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bbc INTERNAL OR EXTERNAL BALANCE?

56K views 21 replies 15 participants last post by  chevy_69_chevelle 
#1 ·
I havent bought my crank rods or pistons yet and have seen post here about internal and external balance

price difference between the two and advatnages disadvantages between the two...

this question is based on a street/strip engine that will see less than 1000 miles per yer and hoping to build this one time and keep it alive for as long as possible with minimal maintenance besides checking/adjusting valves once a year. and on pump gas my compression will be under 12 to 1 and hoping to get around 10.5 to 1 or as close to 11 to 1 as possible
 
#2 · (Edited)
396/402/427 BBC are internal ballance & 454 and larger BBC are externally ballanced .

But with that said i feel its a little $ very well spent in the scheme of things meaning motors cost a lot of money to rebuild and the few hundred to ballance isnt that bad .

I would do it,any motors i have rebuilt that were ballanced (recipricating assembly) felt/ran very smooth but when someone would chince out and not want me to have it done when doing a rebuild for them i have noticed those motors didn not run/feel as smooth.

Ask our resident engine builders/machinests experts like MikeWolfplace/billK/gofast/Carl from CNC what they think.

You could try a search titled ballancing becausee i beleive there was the same question asked/posted here a while back and these experts already chimed in with their good expericned answers.

Just my 2 cents.

Scott
 
#3 ·
Internal balance is easier to find parts for, and to interchange.

As stated, the short-stroke motors are internally balanced, and the 454 is externally balanced, from the factory.

The reason the 454 is externally balanced is that there is not enough room between the crank centerline and the bottom of the pistons when they're at BDC, to fit a large enough counterweight, without moving the wrist pin up and thereby compromising the ring package (in the factory's opinion, at the time it was originally designed in 1970 or so), either by spacing the rings too close together, using narrower rings, making the oil ring overlap the pin bore, or a combination of those.

A 454 or larger can be internally balanced by using longer rods and an appropriately designed crank. Obviously the wrist pin must be MUCH higher up in the piston; and the piston made MUCH less tall. With modern rings and care during assembly though, the ring sealing issues are minimal. People do it all the time, especially in small blocks, where the 400 has basically the same situation.
 
#4 ·
Hi! have a Q on this had rebuilt my 396- .060 over steel crank cast rods -heads installed 2.18/1.88 valve new guides and sprng,vlvs shimmed. 7/16 psh rds. anyway did have it balanced G.M. balanced shop says checked # of 1 rod & # 1 piston set to get bob # and I seen were he added weight to the crank in a couple of the counter weights. and got a print out of all the weight and reciprocating factors. He stated that would be good for what I was doing running the streets. Ths is better than doing nothing I would think!
 
#7 ·
Hi! have a Q on this had rebuilt my 396- .060 over steel crank cast rods -heads installed 2.18/1.88 valve new guides and sprng,vlvs shimmed. 7/16 psh rds. anyway did have it balanced G.M. balanced shop says checked # of 1 rod & # 1 piston set to get bob # and I seen were he added weight to the crank in a couple of the counter weights. and got a print out of all the weight and reciprocating factors. He stated that would be good for what I was doing running the streets. Ths is better than doing nothing I would think!
=
Not even close in my world :sad:
As soon as you change parts in an engine there is no such thing as a so called "Detroit balance"
GM has specs they use for FACTORY PARTS
Now even though the tolerances are very loose by performance standards they do have a tolerance
What do you suppose happens when you install aftermarket parts that are not somewhere near the original weights?
And what is the point in calling something balanced if you do not check & match all the parts not just one
I am sorry but to me this should not even be called balanced

I do not care if you buy a $300 crank or a $3000+ billet from me
If I am balancing your rotating assembly the same rules apply or I don't do it.

As for internal or external
If at all possible I balance everything internally.
While there are many engines out there externally balanced that appear quite happy if you understand balancing you will understand that the closer you balance to the point of unbalance the happier the part will be

If you do not believe this take your trick shiny wheel & static balance it by adding weight to the inside when it wants weight to the outside
On a static balancer it is fine
Now stick it on your car & go drive the thing & tell us what happens,,,
This is only 8 inches or so from the point of imbalance

What do you suppose a crank does when you hang the weight off the end ;)
You may not feel it but your bearings can
 
#8 ·
I would go internal. I've had both but I think it works better when you put the balance weights closer to the imbalance. Also it makes it easier if you every damage a flywheel. You don't have to worry about balance, just go neutral.
 
#9 ·
I have a question. David Vizard in his small block book says that an unbalanced Rotating assembly does not reduce the longevity of the engine. is this true?

I just balanced my rotating assembly then decided to change the pistons.
these are 640 + 150 pin = 790
the sheet from the old balance says those were 893.1 for piston and pin.

will I reduce the life or performance of my engine if I just stick the new pistons on there?
 
