Crankshaft balance Physics question

[west coast]

My physics question is which is a better route to go with a crank? I know if two cranks weight the same and one was externally balanced and one internally balanced you would choose the internal balanced one. So here is where you have option on balancers for the engine, internally balance you usually have more option on the weight of the balancer. Being in a performance based classes you want things as light as possibly to a point, you can always go to far on weight reduction. So for this question I am looking at total rotating weight crank + Balancer + flexplate or flywheel lets just ignore the flexplate end for now. I know they will be different weights but not enough to make up the difference in the option available in the balancer weight because the weight on the flex plate will be farther away from the crank center. Lets compare two different cranks by total weight first, is a lighter crank that is externally balanced crank weight 40lbs with a balancer of 11lbs total 51lbs. Second crank internally balanced crank weight of 50lbs and balancer that weights only 6lbs for a total of 56lbs. So which would be the choice of combination for better performance. I am looking for the science behind this to understand it. A friend of mine has a doctorate in physics and is a professor at a college I am going to ask him this question also. I am no expert at this just trying to understand. this question would apply to the big 3, GM Ford and Mopar they all have externally balance cranks that could be nuetrally balanced.

[Adger Smith]

James,
In the racing world you stay away from the external balance at all cost.
An external balanced crank can be balanced while it is sitting on the engine balancer. It is when you put it in use and the 100+ HP per cylinder power strokes are bending and twisting the crank all different ways the lateral run out of the external weight becomes questionable. There goes the balance!
You also have the "Hit" that the torque converter puts on the flex plate , yes, flex plate is the name of the thing the external weight on the back of the engine is called. Yes, it flexes and there goes the lateral run out of your weight. Your weight conclusions are based on a correct theory, but that is splitting hairs and with a race car it would brobably be a wash with the other balance/flex issues. What happens in the lab under controlled conditions is not what usually happens in the real world. Over the years Racing has shown me just that! Ask any racer that has spent thousands of dollars on the gundrilled, lite weight, exotic weight everything just to gain nothing. I hope this didn't start a firestorm about balancing! Yikes!

[art leong]

Adger you can tell them about my "super trick LIGHTWEIGHT" crank shaft.
And the myriad of problems it gave me.
It was balanced. But the 4 cylinders don't use bobweights, like the V motors when balancing.

[Adger Smith]

That was a lesson in extreme flex and twist! Yep,
I forgot about that one! :~)
Art, That is a good example of what he is wanting to do, but it is an extreme example...!

[SSDiv6]

Originally Posted by Adger Smith

James,
In the racing world you stay away from the external balance at all cost.
An external balanced crank can be balanced while it is sitting on the engine balancer. It is when you put it in use and the 100+ HP per cylinder power strokes are bending and twisting the crank all different ways the lateral run out of the external weight becomes questionable. There goes the balance!
You also have the "Hit" that the torque converter puts on the flex plate , yes, flex plate is the name of the thing the external weight on the back of the engine is called. Yes, it flexes and there goes the lateral run out of your weight. Your weight conclusions are based on a correct theory, but that is splitting hairs and with a race car it would brobably be a wash with the other balance/flex issues. What happens in the lab under controlled conditions is not what usually happens in the real world. Over the years Racing has shown me just that! Ask any racer that has spent thousands of dollars on the gundrilled, lite weight, exotic weight everything just to gain nothing. I hope this didn't start a firestorm about balancing! Yikes!

X10!

There are also a lot of dynamics involved that are not constant due to the acceleration rates of a drag racing engine. Bobweight numbers and size also an effect on the inertia and acceleration of the crankshaft.

Like Adger and Art have shared, a crankshaft design may work in one application and may not work in another.

So in a nutshell, go internal balance in a race engine and leave external balancing to low RPM street engines.

[ss3011]

Another plus to having an internally balanced engine, is when you have to replace a flexplate or a balancer, you don't need to rebalance the assembly. Externally balanced assemblies are balanced with the flexplate and balancer bolted on to the crankshaft. Even though the flexplate or the balancer are not modified during the balancing procedure, there maybe slight differences with the replacement parts which could ruin the balance. You have a better chance with a replacement parts that are neutral balanced.

[Mike Taylor 3601]

Any one that has ever ran ext. balance like 400 sbc or 454 bbc can tell you how much better they are int balanced, cranks last lot longer.
I would rather have heavier crank internal balanced than lighter one ext.
Counterweight placement has alot to do with harmonics also,like Art spoke of I think his crank had none are little counterweight,you can have inline crank balanced without counterweights, but it does'nt work and won't last.
I have noticed some of the China cranks when I get down to 3-4 grams I have to work to get them on down to around 1 gram or less, I believe it has to do with counterweight placement,don't have that problem with good crank a Callies or Crower etc.
Mike Taylor 3601

[west coast]

Thanks everyone. This is a stocker application I wasnt talking about lighten up to max cranks or aftermarket cranks. Some application your option on what crank to use some are lighter than others. So looking at haveing an external crank balanced to be internal with the use of mallory. I would love to have 100hp per cyl. Now dont all cranks flex some? If so it would have less affect on a internally balance crank?

[Adger Smith]

James,
Yes all cranks flex. How much depends on a lot of factors. Yes an internal will probably flex less when all else is equal. I'll give you a real life example. There are a lot of guys on this forum that have been around long enough to have lived this example. Small journal 327/283 style cranks needed a lot of clearance to keep bearings alive and friction to a min.
Along came big journal cranks. GM added 1/4 in in stroke from 327 to make the 350. GM engineers decided they needed more beef in the overlap of the rod and main. so they added material to the mains and rod journals of the 350 cranks. Instantly the sharp engine guys say no need for those gigantic rod and main clearances. The cranks didn't flex as much so no need for it. Now we have better materials and the clearances are even tighter allowing the use of 0 weight oils. Another thing you will see in the to be heavy or lite on crank weights is what happens at a gear change. The heavier crank can actually act like a flywheel and put some inertia into the pull off the gear change. You have the engine at, say, 7,000 and you make a shift. The shift tries to pull it back to converter stall and the weight of the crank acting like a flywheel tries to keep it @ the 7,000 RPM, but can't so it's effort transfers to the tq converter where the converter does it's job and multiplies torque... see. Now what you have to decide is if that extra Tq/inertia converted at the shift is more important than the few pounds of weight trying to pull back up to the next shift point. Since this is your first, just build and don't get caught up in all the trick of the week science. You will have a steep enough learning curve without working to reinvent drag racing. You will always hear racers talking about lite weight because it is easier to understand the effects, but that is "The rest of the story"...
BTY: You had better read the rules about weight of parts in stocker engines. There is a clause in the rule book about excessive weight removal, even for balancing. Wrist pin and above is where you need less weight. All that weight stops and starts twice with each stroke.

[west coast]

Not my first rodeo. Know the rule book very well. Have not had problems going back into the converter on my current combo, graghs from the data logger are good.