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Old 05-27-2003, 03:51 PM
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Question Internal vs External Balance

Ive got a desicion to make here and Ive got to make it soon. What I have is a 2.8 v6 Chevy.
I just bought a new shortblock and am using upgraded parts from my original 2.8 to complete the engine.

The problem for me is in the crankshaft. The new shortblock has a 817 nodular large main crank which is externally balanced. The crankshaft features two large counter weights on each end and is balanced on the flywheel. This is NEW.

However on my 1992 Spec block out of my S10 it contains an Large main Internally balanced nodular crankshaft which has multiple weights.

Now Obviously I cannot place the old flywheel on the new shortblock or else my teeth would rattle loose from my gums and I would have about a month life for my engine.

In mid 1987 GM Introduced the GEN II 2.8 L which featured the aluminum heads and MPFI for FWD applications. The Gen II motors featured DIS ignition and were triggerd by a reluctor wheel on the crankshaft. The reluctor is simply a large round throw in the middle of the crank and has notches on it to referance timing to the trigger and ecm.

Now on a RWD 2.8 after 1987 the same crankshaft is used and is Identical to the GEN II except the Reluctor throw has no timing notches cut into it.
Rwd being s10's, f -bodys,etc.

Mine being a 1992 s10 contains this style crankshaft and thus this is the only information that would suggest why the crankshaft was changed to Internal balance from External. It makes sense since producing two seperate cranks for the same blocks would mean more production cost for GM at the time.

(Note , these are not to be confused with the problem prone early crankshafts with the smaller mains that led the intial negitive opinion of the GM 2.8 v6)

So heres the deal , I am not a engineer. And I did not go to college and was never fond of Mathmatics.
My question for anyone out there is this.

What are the pros and cons of Internal and External balanced engines?

Is there a difference or do they both provide the same result.

For example does the internal mass of the internal balanced crankshaft have any negitive or positive affects and vice versa for a crankshaft which is externally balanced and contains mass at the ends of the crankshaft.

P.S. If this were a non performance engine I wouldnt care, but it is.
No 2.8 flames I could care less if you think you know something negitive about the 2.8 , keep it to yourself. <img src="graemlins/crash.gif" border="0" alt="[crash]" />

As always thanks - Will

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Old 05-27-2003, 05:08 PM
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as a general statement,you wouldn't notice any difference between an internally or externally balanced motor. The only difference would be bearing loads under extreme rpm, most modern engines can handle the loads with ease.
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Old 05-27-2003, 06:43 PM
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Bob is right, you won't notice any difference in your application.
Any time you have a shaft {like a crankshaft} and there is an imbalance in it, weight can be added to it to balance it while it is stationary {called static balance}. Static balance would be similar to putting a tire and wheel on a tire balancer, but instead of spinning the tire, you would just let gravity do it's job and the heavy end would turn until it was at the bottom. To correct the problem, you put a weight 180 degrees from the heavy end, now if the tire is balanced it will not turn under gravity while it is stationary.
Automotive crankshafts can be balanced in a similar fashion, the corrections to the imbalance can be made on both ends of the shaft {which would be external balancing}, and in most instances this works fine, but in high performance or racing applications this is not the hot set up. When the engine that is externally balanced is turned to high RPMS and high horsepower, the flaw in external balancing becomes evident. There are weights at each end of the shaft that are unbalanced, and one end can actually work against the another. Have you ever seen a 400 small block snap the front snout of the crank just behind the harmonic balancer? This is why it happens. An internally balanced engine in my opinion is a better deal in extreme environments, but in your case the externally balanced engine will do exactly what it's designed to do, give great everyday service in a commuter.
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Old 05-28-2003, 01:25 PM
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Ok.

The motor will be spun to 6k.

Its a roots blown 2.8 v6 , Im definately leaning towards the Internal balance unit now. But I belive your right, either unit should handle 6000 rpms, the internal is probably what I'll use.
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