pros and cons of rod length
12-16-2007, 09:22 PM
#1
2nd Gear Member
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Join Date: Oct 2007
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Posts: 234
pros and cons of rod length
What are the benefits and negative effects of rod length?
I'm plannig on having a stroker crank with a stroke of 3.850, but there are a variety of rod lengths available - 6.000, 6.125 and 6.200. Obviously they all stroke the same length so should travel the same speed from tdc to bdc, but they will use different height pistons, the longer the rod length the shorter the piston. Is a short piston a good thing? Lower weight/less energy to move/lower friction surface?
Any thoughts?
I'm plannig on having a stroker crank with a stroke of 3.850, but there are a variety of rod lengths available - 6.000, 6.125 and 6.200. Obviously they all stroke the same length so should travel the same speed from tdc to bdc, but they will use different height pistons, the longer the rod length the shorter the piston. Is a short piston a good thing? Lower weight/less energy to move/lower friction surface?
Any thoughts?
12-16-2007, 09:33 PM
#2
Yukon Cornelius
Join Date: Oct 2005
Location: deep in the heart of dixie GEORGIA
Posts: 11,808
RE: pros and cons of rod length
the longer rod will create more torque thanks to its larger mechanical advantage but will lose a slight amount or rpm range but nothing too significant
12-16-2007, 09:41 PM
#4
D.R. THE PATHETIC DORK
Join Date: Jun 2007
Location: West Virginia
Posts: 30,863
RE: pros and cons of rod length
here is an example like on a 347 alot of people now like to use a 5.315 rod instead of a 5.4 rod...why because the shorter rod is in theroy suppose to create a better rod ratio and not put as much side inertia to the skirt causing piston rock in the bore......that is the theroy but in a street/ strip car usually you'll run into other probs before that'll be an issue
...and I know your not using a stroked 302 but it's just an example
...and I know your not using a stroked 302 but it's just an example
12-16-2007, 09:49 PM
#5
2nd Gear Member
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RE: pros and cons of rod length
So is it worth going for the 6.200 rods or should I stick to the 6.000 rods? How much more torque will it give me? I've read alot of stroker threads that stick to the 6" rods so I assume they are better?
12-16-2007, 10:05 PM
#7
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RE: pros and cons of rod length
I'm the good engine builder with no creds hehe. I've been building engines for years, but havent built one with so many options. Never stroked an engine before either, they've all been stock replacements. I think i'm going to stick to the 6.000 rods with the stroker crank. Now I need to find some decent H beam rods.
I want the higher rev limit for strip use. I will need some hp lower down the rev range, but its a 4V and thats why i'm stroking it.
I want the higher rev limit for strip use. I will need some hp lower down the rev range, but its a 4V and thats why i'm stroking it.
12-16-2007, 10:26 PM
#8
4th Gear Member
Join Date: Mar 2006
Location: Concord, NC
Posts: 1,261
RE: pros and cons of rod length
If your building a 351W you may have a problem with the rod to counter weight clearance with the 6.000. the shortest rod we use is the 6.200 and thats with the 4.000 crank.
12-17-2007, 01:22 AM
#10
6th Gear Member
Join Date: Apr 2007
Location: California
Posts: 10,468
RE: pros and cons of rod length
Longer rod pros - longer dwell time so you get more complete combustion, and more torque with better mileage, emissions and more detonation resistance, so you can run cheaper fuel or more ignition timing etc. Lighter pistons mean increased efficiency. Shorter pistons means less skirt area which means less friction, that combined with less piston side loading due to a more mellow rod angle means better longevity.
Cons - cost, availability, can you fit it into your engine.
The more advanced stuff....
The piston doesn't travel the same speed through the length of the cylinder, even at the same rpm. That's because the rod ratio has an effect on piston speed based on crankshft vs rod/piston position(it gets into effects of geometry). The piston travels at a higher velocity at the top of the bore than it does at the bottom(in most engines). The lower the rod ratio(the shorter the rod) the higher the velocity at TDC and the lower the velocity at BDC. This has several effects. First since velocity is highest at TDC when the piston changes direction, the rod will see the most force here as the inertia of the piston wants to keep going in one direction when the crank forces it to move the other direction by pulling on the rod. This causes the rod to stretch. Higher velocity at TDC means more inertia and therefore more stress on the piston/pin/pin boss. A longer rod will increase velocity at BDC and decrease velocity at TDC, this will put less stress on the rod/piston for the same rpm, either extending engine life(if a rod failure is a possibility) or, primarily, will extend rpm range. edline gets higher due to less inertial load on the internal components. The longer rod, even though it decreases TDC velocity and increases BDC velocity, still has a higher TDC than BDC velocity(generally). In the end though, the longer rod decreases TDC velocity, which may be desirable depending on the build.
