Question concerning the balancing of the crank & rods...

[Missourian]

I keep reading about folks milling the rods of their balance weight and then spin balancing their crank by removal of weight from the crankshaft weights.

If there is additional weight removed from the crank and rods and I have gone from an 8 weight 4.0L crank to a 4 weight 4.2L crank and the desired 4.2L crank is a 12 weight... what’s the point of removing even more weight from crank and rods when I am taking this engine off road?

[LeviPittman]

you want your reciprocating weight to be as light as possible without sacrificing strength. some power can be gained by reducing the rotational weight in your engine but will rev much quicker and youll gain more power by shedding mass in the rods pistons and anything else that moves back and forth in your engine and a heavier crank can in some instances will help your engine regain rpms after a gearshift because the heavier crank will store more inertia

make any sense?
if not pick up a copy of "the horsepower handbook" its put out by hot rod magazine and explains that and all kinds of other neat stuff

the spin balance is more for longevity through a smoother running engine and less about weight reduction

[Missourian]

My machinist that has performed all of the machining on my STROKER builds race engines, typically. The engine in his personal race car is "over balanced" to enable the motor to run higher RPM's. He says the motor smoothes out around 5500 RPM and will continue through 9500 - 10k RPM.

I understand the desire to minimize the weight (without sacrificing strength) of all the moving parts however, when the engine rarely sees in excess of 3k RPM and it is understood that rotating mass keeps the engine running at low RPM's, why would I want to lighten everything up?

[IH 392]

Lighter rods and pistons will cause less stress on the parts, a heavy (ballanced) crank will increase rotational inertia, don't worry qabout lightening the rods and pistons, but a GOOD ballance job is a good thing in ANY engine.

[gonridnu]

You are asking a simple question that has very complex answers, and although I am not an expert on rotational mass I can offer you this....

Engines that are otherwise identical but utilize lighter components do not make more horsepower in steady rate tests. They do accelerate faster as there is less rotational mass to accelerate. Essentially the power savings realized by the lighter components equate to more power available to the drivetrain that is not used to accelerate the mass. Again, this is only realized in transitional states and not at steady speeds. They also decelerate faster as there is less mass to slow down.

The amount of material people are taking off rods and cranks to balance them, or even for clearance, is of little consequence to engine performance due to the small amounts required to balance and their proximity to the axis of rotation. While the same amount of weight at a much greater distance, such as a flywheel, would have a measurable effect on performance, it would still be insignificant in our application.

The only real advantage to the heavier crankshaft is that an object in motion tends to stay in motion and therefore the heavier crank will usually exhibit better idle quality. This is because the heavier rotating assembly "carries" the engine between ignition events at low RPM. The effect is present at higher "operating" RPM's as well but at an increasingly diminished amount in direct proportion to the increase in the rate of ignition events. A small increase in idle speed, perhaps even 100 RPM would offset any difference between the 12 and 4 c.w. cranks with regards to idle quality.

The argument can be made that heavier rotating assemblies, and certainly flywheels, have an effect on the initial launch of the vehicle as an effect of the stored energy. Again, this is really getting "out there" for our application and is more useful in drag racing applications. Even then it is offset to some greater or lesser degree by the lighter engines ability to transition to a higher RPM at a faster rate.

Bottom line is don't sweat the small stuff.....

[Missourian]

My engine is assembled. I left the balancing tabs on the 4.0L connecting rods and additional space was needed to clear the girdle as a result. I have read and had responses saying the removal of these balancing tabs on the connecting rods eliminated the need for clearance washers for the girdle. So, if the balancing tabs are not needed and the delay of getting this engine back into my Jeep because of the need to modify my oil pan... this is driving me crazy. I am really wanting my Jeep back running!

There is SO much information to be absorbed to build a STROKER.

My thinking was 4 wt Crank and connecting rod balance tabs rotating close to the center of rotation would better balance the idle not having used a 12 wt Crank.