I have changed piston and rod types in my motor w/o rebalancing. It's only been turned to 6700 though, so mine may not be a good example.
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| Crank balance? https://www.slantsix.org/forum/viewtopic.php?t=43797 |
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| Author: | polkat [ Fri Feb 25, 2011 7:56 pm ] |
| Post subject: | Crank balance? |
As an inline engine, is the 225 crank (by itself) neutural balanced at the factory? Or to put it differently, can heavier or lighter pistons (assuming they are all of equal weight) be used without rebalancing? Thanks! ________ Fix ps3 |
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| Author: | Joshie225 [ Fri Feb 25, 2011 8:51 pm ] |
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Yes, an in-line 6 crank is neutrally balanced without bob-weights so changing the reciprocating weight doesn't change the balance. Stated another way, the crank itself is balanced and the rods and pistons balance each other. Six cylinder cranks have counter-weights though to reduce the bending stress in the crank. If you have heavier parts going up and down you need heavier counterweights to keep the crank stresses in check. That said normal small variations by just changing pistons are fine so long as the pistons weigh the same. There have been several instances of Chrysler taking weight out of the slant six cranks. Weight was taken out in '62/'63 and again when cast cranks were introduced in mid '76. I think there may be different cast cranks as well, but I'm not too well versed on the later stuff. |
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| Author: | polkat [ Sat Feb 26, 2011 12:13 am ] |
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Well, I'm a bit confused here. First you said... "Yes, an in-line 6 crank is neutrally balanced without bob-weights so changing the reciprocating weight doesn't change the balance." But then you said......... "If you have heavier parts going up and down you need heavier counterweights to keep the crank stresses in check." I assume by that second line you mean heavier or lighter then stock pistons/pin/rods (as I was asking about) will require rebalancing, which contradicts the first line. But if the second line is true, it tells me what I need to know. Thanks! ________ Box Vaporizers |
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| Author: | slantzilla [ Sat Feb 26, 2011 5:00 am ] |
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I have changed piston and rod types in my motor w/o rebalancing. It's only been turned to 6700 though, so mine may not be a good example.
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| Author: | Joshie225 [ Sat Feb 26, 2011 9:42 am ] |
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Quote: Well, I'm a bit confused here. First you said...
V8 cranks are balanced with bob-weights while I-6 and I-4 cranks are not. If you change the piston weight on a V8 the bob weight changes so you generally rebalance. If you don't use bob weights on an I-6 what does a different bob weight matter to the balance? Practically nothing. "Yes, an in-line 6 crank is neutrally balanced without bob-weights so changing the reciprocating weight doesn't change the balance." But then you said......... "If you have heavier parts going up and down you need heavier counterweights to keep the crank stresses in check." I assume by that second line you mean heavier or lighter then stock pistons/pin/rods (as I was asking about) will require rebalancing, which contradicts the first line. But if the second line is true, it tells me what I need to know. Thanks! I-6 cranks have pretty serious bending stresses. This was learned the hard way (failures) in the early days when they tried to make I-6 cranks like early I-4 cranks without counterweights. If we put weights on the crank we can reduce the bending stresses in the crank and it lasts longer. The problem here is I gave you 10 lbs of answer to a 5 lb question. Change your pistons, don't worry about the balancing. |
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| Author: | wjajr [ Sat Feb 26, 2011 10:09 am ] |
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Josh, do straight 8 engines share this naturally balanced trait as well? A straight 8’s song is not as harmonious or smooth sounding as a straight 6 sound, but a lot smoother than a V8’s sound… I’m just wondering. Bill |
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| Author: | Joshie225 [ Sat Feb 26, 2011 1:30 pm ] |
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Yes, I-8s are first and second order balanced naturally. |
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| Author: | wjajr [ Mon Feb 28, 2011 7:23 am ] |
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Josh: Quote: Yes, I-8s are first and second order balanced naturally
What do you mean by first & second order?
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| Author: | olafla [ Mon Feb 28, 2011 7:53 am ] |
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Maths. Engine_balance Follow the links at bottom of page. Olaf. |
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