Post
by amcenthusiast » Sat Dec 23, 2017 11:31 am
Back to the external vs. internal balance debate:
AMC's high performance modifications book named "Performance American Style" recommends that all external balanced '66-'91 AMV8s should be internally balanced for higher performance use.
"P.A.S." recommends swapping the forged steel 390/401 crank and rods into the cast nodular iron crank and rod 290/343 & 304/360 engines and even provides a chart showing the alternative stroker combinations for the smaller CID engines.
Past 6500 rpm, "P.A.S." recommends a custom billet steel crank should be made & recommends Moldex Crankshaft Company by name. And they recommended having custom rods made by a company like Carillo.
This makes a strong statement against running an externally balanced crankshaft by quoting the AM engineers who wrote the book.
Furthermore, whenever I search the Internet for information on crankshaft balancing, the articles seem to unanimously agree that internal balancing is preferred vs. an externally balanced rotating assembly.
This appears to be indisputable, without need for further explanation.
So why would a manufacturer ever choose to make an externally balanced rotating assembly in the first place?
My first reaction to answer this question is 'to reduce the cost of making the engine; they can eliminate the time it takes to detail balance the internal parts'.
But is that true? That's the whole answer; cost reduction?
Nope. And here's an answer to what 'they' never say:
There is a very real advantage to an external balanced rotating assembly, even a power making advantage... for engines that are run in the rpm levels below the stock factory 5000 rpm redline.
What all the 'engine balancing articles' never say is this: when we take a balancing weight and move it further away from the center axis of the crankshaft, that weight effectively becomes heavier. By Newton's Laws, this means the crank and rod rotating weight can become lighter and have a lower moment of inertia and this will require less power to accelerate, reducing parasitic loss, with a corresponding increase of the engine's efficiency level.
Put in layman's terms, 'external balance' is a sneaky way to lighten the rotating assembly so the engine can 'wind up' faster.
I don't think you'll read this anywhere else (I never have, anyway)
The basic understanding of the concept can be described by visualizing two same-weight fat boys sitting on a see-saw, each one the same distance, about midway from the fulcrum.
As such, the see-saw is balanced.
(move one fat boy in or out on the see-saw and it easily goes out of balance)
~But~ we can alternatively balance the see-saw by replacing one fat boy with a lighter weight boy, by moving the lightweight boy further out -further away from the fulcrum.
The lighter boy, seated further away from the fulcrum has more leverage which makes him effectively heavier, but the total/combined weight becomes lighter.
This is the basic concept anyway and the net result is a lighter rotating assembly having a lower moment of inertia which is easier to accelerate, which is based upon Newton's Laws & the rather simple geometry of mechanical advantage.
Moreover, by reducing the total weight of the rotating assembly, the engine becomes a few pounds lighter which increases the basic power to weight ratio of the car.
The main problem is 'crankshaft flex'; more flexing strain is placed on the beam of the see-saw since more leverage is used to replace the weight of the fat boy who was seated closer to the fulcrum before.
What is less obvious and forces the see-saw analogy to be abandoned is the increased tension between the external balancing weight and the internal parts as they spin.
Then we need to use a 'round the world' yo-yo analogy; spinning the weight of the yo-yo applies tension to the string. The faster we spin the yo-yo 'around the world' the more tension it puts on the string (until the string breaks)
For any given speed of rotation, if we make the yo-yo's string longer, the yo-yo's circular path becomes longer and in order to travel the longer distance the yo-yo's speed increases, increasing tension on the string. Likewise, the shorter we make the string, the less tension there is on the string. -this makes the string weaker or stronger depending on length.
This is why for race applications, the AM engineers recommend internal balancing; to shorten the length of the yo-yo's string effectively makes the the rotating assembly stronger -the closer we can place the weight of the yo-yo to the center the stronger the string becomes -which is speed related.
So an externally balanced crank even has a higher performance advantage over an internally balanced crank in the lower rpm ranges -which works out great for going to the grocery store where the engine is never made to spin more than about 3000 rpm (never approaching 80 mph in first gear!) The engine will be making more power by reduced moment of inertia in the lower rpm ranges and not apply excessive tension to damage it's internal parts (oversimplified but true)
But if we are going to race the engine and force the yo-yo to spin as fast as possible on a regular basis, we definitely want to increase the strength of the string and this is alternatively done by shortening the string's length -or what is done by internal balancing.
Hence, while external balancing can be used for performance advantage in the lower rpm ranges, for race engines that will be made to spin in the higher rpm ranges, internal balancing is how we keep the parts from sending the yo-yo into orbit; ie: how to avoid catastrophic engine failure at high rpm -past the stock 5000 rpm redline.
All this expressed in the simplest terms possible (with much oversimplification) but the gist of the debate is made understandable:
Despite what all the engine balancing articles say (or don't say) external balancing does have a power making advantage for engines that won't spin past their stock 5000 rpm redline by reason of lower moment of inertia.
But for a race application, internal balancing is mainly done to effectively strengthen the rotating assembly to avoid catastrophic engine failure in the upper rpm ranges past the stock redline.
The internal balanced engine will have slightly more 'moment of inertia' but much more resistance to parts failure in the higher rpm ranges.
Scientifically speaking, internal balancing is actually a disadvantage for a lower rpm engine because external balanced rotating assemblies are designed for reduced moment of inertia -which is not done to 'cheapen the engine' as one might initially suspect.
(the engineers are going to calculate the harmonic frequencies of the crank in either case and make the correct torsional absorber for either type... in most cases! -there are notorious bad examples but this is not the target topic of internal vs. external balance differences)
It's a subtle nuance to describe, but I've never read any engine balancing article that explained why external balancing can be used for high performance advantage on lower rpm engines. Assuming basic component part strength is not compromised by the use of inferior materials, it's simply less efficient to lug the extra weight around in the lower rpm ranges where most average consumers run their engines.
Thank you SpeedTalk for the space to talk about speed.