Another primary effect this has deals with induction pulse. The higher the velocity at TDC, the stronger the induction pulse will be when the intake valve first begins to open. The vacuum generated will be stronger and try to draw more air/fuel in during the earlier stages of the valve events, but as the piston moves towards BDC and velocity decreases in the piston while air/fuel is filling the cylinder, the pulse grows weaker. The shorter rod is preferable with cams that close the intake valve earlier(shorter duration low rpm type cams), since it generates the strongest pulse during the period of time that the cam is operating most effectively. The longer rod has an opposite effect. The lower velocity at TDC serves to generate a somewhat weaker induction pulse during the initial stages of the valve event, but as the piston moves towards BDC it maintains more velocity in the bottom of the bore, serving to help draw air/fuel in with a stronger pulse than a shorter rod would at the bottom of the bore, allowing you to draw in the induction pulse just a bit longer. This works well with cams that operate at higher rpm where the intake valve closes later, since the stronger pulse at the bottom of the bore(relative to a shorter rod) helps to continue to draw air in longer after the piston passes BDC.
It has a similar effect on exhausting, the higher the velocity at the top of the bore, the better the scavenging at lower rpm(a shorter rod). The longer rod won't scavenge as well until the engine rpm is sufficiently high enough to generate the necessary piston velocity at TDC to get good scavenging. Again, longer rod favors higher rpm cams, shorter rod favors lower rpm cams.
Basically, a longer rod is more preferable for a "bigger breathing" higher rpm engine, but a shorter rod is better for breathing at lower rpm. Longer rodscreate more rpm resistance(higher redline)than shorter rods, but the longer rod also needs more rpm to take advantage of some of it's strengths. Longer rods always have an advantage in terms of engine wear characteristics, but depending on the change in rod ratio, this may not be noticable except in extreme cases, such as a long rod 302 like I have, vs a short rod 347...but that's comparing basically the longest rod 302 block setup(long rod289/302)to the shortest rod 302 block setup(a 347)(with the 8.2" deck on both), the extremes at both end. The big advantage in some engines come in weight savings, which is a primary reason I chose it....Ford's crackhead engineers seemed to think it was necessary to use a ginormous and heavy 1.6" compression height piston, on a short stroke engine that benefits greatly from reduced internal weight.
Also, the greater the change in rod ratio towards either end of the spectrum, the greater the effect it has on all the things I've listed. But everything has to be factored in to see if it's worth it. For me I chose it for the greatly reduced piston weight, less friction/inertia was an advantage, and the longer dwell would help with overall power production across the entire rpm range and allow me to run on pump gas with a healthy tune and still maintain a good safety margin. I also liked the longer rod for the reduced internal loading, so I can beat on it more often than a standard 302 setup for the same amount of wear. The biggy though was that it all went right in without any essoteric modifications. Just a fewer minor machining things here and there and it "bolted in" for only slightly more cost than if I'd used a standard Ford rod/piston setup(I used a pistons and rods for Chevy setups).
Some things can offest the others as well, like the longer dwell can help to pickup some of the bottom end torque lost due to less efficient breathing at lower rpm etc. Ultimately everything needs to be factored in to see if it's worth it. I like the "stuff the longest rod into it that you can" philosophy, but within reason. If it's way too difficult or costs too much, then it's not worth the time/effort/money. And long rods certainly aren't necessary for power, obviously when you look at 347 strokers, or even GM's new LS7 from the factory, both of which have what many would cosider totally crappy rod ratios(like 1.5-1.6 i think for both), but no one disputes the ability to make power with either. It's just one more piece of the puzzle when you build an engine.
Cons - cost, availability, can you fit it into your engine.
The more advanced stuff....
