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CAMARO 03-01-2003 12:19 PM

383 stroker balance
Hello, Just wondering if anyone knows some info about balancing engines.When I recieved my externally balanced 383 from my engine builder I noticed that it had a flex plate instead of a flywheel like I ordered. He claimed that it could be easily resolved since he had all the "bob" weight measurements he took from the motor. So he just balanced a flywheel using these weights. I haven't fired the motor yet, just wondering if I truly have a balanced motor?

[ March 01, 2003: Message edited by: CAMARO ]</p>

4 Jaw Chuck 03-01-2003 03:34 PM

Yes, his weight calculations will adjust for your particular situation.

Deuce 03-01-2003 04:28 PM

[quote]Originally posted by 4 Jaw Chuck:
<strong>Yes, his weight calculations will adjust for your particular situation.</strong><hr></blockquote>

In my opinion .......NO

Bobweight does not include the flywheel (or flexplate)specs.

Bobweights (the weight of the pistons/rods/rings and rod bearing) are placed on each throw of the crankshaft to be balanced. All of these items were made to weigh the same prior to this step. The harmonic balancer, the lower front pulley, the bottom timing chain gear and the flywheel are installed. The pressure plate also. The entire rotating assembly is then spun and depending on certain factors......weight is added or drilled out. Most balancers add weight or drill out on the crankshaft. Sometimes weight is added to or subtracted from the flywheel or harmonic balancer.

He CAN duplicate the Bobweight but without your crankshaft and other pieces ......he cannot balance a flywheel to your rotating assembly.

Kind of long and involved...... Hope this helped.

:D 03-01-2003 04:48 PM

my balance guy records all specs and can rebalance any replaced part back to its orig specs, especially balancers and flywheels, he does this on a regular basis for the drag race people who change flywheel weights.

Deuce 03-01-2003 05:05 PM


I have read your answers and have always always found your answers to be informative and CORRECT.

Could you explain out he balances the flywheel without any of the other pieces? Not saying it cannot be done........just want it to be explained HOW to me. Inquiring minds want to know.

I did a lot of engine balancing years ago when I worked at the local speed shop and there may have been advances made that I am not aware of :D

Deuce Roadster....

4 Jaw Chuck 03-02-2003 01:34 AM

The main reason why doing this is acceptable is because all flywheels come static balanced with correct bobweight already. Since the flywheel is a narrow disc there could not be much of a dynamic imbalance especially on a billet or even machined castings.

Sure you could gripe about it not being actually attached to the crank during the balance procedure but in reality it does not matter one bit.

The physics behind it deal with masses of centroid shaped bodies and other phenomena when approaching thin bodies or bodies shaped like flying saucers (there is a reason why there shaped like that you know! :p ). Flywheels are very thin compared to a crankshaft.

We can get further into the discussion with actual calculations if you like but let me get my books first so I don't mislead you. You would see that the equations end up as very minor amounts &lt;0.5 gram, even considering ridiculous porosity in a casting. Billets and forgings are nearly perfect anyway if you run an aftermarket flywheel, you get what you pay for.

Maybe get him to throw in a better flywheel to satisfy your need for perfection? <img src="graemlins/mwink.gif" border="0" alt="[mwink]" /> :D

Deuce 03-02-2003 03:01 AM

[quote]Originally posted by 4 Jaw Chuck:

Sure you could gripe about it not being actually attached to the crank during the balance procedure but in reality it does not matter one bit. Flywheels are very thin compared to a crankshaft. You would see that the equations end up as very minor amounts &lt;0.5 gram, even considering ridiculous porosity in a casting.</strong><hr></blockquote>

IF we were talking about the differences between flywheels OR the differences between flexplates.....there could be a small diffence (maybe grams) but he is talking well over 10 pounds or more difference between a flexplate and a manual shift flywheel. Most stock 168 tooth (14") flywheels weigh over 30 pounds. :D

CAMARO 03-02-2003 03:53 AM

It seems there are many different opinions on this subject. Let me just ask this, Should I have changed my balancer when I went from a flexplate to a flywheel?

