Rod Lengths
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Elder User
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Rod Lengths
I got to thinkin. I drew up some cad drawings and did some chin scratching and thought I would drop a post.
Do you think that for a given stroke, bore, etc. The shorter rod length producing greater dwell time at BDC giving ability to lengthen the intake duration on cam timing is significant?
Furthermore, would it be significant enough to outweigh the losses incurred by friction of the piston against the wall due to the increased rod angle?
If so, do you then think that Desktop Dyno or Dyno2000 are not accurate when comparing cams in stroked engines to stock engines unless the rod/stroke ratio were the same? (since they don't ask for rod length)
OR, is it just as significant for the longer rod to dwell more at the top, lending more towards a longer exhaust duration and/or greater overlap? (Especially in these 460's)
I was hoping to *atleast* hook George on this one, but everyone should throw in their 2 cents.
Do you think that for a given stroke, bore, etc. The shorter rod length producing greater dwell time at BDC giving ability to lengthen the intake duration on cam timing is significant?
Furthermore, would it be significant enough to outweigh the losses incurred by friction of the piston against the wall due to the increased rod angle?
If so, do you then think that Desktop Dyno or Dyno2000 are not accurate when comparing cams in stroked engines to stock engines unless the rod/stroke ratio were the same? (since they don't ask for rod length)
OR, is it just as significant for the longer rod to dwell more at the top, lending more towards a longer exhaust duration and/or greater overlap? (Especially in these 460's)
I was hoping to *atleast* hook George on this one, but everyone should throw in their 2 cents.
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Elder User
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Rod Lengths
Are you asking or saying?
Maybe my eyes are strained, but it looks like over the 45 degrees at BDC, the 3.85 inch stroke of the 460 with a rod of 6.605 inches, the piston will go down .1266 inches and back up .1266 inches. The 6.8 rod will go down .1315 inches and of course same back up. These happen in the same amount of time for a given rpm, but the long rod goes more distance, so I am *assuming* it *dwells* less since it moves more. And the shorter rod *dwells* more since it moves less at BDC.
I don't know if my meaning of dwell is right, but I think (hope) I got the motions right. AutoCad R14 did the numbers.
Maybe my eyes are strained, but it looks like over the 45 degrees at BDC, the 3.85 inch stroke of the 460 with a rod of 6.605 inches, the piston will go down .1266 inches and back up .1266 inches. The 6.8 rod will go down .1315 inches and of course same back up. These happen in the same amount of time for a given rpm, but the long rod goes more distance, so I am *assuming* it *dwells* less since it moves more. And the shorter rod *dwells* more since it moves less at BDC.
I don't know if my meaning of dwell is right, but I think (hope) I got the motions right. AutoCad R14 did the numbers.
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Elder User
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Rod Lengths
and the numbers for the top are:
.4595 up and same down over 45 degrees for 6.605 rod
.4546 up and same down over 45 degrees for 6.8 rod
Meaning the longer rod will appear to move less over 45 degrees centered at TDC for a given rpm; or, dwell more.
And, to look at it another way, for the 6.8 rod to cover the same distance (ie .4595 up and down), the crank will have to rotate more than 45 degrees (54.2745 to be exact) and more than the 6.605 rod would at the top.
.4595 up and same down over 45 degrees for 6.605 rod
.4546 up and same down over 45 degrees for 6.8 rod
Meaning the longer rod will appear to move less over 45 degrees centered at TDC for a given rpm; or, dwell more.
And, to look at it another way, for the 6.8 rod to cover the same distance (ie .4595 up and down), the crank will have to rotate more than 45 degrees (54.2745 to be exact) and more than the 6.605 rod would at the top.
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Elder User
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Rod Lengths
I did make the silliest of mistakes. I put the stroke in as a radius instead of a diameter. So, that 460 has a monster stroke of 7.7:-staun
, but the idea is the same. Just the numbers are bigger than they should be.
, but the idea is the same. Just the numbers are bigger than they should be.
Posting Guru
Joined: Aug 2000
Posts: 2,416
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From: PacNW
Rod Lengths
>I got to thinkin. I drew up some cad drawings and did some
>chin scratching and thought I would drop a post.
>
>Do you think that for a given stroke, bore, etc. The
>shorter rod length producing greater dwell time at BDC
>giving ability to lengthen the intake duration on cam timing
>is significant?
