Early Hemi Crank Balancing

Okay, I'm sort of past this stage with my recent build, but I always found this to be a good topic. Someone asked me this morning about this very thing and I told them my experience with the topic, but since I don't consider myself a professional, by any means, I thought I would put it up for discussion. Seems there is a lot of conflicting information, so I thought maybe a thread on this very topic might be of some value for anyone who has there rotating ***embly going out for balance.

Let's look at the basic idea first... The ultimate goal is a rotating ***embly that's balanced so it doesn't shake itself to bits, right? Rotating ***embly including harmonic balancer/dampener, crankshaft, rods, pistons, wrist pins, related bearing shells, and all the way back to the flywheel. I suppose you could go so far as mounted hardware, crank pully and the clutch, but I suppose that's not normally considered.

Knowing the components there seems to be two general ways...

1) Dynamic balancing including all of the above components

and...

2) Dynamic balancing including all of the above EXCLUDING harmonic balancer/dampener and flywheel; treating them as "zero en***ies" and checking balance on those individual components.

I suppose the benefit to the second is that you have the ability to change those components without pulling the whole motor apart and re-balancing everything.

SO! Let's talk. Again, I'm not an engine machinist nor am I a professional engine builder, so I'm discluding my own experiences and/or opinions, but would like to hear what the "experts" are saying on the topic and for what applications.

So what do you say folks?

 

Ahh yes Engine Balancing !! ""The Black Art"" as spoken from old time engine builders. This is one of my favorite subjects. In the therory that you have mentioned I'll comment as so. IMO the best way to attack the balance job would be to balance all of the internals as perfect as possible. Then to use all of the bolt on components as a fine tuning job so to speek as taking the balance job one step further. This will give you the smoothest final ***embly. If you look at some of the older H-P engine ***emblies you will see tweek drilling spots on flywheels and harmonic balancers and I'm sure this is what was being done. So if you start with the internals and balance them first you will not be relying so much on the external parts to do most of the work in the balance job but as to fine tune the job itself. Now if you have an external balanced engine that will be another story !! >>>>.

 

I've been very curious about this same subject with two build in the distant future. For the "perfect" balance, wouldn't the pistons/rods be weight matched, crank/pistons/rods balanced, then the flywheel, then harmonic balancer? Would it be possible to get rid of the factory harmonic balancer if the internals are all well matched? I would really like to hear from a builder that does top notch balancing on how the process goes, and what it runs to have something like this done.

 

Balance card. Weights on the Bobweight Total are the combined weights of the internals. The fact of the harmonic balancer being used depends on, If the engine is external balanced, personal preference on the fine tune of the balance as on an internal balance engine it is not needed to balance the rotating ***embly. Most blown injected engines had a crank hub and not a harmonic balancer. Now most of these engines had better crankshafts and along with the precise balance job they could get by without a vibration dampner >>>>.

 

"Black Art"....I like that... As said, there are two procedures; one for internal systems and one for external. We balance everything within the engine as a unit and like Engine Pro, we use the outside stuff as a fine tune trying very hard to keep all of the outside stuff at 'zero'. This makes parts replacement alot easier.

As for cost, we charge $175 for most applications. Mallory metal is extra.

This subject is not just for EarlyHemis, but all rebuilds.
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As far as the info I have gathered today, you guys are all pretty much on the mark. Im doing my first early Hemi rebuild currently (DeSoto), and I have learned an amazing amount of info on the whole process. Im at the balancing stage so I pm'ed Scooter this morning. Im running a HotHeads flywheel and balancer, so I called them. They said, as you all have stated as well, that the balancer and flywheel are to act as "neutral", or "zero". So I am going ahead with the internal balance this weekend. I guess Ill let you all know how it turns out soon.

 

Right on man!!! Can't wait to see that thing!! Keep us posted!!

 

Awesome Mark- I balanced my 392 without the flywheel and balancer. I wish I would have thought to bore the end of the crank for a pilot bearing though.. The crank might just have to come out again..