Deuce 03-02-2003 06:08 AM

[quote]Originally posted by CAMARO:
<strong>It seems there are many different opinions on this subject. Let me just ask this, Should I have changed my balancer when I went from a flexplate to a flywheel?</strong><hr></blockquote>


:D 03-02-2003 08:28 AM

the weight of the flywheel means nothing it is the amount of the imbalance that is recorded, if you have this figure you can balance a 14" flexplate to a 35" 100lb. wheel. its the amount of the imbalance that you are duplicating, not the size or weight of the object.
a good balance shop keeps records of all work done and all weights involved, using these figures a new component rematch is a breeze. In the case of the heavy wheel, it is mounted to a mandrel and spun up, then modified to match (drill or weld) the exact imbalance of the original.

[ March 02, 2003: Message edited by: ]</p>

4 Jaw Chuck 03-02-2003 08:32 AM

Deuce? how would the total weight of the flywheel affect the balance of the engine? There would be a change in inertia and harmonics frequency but not balance. Harmonics you can't do anything about and inertia is a fact of life, this would not change your balancer weight at all?

Can you explain, I'm confused? <img src="confused.gif" border="0">

reddwarf 03-02-2003 11:28 AM

Seems like there might be some confusion here over internal vs. external balance.

His 383 obviously uses the 400 crank (not a dedicated internal balanced 383 crank). It is EXTERNALLY balanced.

That said, I don't see why it would not be balanced by adding the same amount of weight that had been added to the flex plate since a flywheel with no balance weights is NEUTRAL <img src="confused.gif" border="0">

4 Jaw Chuck 03-02-2003 11:42 AM

I think he is concerned that the thickness of the new flywheel would cause a dynamic imbalance due to the spacing of the weights between the two types. They are a different shape and he does have a point, but the effect is extremely small if you do the calcs (I used assist balancing GM locomotive cranks and blower rotors), I wouldn't worry about it. Balancing is not an exact art and the fact that a crankshaft is flexible except when installed makes it all really a guessing game of pin end weight percentage changes, over balancing and underbalancing blah, blah, etc.etc.etc.

Too much importance is placed on balancing engines especially in magazines, matching piston weights and rods is all you ever need and as long as your crank/rod combo is compensated for (you said he balanced the rotating assy) what more could you ask for, the flywheel is a non-issue if you ask me.

Correct me if I'm wrong on any of these assumptions. <img src="confused.gif" border="0">

CAMARO 03-02-2003 12:52 PM

I love this forum. Thanks guys for all tour input. I feel much better now, I was worried to death that my engine was gonna come apart from shaking. The general feeling I get from everyone is that he did it correctly or it's close enough.

Deuce 03-02-2003 01:58 PM


This is the paper on my little 283 for my 40 Ford coupe. This is the inital spin. This is with the 4 Bobweights, flywheel, pressure plate, harmonic balancer, lower front pulley and lower timing chain gear installed on the crankshaft. This is everything that rotates.

Now the sheet after the crankshaft, flywheel and harmonic balancer are balanced.

Notice that the out of balance at 1000 RPMS on the inital spin is 5.44 lbs. Then compare it to the 5000 RPMS out of balance after balancing the crank, flywheel and harmonic balancer (4.87 lbs.)

It is less at 5000 RPMS than before at 1000 RPMS. The Bobweight did not change. All the pistons, rods, rod bearings and rings were already made the same weight (Bobweight).

What did change was the crankshaft throws (drilled) and the flywheel drilled and the pressure plate rotated 120 degrees to a more in balance location.

To try to make this simple....... when you speed balance a tire and rim on your balance it to the hub and rotor also to get the best balance.

When you just spin the rim and tire on a tire machine...... it can be balanced but MAY not ride smoothly because it is out of balance after it was bolted to the hub and rotor.

I know of nobody who records the weight of a flywheel or flexplate after balancing.

A 383 is externally balanced. This means that the rotating assembly is balanced by the harmonic balancer and flywheel. Just try a 350 flywheel. HA HA HA


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