#10 ·
I have a question. David Vizard in his small block book says that an unbalanced Rotating assembly does not reduce the longevity of the engine. is this true?

I just balanced my rotating assembly then decided to change the pistons.
these are 640 + 150 pin = 790
the sheet from the old balance says those were 893.1 for piston and pin.

will I reduce the life or performance of my engine if I just stick the new pistons on there?
=
I do not agree with that at all but what do I know :D

Considering even the factory has tighter tolerance than 100 gms by a bunch
I don't think I would just stick them in.
The factory is ± 15 or so grams as I recall
I don't really remember as I don't care what the number is but I know it is not 100 grams, that is a bunch
They even change the balance factor on the same casting crank for different weight pistons
Like the 366 truck vs the 396/427
Or the 305 vs the 350
Same crank forgings or casting, different counterweighting
I don't think they do this because they have nothing better to do with their time ;)
 
#13 ·
My 461 is externally balanced still. I picked up a factory GM 1053 crossdrilled steel crank that was like $300. It just didn't make sense to spend upwards of $300+ to internal balance a stock GM crank. If I was gonna spend that kind of coin, I would have purchased a lower end aftermarket crank that was already setup for internal balancing.

You need to consider what parts you have and how much tungsten it's gonna take to intenal balance as the slugs aren't cheap.

My little 461 has been running 7600+ rpm for several seasons now and knock on wood holding up fine.

In an ideal world, yes, internal balance for sure. But with my setup, I don't regret the decision I made. Even if the crank cracked down the road, I still doubt you could blame the balance method, rather the material, lack of journal radius and the crossdrilled configuration which is sometimes thought to weaken the crank.
 
#15 ·
One more thing about "external" balance:

The factory chooses a certain amount of counterweight to add to the flywheel (flexplate) and/or damper. That weight is the same for every engine of that type--it's a standard amount of offset weight.

Each engine gets balanced at the factory, and they carve additional weight out of the crank counterweights as needed for that particular engine. They do NOT screw with the "standard" out-of-balance weight at the damper and flywheel.

Some "engine balancers" weld or grind weight off the flywheel or damper rather than doing it right by adding/subtracting at the crank counterweights. That upsets the "standard" weight--and makes it impossible to replace the damper or the flexplate with another unless you pay to have the new one matched to the old one.

DO NOT allow the people doing your engine balance to screw up the damper/flywheel!
 
#16 ·
After my experience I would HIGHLY recommend internal balancing.

below is a picture of the results that can happen from externally balancing a motor.


This motor had seen 300 passes at a maximum RPM of 7200 in the lights, shifting at 6300rpm.

The counterweight had broke off the flexplate and caused a vibration in the transmission case which in turn did the damage that you see there. If the motor had been internally balanced, this wouldn't have happened.

I also pulled the motor and put a torque wrench on all the rod and mains to see if they had loosened up and low and behold I had one rod bolt that was loose.
 
#18 ·
The best thing I am saying to do is remove the weight from the damper (if removeable, like mine is) and get a flexplate with no counter ("out-of balance") weight on it and balance it all in the crank, therefore eliminating the change of a weight flying off the flexplate
 
#19 ·
Ok well here is my thought I saved money by buying a GM forged nitrited and crossdrilled crank got it for $250 new. Iam having everything internal balance because I am running 14:1 comp and I have a very heavy foot lol so my builder after seeing me race the last blower motor car told me with my tendency to beat on stuff to go internal balance as the motor would last longer. so I ponied up the $200 he charges to have it done. now I still think that is cheaper then some of the aftermarket forged cranks.so take it for what its worth. my 2 cents
 
#21 ·
Mike, sorry to see your misfortune, but that's not the result of having an externally balanced motor, it's the result of having a counterweight come off. If you'd had a stick car or your flexplate hadn't "come from together", you wouldn't have broke.
That said, 7200rpm is higher than I'd want to run an externally balanced engine.
 
#22 ·
Busted I agree with you. :) All I was saying is that internally balanced = no counterweight on flexplate = no chance of counterweight coming off haha anyway my motor has gone as high as 7800rpm but I only did that once. Surprisingly it was still pulling at that rpm.

After talking to my tranny guy (NHRA division 7 Stocker) he said that this is a typical thing he sees with drag cars that have externally balanced engines (i.e. flexplate weights coming off).

Live and Learn . I am actually lucky about this. I didn't actually notice anthing wrong with the car until I pulled the starter to get the headers out since I had 5 weeks between races and was getting some Dawson Racing Headers made for it. While I had the starter out I noticed a dent in one of the shims and looked around and saw the flexplate weight missing. Then I saw the crack. If I hadn't decided to get the headers done, I would have been waiting for the next race at which point, I would have done a lot more damage.

Next time its internally balanced for me.
 
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