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Old 11-26-2010, 09:47 AM
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Balancing Questions

Being a complete nerd, and a suspicious nerd at that, I have always sort of distrusted the common method of determining bobweights for balancing a V-8 rotating assembly; i.e. 100% of the rotating + 50% of the reciprocating. I know there are a lot of ways of doing things out in the world that remain the same because "that's just the way we do it", without any real understanding of why.

So I threw together an Excel spreadsheet to analyze and graph the motion of the engine components.

The first thing I discovered was that the motion of a pair of pistons, eg. #1 & #2, when combined, act like a single rotating mass, generally centered about where the camshaft centerline is. The path of that single mass formed a perfect circle, with a diameter of exactly half the stroke. That was kind of eye-opening, as the "half the reciprocating mass" started to make sense to me. Half the diameter requires half the mass at full diameter to balance....a-ha!

The connecting rods really surprised me. I know the common technique is to include half of their weight with the rotating mass, and half with the reciprocating. First I had to find the center of mass of a connecting rod, so I threw some typical SBC 5.7" numbers at it. 600 grams total weight, 200 grams small end weight. This would put the center of mass at one third of the rod length from the big end center. I hoped by calculating the center of mass of the rod, it would can eliminate the whole "weight of the big end vs. the weight of the small end" thing. It also has the added benefit of removing the motion of the rod about its own center from the equation, as by definition, anything moving about its own center of mass is inherently balanced.

The graph blew my mind. The rod mass centers follow ellipses, instead of circles. And wouldn't you know it...the combination of the two rods produced another perfect circle, centered close to the camshaft centerline, with a diameter slightly smaller than the stroke.

When I did the math to figure the amount of bobweight to re-create the forces of two rods, I got 1000 grams.... Quickly, I calculated the weight by the "old-fashioned" method, using 600 grams each...2 x (100% of 400g + 50% of 200g) ... and got 1000 grams! Whoa...I guess engine builders actually know what they're doing. I was actually sort of disappointed at this point...having hoped I might discover some secret that all other automotive engineers had overlooked for over a century

Now, mathematically, a circle is just a special type of ellipse, so it got me to thinking...what would the effect of changing the rod weight, and the "balance" of the rod....by taking more weight from the small end vs the big end balancing pads do? Would it change the shape of the ellipses? Would it require more or less bobweight? I have heard some machinists sort of roll their eyes at customers who want their rotating assemblies under or over balanced before, but I wanted to know if their was some method to the madness.

I was not disappointed. I discovered that I had just gotten lucky and picked the "magic" combination that makes the "100% + 50%" rule work perfectly, by using 600g/200g. I found that if you keep the total-weight to small-end weight ratio at 3:1, you preserve the traditional method. Changing rod length and/or rod mass didn't change this outcome as long as the 3:1 ratio remained intact. The 100%/50% rule always matched the more detailed analysis.

However, if the ratio changed...it called for a small percentage change +/- to the bobweight amount. Frequently, when I see rod weights being matched, nobody seems to talk about the ratio. They are always more concerned with just the weight being the same. Honestly, it would take a bit of algebra to figure out how much to take off each rod, and it would slow the process down, but I feel it would definitely make for better balancing.

Have any of you guys who actually balance engines found this to be true in the real world?

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Old 11-26-2010, 03:34 PM
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I overbalance a few engines by small percentage. Otherwise the standard formula works fine. Read this http://victorylibrary.com/mopar/crank-bal-c.htm
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Old 11-26-2010, 04:08 PM
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Thanks for the link, Bob. Lots of good egghead reading there for me. The guy seems to agree with me on a couple of things, and maybe disagree on the others.

I'm going to have some fun playing around with my 496 project pretty soon...working with some real numbers, and maybe pestering my local machinist to do this a little differently.
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Old 11-26-2010, 05:22 PM
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Fresh look at balance.

I'll follow this thread in hopes there will be more. I believe there is more to consider, given your initial approach to it.
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Old 11-26-2010, 07:46 PM
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Quote:
Originally Posted by nofearengineer
, when I see rod weights being matched, nobody seems to talk about the ratio. They are always more concerned with just the weight being the same.

Isnt that because the ratio is always neutralized when the crank is ballanced to the moving part weights?
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Old 11-26-2010, 09:16 PM
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This is interesting.

I had a crank machined and polished and a I took the rods and pistons to get pressed on. When I picked it up I thought it was going to get balanced right away but didn't.

I was told that they balanced the rods so the pistons would have to be taken off.

The engine is a 283 and the pistons I have are Federal Mogul Power Forged.
With that short of stroke and decent pistons I'm not sure if it really matters.
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Old 11-26-2010, 09:45 PM
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Quote:
Originally Posted by 65smallblock
Isnt that because the ratio is always neutralized when the crank is ballanced to the moving part weights?
I'm not sure I understand your question.