 

Good post. Yes the mallory metal is extra!! We run an innovators west balancer on our injected nitro 354. A blown engine will use the blower and belt to dampen most harmonics. Lippy

 

Just remember that a 3/8 lockwasher (weight 3 grams) swinging 6 inches from center at 8000 rpms will weigh 120 pounds. Scarry thought.

 

...but you still need to check the parts before bolting them on....

Good Luck.

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I'm no expert on balancing for sure.I have been told and read info by engine tuners saying balancing is far from an exact science.Some claim the perfect balance is only over a narrow rpm range.And others,Smokey Yunick was one,say the as the crank spins a huge m*** of oil is wrapped around the crankshaft in and ever changing pattern,and this must affect balance .

 

EXCELLENT TREAD!!! Thank God for Smoky Yunick, too!!!

Now, wouldn't this CALL FOR a baffled oil pan as an , almost, necessity????

Dead serious here, as what goes on in the oil pan -- seems to me -- could be a much overlooked issue (except, of course for racers).

 

****! I meant "thread," not "tread."

THREAD is TIRES!!!!

 

MR. FORD is building a Chrysler!?

 

swinging 6 inches from center
Which hemi has the 12" stroke?

A V8 cannot be "perfectly balanced", it's not a question of quality control, math or making the parts equal to .000001 gram: it's not possible. All you're going to get is a reasonable compromise that works well in your operating range.

And since the subject comes up every other day I have a question that no one responds to.

The top of the rod is reciprocating weight, and 50% (if using 50% factor) is added with piston etc. weight.
The bottom of the rod is rotating weight and is added to the crank weight.
The line separating the 2 halves is the center of gravity (balance point).
I can build an engine with the balance point inside the big end, showing that it has no reciprocating weight at all - clearly impossible.

Why is the center of gravity used?

The most common "answer" (non-responsive comment): "that's the way it's always done", "it says so in this article", "the balancing machine manual says so".

(please: Explaining how it's done doesn't address the question. I know how it's done.)

 

Can I phone a friend?

Okay, I'll throw this out there, but I'll warn you... I've had no coffee yet! At least this will get an interesting conversation going, related to this topic. I've never actually looked at a manual, but this is my guess...

First the definition of CG (have to know what we're talking about here):
http://en.wiktionary.org/wiki/center_of_gravity

Each of the parts mentioned has a CG; the piston rod, wrist pin, piston, and the crankshaft itself. The crankshaft being a little more complicated with it's multiple counterweights, will have several CG's on various planes (one for each counterweight).

NOTE: ***uming crankshaft has already been balanced for the below. Also ***uming the flywheel/flexplate and harmonic balancer have been omitted from the equation.

The idea of balancing a rotating ***embly is to get the combined CG of all the components as close to the axis center (center line) of the crankshaft. In order to do this, weight must be added to offset the CG related to the reciprocating components (piston, pin, and rod components). The closer the CG is to the center of axis, the more efficient the crankshaft can spin. Thus reducing vibration in it's dynamic environment.

The challenge is to find the ideal location of the counterweight to be added. That's where technology helps us, but the idea is to place a m*** of a given value at some distance away from the crankshaft center and at a certain degree in the selected plane (usually the counterweights of the crankshaft). Once the weight is place in the correct locations, theoretically, the reciprocating components should appear "invisible".

How was that? Am I close? That was my purely uncaffeinated response which is hopefully better than "no response".

 

The Reason you balance a crankshaft is ""When you rev your **** to 9000 RPM the crank doesn't fly out of the engine and break all of your cool ****"" Nuff Said ?? >>>>.

 

Word! I like that better!

 

Thanks for the effort, but not close.

 

Oh. Bummer. Do I get to see the right answer so I can edit my post to look like I actually know what I'm talking about? Should I erase my post all-together so there's no evidence left of my stupidity?

Did I miss-read the question?

