There are a few things that I would like to point out.
First and foremost is the difference between internal and external balance. The first picture is of the external balance counterweight on an OEM 454 flexplate. External balancing is done because of improperly designed counterweights on the crankshaft. Not enough mass was added to the counterweight, most likely because of the manufacturing process, but could also be because heavier (larger) pistons and rods were installed.
External balancing is, by far, much more economical than internal balancing. External balancing can be achieved by adding a welded or bolted (2nd picture) counterweight to a flywheel of flexplate, drilling holes or machining slots 180-degrees opposite of where the mass needs to be added, or a combination thereof. Removing material rather than adding material allows for an ultimately lighter rotating assembly.
IMO, external balance is acceptable for stock and mild performance engine builds only. This is because of the imbalance of the flywheel/flexplate (and damper) being placed on the ends of the crankshaft, unsupported by bearings on both sides, as is the case with the crankshaft's integral counterweights. Any serious effort build should be internally balanced.
A note on balancing an engine. If I am building an external balance engine, I will balance the crankshaft with the damper and flywheel/flexplate installed. In an internally balanced application, the flywheel/flexplate and damper/hub get balanced independently of the crankshaft. I want to make sure that all are a true neutral balance, should I ever need to change a component.
Some engines have options to run different diameters (tooth count) flexplates/flywheels. My 3rd and 4th pictures illustrate the difference between the Chevy 168 tooth wheel and the 153, which equates to about 1.344-inch difference in diameter. 12.844-inch diameter for the 153 tooth and 14.188-inch diameter for the 168 tooth wheel. I prefer the small wheel for two reasons, 1. Reduced inertia and 2. Increased cranking RPM.
The two flexplates pictured weigh about the same because the smaller SFI approved plate is made from slightly thicker 4130 chromoly, however it will theoretically allow the engine to rev faster because of the reduced inertia. "Inertia is the resistance of any physical object to any change in its state of motion including changes to its speed and direction or the state of rest" (
https://en.m.wikipedia.org/wiki/Inertia). More specifically, "rotational inertia, the property that a rotating rigid body maintains its state of uniform rotational motion. Its angular momentum is unchanged, unless an external torque is applied; this is also called conservation of angular momentum."
Inertia = (1/2)(Mass)(Radius^2)
Basically, the more weight that can be removed from the outside diameter, the faster it can be accelerated.
I don't mean that someone could look at any one component of a system, such as a lightened flywheel/flexplate and say, "that is why this engine has such good throttle response, it has a lightened flywheel!" All else being equal, light weight pistons will have the most dramatic affect as will anything that can be done to reduce reciprocating mass. Reducing the weight of the rods and drilling the crankshaft rod journals allows for lighter counterweights. And so on and so forth.
Regarding flywheel/flexplate cracking/breaking, I have seen a surprising lot of broken wheels of all kinds at NJBA events. I think this has to do with the conservation of angular momentum where the axis of a flywheel is not easily changed. This axis changes every time the direction and/or trim of the boat changes. I have not witnessed the same in road racing or wheel-standing drag cars. Maybe the transmission input shaft provides enough support to prevent that.
Since it took me so long to type all this on my cell phone, please excuse any redundant information.
Cheers,
Joe