My point is.....yes, when they put the bobweights on it, they do indeed balance the crank. Some shops and/or sellers of pre-balanced kits often advertise "within half a gram" or even less. The problem is...they might be starting with bobweights that don't exactly represent the true balance weight of the rods, pistons, etc that are actually being used. Close, but not exactly. I think the "big end / small end" thing may just be a "close enough" technique.

And all because the "science" of engine balancing started long before there were computers, spreadsheets, etc. Just the calculations in my little sheet would have taken a long, long time to do by hand. But how many extra percent of engines might live just a little longer, or achieve one or two more horsepower if calculated on a case by case basis?

I'm going to try to make some decent graphical representations of this soon and get it posted.
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Old 11-26-2010, 10:16 PM
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Quote:
Originally Posted by nofearengineer
I'm not sure I understand your question.

My point is.....yes, when they put the bobweights on it, they do indeed balance the crank. Some shops and/or sellers of pre-balanced kits often advertise "within half a gram" or even less. The problem is...they might be starting with bobweights that don't exactly represent the true balance weight of the rods, pistons, etc that are actually being used. Close, but not exactly. I think the "big end / small end" thing may just be a "close enough" technique.

But how many extra percent of engines might live just a little longer, or achieve one or two more horsepower if calculated on a case by case basis?

.
You are clearly coming at this from beyond my understanding, very interesting though. So basically you are trying to get a "tighter" ballancing job all around. ?? Seems worth while too me. Especialy since there is such a trend toward longer strokes, even with smaller cube engines.

When I did my 327 I did the small end big end thing and found as much as 7 grams discrepancy with the factory rods. These were from a 70's circa 350. But the point is I was stunned to find the "lack" of concern with factory ballancing. Maybe that has improved since then.

Id like to be a fly on the wall when these things are disgused among certian circles, like the builders of funny car engines or nascar motors..
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Old 11-26-2010, 10:36 PM
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Quote:
Originally Posted by nofearengineer
I'm not sure I understand your question.

My point is.....yes, when they put the bobweights on it, they do indeed balance the crank. Some shops and/or sellers of pre-balanced kits often advertise "within half a gram" or even less. The problem is...they might be starting with bobweights that don't exactly represent the true balance weight of the rods, pistons, etc that are actually being used. Close, but not exactly. I think the "big end / small end" thing may just be a "close enough" technique.

And all because the "science" of engine balancing started long before there were computers, spreadsheets, etc. Just the calculations in my little sheet would have taken a long, long time to do by hand. But how many extra percent of engines might live just a little longer, or achieve one or two more horsepower if calculated on a case by case basis?

I'm going to try to make some decent graphical representations of this soon and get it posted.

Still trying to get my head around the ratio thing.. As I understand the science of ballancing, (and as bighotroddin mentioned) Overall ballancing must be done in sequence. The rods cannot be "matched" with pistons installed and therefore they (the rods) are done first. (of corse the weight matching applies to pistons and wrist pins as well.)

Then you do some math to find the proper crank counter weight. Here is the point I was making earlier. The small end is calculated as part of the piston and the big end (and bearings) as part of the crank weight. So once the math is done and crank counter weight determined, you will still be @ the 100%+ 50% formula reguardless of the ratio..

Or am I missing something?

Just another thought. Seems to me that a finer ballancing job is largly a matter of lightweight parts and quality machine work. If a machinist pays a lot of attention to detail and has quality equipment that might be more usefull than advancing the science of ballancing.. In this age of drive thru service Im sure there is a lot of shoddy ballancing jobs being passed off as the real thing.

Last edited by 65smallblock; 11-26-2010 at 10:52 PM.
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Old 11-26-2010, 11:15 PM
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You're almost there.

What I'm saying is that for the connecting rod mass (and the oil that clings to it, but that's a discussion for another day), adding the big end weight to the rotating portion, and the small end weight to the reciprocating portion, does not produce the "correct" bob weight amount, except in the case where the small end weighs exactly 1/3 of the total rod weight.

And I have seen "matched" sets of connecting rods, where there was a large range of balancing pad left on the rods.

The way it's done is they match the small ends of the rods to the lowest, and then match the totals by shaving the bottom balancing pad. But when you shave the bottom balancing pad, you invariably change the "weight" of the small end as well by a small amount. To boot, the ratio might not have even been 3:1 to begin with.

It would only take about 90 seconds to write an Excel formula that would tell you exactly how much to remove from both ends to match small end and total weight, which would also produce that magical 3:1 ratio. However, at that point, it wouldn't necessarily have to be the 3:1 ratio anyway. All that would matter is that all 8 rods have the same ratio. No sense grinding away on your rods if they're already well-matched but not exactly 3:1. That would be a little harder to program, but it could be done....Of course, in that event your would have to calculate the bob weight component by my method rather than the "big end /small end" method.

Good Lord...I'm digging myself a hole here.
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