>
>Furthermore, would it be significant enough to outweigh the
>losses incurred by friction of the piston against the wall
>due to the increased rod angle?
>
>If so, do you then think that Desktop Dyno or Dyno2000 are
>not accurate when comparing cams in stroked engines to stock
>engines unless the rod/stroke ratio were the same? (since
>they don't ask for rod length)
>
>OR, is it just as significant for the longer rod to dwell
>more at the top, lending more towards a longer exhaust
>duration and/or greater overlap? (Especially in these
>460's)
>
>I was hoping to *atleast* hook George on this one, but
>everyone should throw in their 2 cents.
Here's what the folks that make DD2K have to say about that very subject:
Question: Everyone talks about how longer rods make more power. Why isn’t rod length one of the choices in the pull-down menus?
Answer: Tests we have performed with the Dyno2000 show that rod length has virtually no affect on power. We realize that many actual dyno tests have shown power increases, but our simulation tests tell us that the power, when found, probably has little to do with piston dwell at TDC (and the associated thermodynamic effects) or changes in rod angularity on the crank pin. The measured power differences are most likely due to a reduction of friction on the cylinder wall from changes in side-loading on the piston. This can vary with bore finish, ring stability, piston shape, the frictional properties of the lubricant, etc. These variabilities are highly unpredictable. Some development, after all, can only be done in the real world on a engine dynamometer.
There's also a good discussion on this webpage:
[link:members.tripod.com/racerjoe/classic_faq/all_classic/rod_length.html|Are long rods worth the cost?]
>chin scratching and thought I would drop a post.
>
>Do you think that for a given stroke, bore, etc. The
>shorter rod length producing greater dwell time at BDC
>giving ability to lengthen the intake duration on cam timing
>is significant?
>
>Furthermore, would it be significant enough to outweigh the
>losses incurred by friction of the piston against the wall
>due to the increased rod angle?
>
>If so, do you then think that Desktop Dyno or Dyno2000 are
>not accurate when comparing cams in stroked engines to stock
>engines unless the rod/stroke ratio were the same? (since
>they don't ask for rod length)
>
>OR, is it just as significant for the longer rod to dwell
>more at the top, lending more towards a longer exhaust
>duration and/or greater overlap? (Especially in these
>460's)
>
>I was hoping to *atleast* hook George on this one, but
>everyone should throw in their 2 cents.
Here's what the folks that make DD2K have to say about that very subject:
Question: Everyone talks about how longer rods make more power. Why isn’t rod length one of the choices in the pull-down menus?
Answer: Tests we have performed with the Dyno2000 show that rod length has virtually no affect on power. We realize that many actual dyno tests have shown power increases, but our simulation tests tell us that the power, when found, probably has little to do with piston dwell at TDC (and the associated thermodynamic effects) or changes in rod angularity on the crank pin. The measured power differences are most likely due to a reduction of friction on the cylinder wall from changes in side-loading on the piston. This can vary with bore finish, ring stability, piston shape, the frictional properties of the lubricant, etc. These variabilities are highly unpredictable. Some development, after all, can only be done in the real world on a engine dynamometer.
There's also a good discussion on this webpage:
[link:members.tripod.com/racerjoe/classic_faq/all_classic/rod_length.html|Are long rods worth the cost?]
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Elder User
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Rod Lengths
Yea, there are some good thoughts there.
Not only does friction increase with the shorter rod, but acceleration also increases, which would steal some more energy.
Not only does friction increase with the shorter rod, but acceleration also increases, which would steal some more energy.
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Elder User
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Rod Lengths
But, has anyone really tested this???
I mean, to optimize a cam timing for a short and long rod setup of equal stroke and see how they differ on the dyno.
Also, in his figuring (the author of that article), he only used .25 from TDC to compute dwell and then took a percentage from there.
I tend to think .25 is not a big enough sample of movement to reflect, really, what is happening.
If you could squeeze a couple more degrees of duration or overlap out of your cam, would you do it?
I mean, to optimize a cam timing for a short and long rod setup of equal stroke and see how they differ on the dyno.
Also, in his figuring (the author of that article), he only used .25 from TDC to compute dwell and then took a percentage from there.
I tend to think .25 is not a big enough sample of movement to reflect, really, what is happening.
If you could squeeze a couple more degrees of duration or overlap out of your cam, would you do it?
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Thread Starter
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Elder User
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Rod Lengths
OK, I went back into cad and cleaned everything up. I got a stroke 3.85 this time.
