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calculating bob weight

General engine tech -- Drag Racing to Circle Track

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talon
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calculating bob weight

Post by talon » Tue Nov 29, 2005 8:14 am

Good Morning Gents

when calculating bob weights for a 90* V8 - is the formula

100% of rotating weight or mass(big rod ends and rod bearings)+ 50% of recipricating weight or mass(pistons,pins,locks and rod small end)

have any of you ever had to balance the CHP SBF 347 cast or forged crank - I heard they were a bit of a challenge.

How difficult is is to make one of these externally balanced cranks a internally balanced unit?

Thanks
Jim

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Post by SchmidtMotorWorks » Tue Nov 29, 2005 10:36 am

100% of rotating weight or mass(big rod ends and rod bearings)+ 50% of recipricating weight or mass(pistons,pins,locks and rod small end)
That;s right.

Not familiar with the specific crank

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Post by Todd H » Tue Nov 29, 2005 11:27 am

On a related topic, have you guys ever seen a V8 overbalanced? We used to do this a good bit, drivers always claimed the engine was smoother at high rpm. Todd H

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Re: calculating bob weight

Post by SupStk » Tue Nov 29, 2005 11:43 am

talon wrote:Good Morning Gents

when calculating bob weights for a 90* V8 - is the formula

100% of rotating weight or mass(big rod ends and rod bearings)+ 50% of recipricating weight or mass(pistons,pins,locks and rod small end)

have any of you ever had to balance the CHP SBF 347 cast or forged crank - I heard they were a bit of a challenge.

How difficult is is to make one of these externally balanced cranks a internally balanced unit?

Thanks
Jim
Jim,
I haven't balanced that particular crank but most 28.5 oz/in fords can be internally balanced with four slugs of 1x1 mallory. Guess it depends a lot on your bobweight size.

I have overbalanced on my own engines. Used 53% as the factor... I did some research on this and it seems to be a universal speed secret because I haven't had any luck quizing others that have done it.

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Post by SchmidtMotorWorks » Tue Nov 29, 2005 12:01 pm

I have overbalanced on my own engines. Used 53% as the factor... I did some research on this and it seems to be a universal speed secret because I haven't had any luck quizing others that have done it.
I have never understood the reason for over balance. It doesn't reduce vibration or bearing loading, it makes both worse. It just adds more weight to the end CWs which are already too heavy.

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Post by Wolfplace » Tue Nov 29, 2005 1:16 pm

On the "overbalance" I use 52% for engines that are above 7500 most of the time.
This has no basis in anything scientific just what I understand a lot of folks I consider a bunch smarter than me are doing.
If you look at Alan Lockheed's "Engine Expert" program it recommends a factor that goes up with RPM as I recall.
Never actually asked him why but I would guess he would have an answer if quizzed.

I just wish I could remember all the questions I don't have answers too when I get to rubbing up against these smart guys hopin some of their intelligence will rub off,,,
So far it hasn't worked all that well,,, :roll:
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Post by Todd H » Tue Nov 29, 2005 1:44 pm

Well there must be something to it, we bought a rotating assy from Hendricks Motorsports once and it was overbalanced as well. They even gave us the balance card to prove it. FWIW, Todd H

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Post by SupStk » Tue Nov 29, 2005 1:52 pm

SchmidtMotorWorks wrote:
I have overbalanced on my own engines. Used 53% as the factor... I did some research on this and it seems to be a universal speed secret because I haven't had any luck quizing others that have done it.
I have never understood the reason for over balance. It doesn't reduce vibration or bearing loading, it makes both worse. It just adds more weight to the end CWs which are already too heavy.
Jon, It is one of those things I "just tried".

From what I understand all V engines are in an imperfect state of balance. If you balance to be smooth at lower RPM it will be worse at higher revs. A Harley i consider a good example, you can make it dead smooth at a particular RPM but everywhere else there will be a certain amount of out of balance (most of it noticeable). Guess the trick is to make it smooth where is is going to be run most of the time. That is the reasoning for over balance.