The piston doesn't travel the same speed through the length of the cylinder, even at the same rpm. That's because the rod ratio has an effect on piston speed based on crankshft vs rod/piston position(it gets into effects of geometry). The piston travels at a higher velocity at the top of the bore than it does at the bottom(in most engines). The lower the rod ratio(the shorter the rod) the higher the velocity at TDC and the lower the velocity at BDC. This has several effects. First since velocity is highest at TDC when the piston changes direction, the rod will see the most force here as the inertia of the piston wants to keep going in one direction when the crank forces it to move the other direction by pulling on the rod. This causes the rod to stretch. Higher velocity at TDC means more inertia and therefore more stress on the piston/pin/pin boss. A longer rod will increase velocity at BDC and decrease velocity at TDC, this will put less stress on the rod/piston for the same rpm, either extending engine life(if a rod failure is a possibility) or, primarily, will extend rpm range. edline gets higher due to less inertial load on the internal components. The longer rod, even though it decreases TDC velocity and increases BDC velocity, still has a higher TDC than BDC velocity(generally). In the end though, the longer rod decreases TDC velocity, which may be desirable depending on the build.
Another primary effect this has deals with induction pulse. The higher the velocity at TDC, the stronger the induction pulse will be when the intake valve first begins to open. The vacuum generated will be stronger and try to draw more air/fuel in during the earlier stages of the valve events, but as the piston moves towards BDC and velocity decreases in the piston while air/fuel is filling the cylinder, the pulse grows weaker. The shorter rod is preferable with cams that close the intake valve earlier(shorter duration low rpm type cams), since it generates the strongest pulse during the period of time that the cam is operating most effectively. The longer rod has an opposite effect. The lower velocity at TDC serves to generate a somewhat weaker induction pulse during the initial stages of the valve event, but as the piston moves towards BDC it maintains more velocity in the bottom of the bore, serving to help draw air/fuel in with a stronger pulse than a shorter rod would at the bottom of the bore, allowing you to draw in the induction pulse just a bit longer. This works well with cams that operate at higher rpm where the intake valve closes later, since the stronger pulse at the bottom of the bore(relative to a shorter rod) helps to continue to draw air in longer after the piston passes BDC.
It has a similar effect on exhausting, the higher the velocity at the top of the bore, the better the scavenging at lower rpm(a shorter rod). The longer rod won't scavenge as well until the engine rpm is sufficiently high enough to generate the necessary piston velocity at TDC to get good scavenging. Again, longer rod favors higher rpm cams, shorter rod favors lower rpm cams.
Basically, a longer rod is more preferable for a "bigger breathing" higher rpm engine, but a shorter rod is better for breathing at lower rpm. Longer rodscreate more rpm resistance(higher redline)than shorter rods, but the longer rod also needs more rpm to take advantage of some of it's strengths. Longer rods always have an advantage in terms of engine wear characteristics, but depending on the change in rod ratio, this may not be noticable except in extreme cases, such as a long rod 302 like I have, vs a short rod 347...but that's comparing basically the longest rod 302 block setup(long rod289/302)to the shortest rod 302 block setup(a 347)(with the 8.2" deck on both), the extremes at both end. The big advantage in some engines come in weight savings, which is a primary reason I chose it....Ford's crackhead engineers seemed to think it was necessary to use a ginormous and heavy 1.6" compression height piston, on a short stroke engine that benefits greatly from reduced internal weight.
Also, the greater the change in rod ratio towards either end of the spectrum, the greater the effect it has on all the things I've listed. But everything has to be factored in to see if it's worth it. For me I chose it for the greatly reduced piston weight, less friction/inertia was an advantage, and the longer dwell would help with overall power production across the entire rpm range and allow me to run on pump gas with a healthy tune and still maintain a good safety margin. I also liked the longer rod for the reduced internal loading, so I can beat on it more often than a standard 302 setup for the same amount of wear. The biggy though was that it all went right in without any essoteric modifications. Just a fewer minor machining things here and there and it "bolted in" for only slightly more cost than if I'd used a standard Ford rod/piston setup(I used a pistons and rods for Chevy setups).
Some things can offest the others as well, like the longer dwell can help to pickup some of the bottom end torque lost due to less efficient breathing at lower rpm etc. Ultimately everything needs to be factored in to see if it's worth it. I like the "stuff the longest rod into it that you can" philosophy, but within reason. If it's way too difficult or costs too much, then it's not worth the time/effort/money. And long rods certainly aren't necessary for power, obviously when you look at 347 strokers, or even GM's new LS7 from the factory, both of which have what many would cosider totally crappy rod ratios(like 1.5-1.6 i think for both), but no one disputes the ability to make power with either. It's just one more piece of the puzzle when you build an engine.