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Topic Review (Newest First)
03-14-2007 08:38 PM
chevy boy
balance confussion

my engine is brand new its not screwed is it
03-14-2007 08:31 PM
chevy boy
balance confussion

hi i have a 1990 chevy 305 and iv heard that 86 and later that the motors are external balanced and have a one peice rear main seal which mine does my question is what will happen if the motor has a internal balanced flex plate on it what kind of damage am i lookin at the engine has 3400 miles on it and i just found out that it might be the wrong flexplate but the plate that is on there has a good size balance weight that doesnt look like it would be small enough to balance just the flexplate itself but iv ben told it was internal balance but iv seen the same exact flexplate with the same wieght for external applications some help on this would be so great im loosing my mind on this one
10-30-2003 09:54 PM
383 stroker balance

Hi everyone, my friend has just had a 383 built from a local engine builder......he was given the wrong flywheel for his Tremec 5 speed transmission. The problem is, we can't find the right flywheel weights. Can someone tell me exactly what to do?
I'm starting from a motor thats in the car, has been driven, now the tranny is out, we have no flywheel on the car. Can someone just tell me exactly what to do with the pressure plate, what flywheel to get, etc...I'm desperate. The engine builder built my motor too and he did a good job, but when we asked him questions he got very mad, and very busy all of a sudden. He proceeded to talk about my friend and I to the local car guys, and he even threatened us! we need some help here thanks a lot....
05-13-2003 11:20 PM
Ken-96GMC K1500 SC What about this?

03-03-2003 03:19 AM
4 Jaw Chuck You are correct in the fact that the balance will change.

Flywheel diameter of course would change the balance, so does the thickness of the flywheel but the thickness change is tiny compared to the length of the crankshaft...this is assuming a same diameter flywheel. Am I missing something from the original question? Is the flywheel a different diameter?

When the factory builds engines they don't balance clutch engines different than automatic equipped vehicles, the flywheels are the part that accomodates that with weights placed strategically on it's diameter.

The amount is so tiny that it would probably entail drilling and removing less than a half gram. At some point you have to apply a tolerance to your machining operation? No?

I guess it all comes down to what tolerance for perfection you subscribe too.
03-02-2003 06:40 PM
reddwarf [quote]Originally posted by Deuce Roadster:
I have changed flywheels (153 to 168) after balancing a assembly and the balance changed (still in the balancer) SO let's just agree to disagree.

Well if you move the same balance weights farther away from the crank center then of course the balance would that what you're saying?
03-02-2003 02:46 PM
Deuce [quote]Originally posted by 4 Jaw Chuck:
<strong> What about windage and the half pound of rotating oil mass attached to the crank at all times. All these are factors that affect balance that are not accounted for in a simple bobweight calculation.</strong><hr></blockquote>

IF you will go back and look at the Bobweight has 3 grams added for oil on the pistons.

I cannot change your mind and I am satisfied with what I have said. I have changed flywheels (153 to 168) after balancing a assembly and the balance changed (still in the balancer) SO let's just agree to disagree.

Have a nice day !

[ March 02, 2003: Message edited by: Deuce Roadster ]</p>
03-02-2003 02:32 PM
4 Jaw Chuck Without trying to sound like I'm trying to start an argument Deuce but what you have done with your 283 is make the crank lighter, thats all...and made the counterweight inertial masses equivalent for that particular bobweight.

We talk about balance like there are not other forces that affect the balance of the engine, combustion pressure, quench clearance, piston diameter, cylinder wall friction I could go on and on. How is your bobweight determined? By using half the rod weight plus the pin right? So essentially you are treating the rod as two separate pieces, not really very realistic since every one knows rods are one piece. What about windage and the half pound of rotating oil mass attached to the crank at all times. All these are factors that affect balance that are not accounted for in a simple bobweight calculation.

In real life of course this is all we can do with the mechanical components to effect any change, but really when you do flywheel calculations and bobweight calc you are assuming a great many things that have no basis in reality. The original engineers who designed your engine took a great many things into account when they designed the original crank throws and much of the knowledge and rules of thumb were developed in the rail era, funny many things have not changed all that much.

Did you notice that the amount of out of balance for your original spin was 200 pds at 6000 rpm? Funny but that just about coincides with combustion chamber pressure during compression and the max safe rpm the motor was designed for?

Have you ever read Smokey Yunicks book Deuce?
03-02-2003 02:09 PM
gaino If it is a 400 crank... you better put external balnce on it .. every 383 or 400 is identified by the balancer..i will
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

03-02-2003 12:52 PM
CAMARO 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.
03-02-2003 11:42 AM
4 Jaw Chuck 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">
03-02-2003 11:28 AM
reddwarf 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">
03-02-2003 08:32 AM
4 Jaw Chuck 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">
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>
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