Now i will sit back and watch this thread, might learn something!!!

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Post by SchmidtMotorWorks » Tue Nov 29, 2005 2:09 pm

Well there must be something to it, we bought a rotating assy from Hendricks Motorsports
Oh, I wouldn't make that conclusion, some of the requests from crank customers (top Pro Stock pros) for the hot tip CW designs were absolutely laughable.

The most memorable was for a while a lot of pro stock teams were using center counterweights that were cut down to half height and positioned 120 degrees opposite the rod pin. All that did was add useless weight to the crank and a little twist to the center main. That idea died after about two years.

When you track the source of the idea down it was copied from an old moldex crank that had the big center CWs to make them cheap to balance and later it was rebalanced for a lighter bob weight. That crank happened to coincidently be in an engine that did well and the new "hot tip" was born that was nothing more than a copy of a hack balance job.

I think the over balance comes from the "more must be better" thinking. I haven't heard a better explaination.

Regarding Harley balancing, when some Harley engines seem to run smooth at certain speeds it is the result of the vibration frequency syncing-up with the resonance frequency of the entire bikeand engine mount. If you mount the engine on a very rigid dyno, you will find there is no smooth out speeds becuase the resonant vibration frequency is higher than the engine can rev.

The bearing loads do not experience lower loads at certain speeds in the middle of the range. They always increase with engine speed.

J.C.

another question for the balancing experts

Post by J.C. » Fri Dec 02, 2005 9:24 pm

when balancing, do you allow for the drag of the pistons and rings in the cyls? :?:

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Re: another question for the balancing experts

Post by SupStk » Fri Dec 02, 2005 10:25 pm

J.C. wrote:when balancing, do you allow for the drag of the pistons and rings in the cyls? :?:
Nope, you balance to weights, drag isn't considered.

J.C.

Re: another question for the balancing experts

Post by J.C. » Fri Dec 02, 2005 11:07 pm

SupStk wrote:
J.C. wrote:when balancing, do you allow for the drag of the pistons and rings in the cyls? :?:
Nope, you balance to weights, drag isn't considered.
another silly question, but without questions, there can be no answers. why not? and why not proportional to max engine rpm and ring and cyl drag? and bearing friction at max rpm :?: could this be why some over balance an engine?

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Post by SchmidtMotorWorks » Sat Dec 03, 2005 1:15 am

and why not proportional to max engine rpm and ring and cyl drag? and bearing friction at max rpm Question could this be why some over balance an engine?
Bearing friction has no influence on balance.

Over balance is out of balance.

Keep in mind, the ammount of overbalance that most people add is a very small percent of the internal imbalance of a V8 crank. Over balance can have no positive consquence to bearing loading or vibration.

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Post by Todd H » Sat Dec 03, 2005 3:51 pm

While we are at it then lets bring V6's in to discussion, Some of them IIRC are balanced at 50% and some of the old Busch stuff balanced at 36.6%. I never could get the logic of why the difference? Rpm's perhaps, Even vs. odd fire? Todd H

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Overbalance

Post by Adger Smith » Sat Dec 03, 2005 4:19 pm

OK Guys,
I'll put it in the universal language we speek here in Texas.
If you are shooting at a slow flying Quail you only "lead" a little.
When that faster flying Quail zooms by you had better "lead" him a bunch.
In my shop the same thing holds true with balancing.
A given Recip. weight gets heavier (inertia weight) as the RPM increases.
The % of overbalance is dependent on RPM & weight. It will make an engine run smoother at extended high RPM. I don't know of any formula to determine the correct % of overbalance. I have worked on it for many years by inspecting engines that come back to be freshened up. Bearing wear seems to be the greatest tell-tale, along with oil temp, of correct overbalance. If you are using an engine that operates from 7,400 to 9,600 who cares if it is smooth at 4,000 RPM? I tend to use smaller %'s than most. I would rather have a counterweight that is a little light than heavy.
Just my .02
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