Now, 90 degrees rotation of the crank centered on TDC (means 45 degrees each side of TDC) will produce .7056 inches of travel up and same back down with the 6.605 rod.
To produce this same movement with the 6.8 rod attached, you must rotate the crank 103.3191 degrees (51.65955 each side of TDC). Subtract the two and you end up with nearly 4 degrees (2 per side). Is this significant in terms of cam timing? Especially on an engine that can use a little more exhaust?
Now, 90 degrees rotation of the crank centered on TDC (means 45 degrees each side of TDC) will produce .7056 inches of travel up and same back down with the 6.605 rod.
To produce this same movement with the 6.8 rod attached, you must rotate the crank 103.3191 degrees (51.65955 each side of TDC). Subtract the two and you end up with nearly 4 degrees (2 per side). Is this significant in terms of cam timing? Especially on an engine that can use a little more exhaust?
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Elder User
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Rod Lengths
OK OK, those numbers are crap. (I only got a few hrs into this - cut me some slack)
I was thinking the 6.605 rod went as high as the 6.8 for some reason, got my wires straight now though.
The right numbers are 90.2899 for long rod, 90 for short. a difference of only .2899 degrees
Basically insignificant, in agreement with what Dyno2k says.
However, due to friction and acceration forces, I think I would rather have a long rod.
I was thinking the 6.605 rod went as high as the 6.8 for some reason, got my wires straight now though.
The right numbers are 90.2899 for long rod, 90 for short. a difference of only .2899 degrees
Basically insignificant, in agreement with what Dyno2k says.
However, due to friction and acceration forces, I think I would rather have a long rod.
Senior User
Joined: Jun 2002
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From: Oklahoma
Rod Lengths
The were a lot of tests done on another brand of engine during the mid-to-late 60's on various rod lengths in a small-block engine and it seems the general concensus was that you should use as long a rod as possible providing the wrist pin didn't interfere with the oil ring groove area. On that brand of motor a r:s ratio between 1.9 and 2:1 was considered ideal. It seems the longer rods don't just have an effect on cylinder wall friction, they also have an effect on the breathing and combustion efficiency of an engine too.
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Rod Lengths
I remember from my bicycle riding days, pedaling with my toes could give me more rpm (or less knee strain); hence toe clips. But on a hill I would flip the pedal over so the clip would hang down and use the flat part of my foot. I just couldn't get the torque I wanted out of my toes.
So, I flipped a bike over on its seat and grabbed some sticks. I found with short sticks, I could get more torque than with the long sticks. With the long sticks, I could get it to spin faster (until the sticks fly off the pedals). Not really a *scientific* experiment, but just some observations.
Oh, and I also know that people with longer legs, tend to have more knee trouble from pedaling. Don't really know why, could be anything.
So, I flipped a bike over on its seat and grabbed some sticks. I found with short sticks, I could get more torque than with the long sticks. With the long sticks, I could get it to spin faster (until the sticks fly off the pedals). Not really a *scientific* experiment, but just some observations.
Oh, and I also know that people with longer legs, tend to have more knee trouble from pedaling. Don't really know why, could be anything.
Laughing Gas
Joined: Nov 2002
Posts: 1,198
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From: Fairmont
Rod Lengths
Seems like this is what I have found in the racing area,
On restricted intake engines, long rods (because of dwell time) are best to take advantage of every bit of power produced.
On unrestricted intakes but small heads a short rod engine is suggested (because of the faster piston velocity) to get the intake charge moving quicker.
Then unrestricted, big flow heads back to long rods again.
I have spent many hours with JE pistons, and Mahle discussing these items.
Besides the dwell time and side force on the cylinder walls of a long rod, I like them because of the lighter weight.
The longer the rod we get, the lighter the piston gets. Yes the rod does gain some weight, but not as fast as the piston losses it.
On restricted intake engines, long rods (because of dwell time) are best to take advantage of every bit of power produced.
On unrestricted intakes but small heads a short rod engine is suggested (because of the faster piston velocity) to get the intake charge moving quicker.
Then unrestricted, big flow heads back to long rods again.
I have spent many hours with JE pistons, and Mahle discussing these items.
Besides the dwell time and side force on the cylinder walls of a long rod, I like them because of the lighter weight.
The longer the rod we get, the lighter the piston gets. Yes the rod does gain some weight, but not as fast as the piston losses it.