 

I think you misread my post.
It's not a trivia question - it's an actual question. I don't know the answer, and I suspect no one else does, because the origin is waayyyy back there.
I think this is one of those cases where what seemed to be a good idea worked pretty well for most engines, and therefore must be accurate. In point of fact, this type of error, which is called a post hoc error (from the expression "post hoc, ergo propter hoc" - meaning "if thing #1 came after thing #2, then #2 must have caused it") is very common, hotrods, medicine, finance, blah. The fact that it's Latin tells how old this problem is: 2,000 years at least.
IMHO the 50% formula combined with the C-G rod weight produce an acceptable result not by physics but by co-incidence, because the range of end for end rod weights, rod ratios, etc. is narrow enough that gross errors are rare.
What I think is happening is that back when this was done by just drilling holes until the vibration reached its lowest level someone realized that % balancing was needed due to the V, then tried balancing a rod to get how much of each - but did it wrong. When 50% was used the results were very good in many V8 motors, leading to the false conclusion.
My hypothesis: the actual end for end rod weight depends on where each gram is vs. its position on the rod, but that the "split" isn't the C-G but half-way up the length on centers. For many engines, the difference is very small, but not always. Look at an H beam for a 5.15" Windsor - the center of the beam is almost as big as the pin, a small change makes no difference. Now look at a stock DeSoto 237 L6 rod - about the same big end but almost 3" longer - it looks like a quarter and a dollar on a pencil, any change at all move the CG an inch.
Even more complicated: supposedly, 50% is because of the V angle. If so, why isn't the size of the V important? It is, 60° motors are different so 50 isn't a magic number. Why isn't the rod thrust angle (based on ratio) a factor?
I think the real math will turn out to be the geo center rod weight × some other % factor, and also include the rod ratio and V angle, resulting in a number close to 50.
The real math may turn out to
This would explain why some engines appear to need over or under balance.

 

OHHH! Yes! I see I DID read your question wrong. I really did think you were just testing the knowledge of HAMBers. HA! Did my reply at least sound good? HAHA!

Very interesting question as well as hypothesis.

 

Balancing made simple Step 1, equal all parts in weight, record weight and place bob weights on rod journals, spin crank and observe out of balance, add or subtract weight from trouble spots, Now here's when it gets tricky, you can spin the crank in and out of balance phase but it still is not smooth. And a V-8 shaft is just 2 planes trying to fight each other for an axis of equal rotation. So this is when common sense takes over. My observation is when I start out with a crank that will literally fly out of the damn machine before drilling or adding weight I know I have a problem but when I'm done and the crank will spin like a GYRO I know that I have improved the balance of the rotating m*** greatly !! >>>>.

 

Fine and dandy, then after the ***embly is perfectly balanced and you have the center of gravity perfect and all the planets align, you throw a three disc clutch behind it, some go juice in it (nitro) and at least your perfectly balanced engine pieces all bounce evenly off the ground. Just kidding. A friend of mine always says...There's a hundred ways to skin a cat and the cat he don't like none of em. Pro 5X, I like it. Lippy

 

I get something a more like 72 pounds of force for this example.

While we are at it, we might as well mention that anything without all the m*** in the same radial plane can't be perfectly balanced. http://www.freestudy.co.uk/dynamics/balancing.pdf

Good question. I am going to take a guess and say that since the math doesn't work out on this deal that the 50% is a compromise. The force due to the reciprocating m*** is always changing magnitude and direction, which is another thing that makes perfect balancing impossible.

 

Ok, before I continue, I need to know what amount of time the piston is stopped at the top and bottom of the cyl at 8,000 rpm.

 

Damn this is complicated !! >>>>.

 

what amount of time the piston is stopped at the top and bottom of the cyl at 8,000 rpm

Is that a theoretical question?
Theoretical answer: zero, for each.

The force due to the reciprocating m*** is always changing magnitude and direction, which is another thing that makes perfect balancing impossible.

But the m*** magnitude doesn't change, and the change in vector (speed + direction) is predictable by simply geometry (it follows the locus of points dictated by the crankpin offset and rod length) - so the average change can be done by calculus.

More headache: for a common n = 2:1 motor, the 90° pin angle is 56% of the stroke length (never = to 50% no matter what unless the rod is infinite), so the motion is asymmetrical ATDC vs. ABDC (and asymmetrical in another way if pins are offset).

 

Panic...Your great and that is not sarcasm!!!!!!!!!!!!!!!
Duane.