:28 AM
#6
Forget all the bench racing and theory.
Reality is it makes for a lighter piston, lighter piston is easier on the connecting rod bolts the most stressed fastener in an engine.
The rest of the answers are just bench racing miniscule differences that might be meaningful at the highest levels of racing but are meaningless on the street.
Reality is it makes for a lighter piston, lighter piston is easier on the connecting rod bolts the most stressed fastener in an engine.
The rest of the answers are just bench racing miniscule differences that might be meaningful at the highest levels of racing but are meaningless on the street.
03-23-2009, 11:50 AM
#7
Little to no benefit in the "real world".
People argue about the longer dwell time at TDC, but it also causes a weaker intake charge on paper as a result.
Lighter pistons can be a benefit, but you may end up with a heavier rod that more then negates that benefit. You may end up with a heavier rotating assembly by accident!
There was a huge very (and very advanced) thread on it in the adv tech page on speedtalk a while back. It may be over the average enthusiasts head though...
People argue about the longer dwell time at TDC, but it also causes a weaker intake charge on paper as a result.
Lighter pistons can be a benefit, but you may end up with a heavier rod that more then negates that benefit. You may end up with a heavier rotating assembly by accident!
There was a huge very (and very advanced) thread on it in the adv tech page on speedtalk a while back. It may be over the average enthusiasts head though...
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03-23-2009, 02:56 PM
#8
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so basically the guy at summit that told me that is a DA? lol
BS aside...should i look into stock length rods or 6"? not looking to get too complicated with anything.
BS aside...should i look into stock length rods or 6"? not looking to get too complicated with anything.
03-23-2009, 03:07 PM
#9
I worked through it this way:
Draw a picture of one piston/rod/crank on a piece of paper. Look at the angle of the rod to the piston at the bottom of the stroke. Now draw another picture a much longer rod, same crank throw and piston. You should be able to see that the angle of the rod to the piston is less with the longer rod. You can accentuate the length of the rod to see it better.
The benefit of the longer rod is that there is less side force on the piston, thus the cylinder wall, versus the shorter rod.
Draw a picture of one piston/rod/crank on a piece of paper. Look at the angle of the rod to the piston at the bottom of the stroke. Now draw another picture a much longer rod, same crank throw and piston. You should be able to see that the angle of the rod to the piston is less with the longer rod. You can accentuate the length of the rod to see it better.
The benefit of the longer rod is that there is less side force on the piston, thus the cylinder wall, versus the shorter rod.
03-23-2009, 03:10 PM
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I worked through it this way:
Draw a picture of one piston/rod/crank on a piece of paper. Look at the angle of the rod to the piston at the bottom of the stroke. Now draw another picture a much longer rod, same crank throw and piston. You should be able to see that the angle of the rod to the piston is less with the longer rod. You can accentuate the length of the rod to see it better.
The benefit of the longer rod is that there is less side force on the piston, thus the cylinder wall, versus the shorter rod.
Draw a picture of one piston/rod/crank on a piece of paper. Look at the angle of the rod to the piston at the bottom of the stroke. Now draw another picture a much longer rod, same crank throw and piston. You should be able to see that the angle of the rod to the piston is less with the longer rod. You can accentuate the length of the rod to see it better.
The benefit of the longer rod is that there is less side force on the piston, thus the cylinder wall, versus the shorter rod.
03-23-2009, 03:27 PM
#11
People tend to over think street setups and over build there motors, half the people on here dont use there setups to the max.
Nothing wrong with using a 5.7 rod over a 6...
Nothing wrong with using a 5.7 rod over a 6...
03-23-2009, 08:54 PM
#12
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03-23-2009, 09:17 PM
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Them silly senile old people :\ lol Twas a good read. I liked this bit:
Originally Posted by Silly Old Fart
In fact, this may surprise you, but I know of a gentleman who runs a 5.5" Rod in a 350 Small Block Chevy who makes more horsepower (we're talking top end here) than he would with a longer rod. Why? Because with a longer dwell time at BDC the short rod will actually allow you a slightly later intake closing point (about 1 or 2 degrees) in terms of crank angle, with the same piston rise in the cylinder. So in terms of the engines sensitivity to "reversion" with the shorter rod lengths you can run about 2-4 degrees more duration (1-2 degrees on both the opening & closing sides) without suffering this adverse affect! So much for the belief that longer rod's always enhance top end power!
03-24-2009, 02:59 AM
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The 5.7" v 6" rod discussion is academic for street street/strip engines.The theory behind it is for racers looking for that last nth degree of Hp; the average joe on the street is not going to know the difference.
03-24-2009, 06:30 AM
#16
Not to play devil's advocate or anything, but for every positive claim about using a longer rod, you will find a positive claim for using a shorter rod.
If you guys know who John Kaase is, you'd be interested to see how he designs his motors for the older Engine Masters challenge. He has won it numerous times, and placed second numerous times as well. I know that these motors aren't "street" motors, but they do have applications that can be used. Most people pick a displacement, then a rod size when building a motor. In his motors from 2004-2006, he would analyze head flow. Then he would use the smallest bore possible that still allowed acceptable air flow, without shrouding the chamber too much. Then it was just a matter of math to get the stroke correct to come up with the displacement allowed within the rules. His motors always had huge strokes, with relatively small bores, and very short rods in comparison to everyone elses setups. His setups were based on preventing detonation, but it is some advanced stuff, far beyond what I know, but interesting nonetheless.
If you guys know who John Kaase is, you'd be interested to see how he designs his motors for the older Engine Masters challenge. He has won it numerous times, and placed second numerous times as well. I know that these motors aren't "street" motors, but they do have applications that can be used. Most people pick a displacement, then a rod size when building a motor. In his motors from 2004-2006, he would analyze head flow. Then he would use the smallest bore possible that still allowed acceptable air flow, without shrouding the chamber too much. Then it was just a matter of math to get the stroke correct to come up with the displacement allowed within the rules. His motors always had huge strokes, with relatively small bores, and very short rods in comparison to everyone elses setups. His setups were based on preventing detonation, but it is some advanced stuff, far beyond what I know, but interesting nonetheless.
03-24-2009, 06:51 AM
#17
A longer Rod influences the piston to dwell a bit longer at TDC than a shorter rod would and conversely, to dwell somewhat less at BDC.
For a street setup, there is no "wrong" rod length. Like joel said, for pretty much every pro there is a con.
