What factors into deciding on whether to over-balance a crankshaft vs. using a standard 50% balance? And when over-balancing, by how much is the balance factor adjusted, e.g. 1%?, 2%?

Is this combination a candidate for over-balancing?
- 650 (est) HP street/strip application w/ emphasis on track performance
- expected shift RPM of 6500+ and peak RPM est. of 7200 on big end
- 3.75" stroke 440 w/ steel rods
- standard bob weight calc = 2496 grams
- highway cruise RPM (driving to & from track) ~3500

Jeeezzzeeeee...............build something already. My motor is slightly overbalanced but not sure how much.

in my humble opinion, NO, you are not a good candidate.
i'll let the others debate the pro's and con's of it.

The attached is from an article in the May 1999 edition of Hot Rod magazine.

IMO, any engine that is going to be used in a performance application that will spin above 5000 rpm or so should be over balanced.

Your going to shift at 6400 and stripe at 7200 or so. Your a canidate.

Further, your more of a test and tune guy, looking for max performance "WHEN" you ever get your car out for a few runs.

Vizard found extra HP everytime when Testing the effects of OB.

Further, I think Every engine can benefit from a slight over-balance.

I feel the benefits from over balance have been out there a loooong time. Lots of builders , maybe even most in the know OB, IMO

The question is how much? There are elite engine programs out there that can calculate that amount based on your componet weights. For most engines 51 to 52 %

Further it is known that a engine can never be Perfectly balanced at All RPMs. So I say balance it at the point where it can give the most performance benefit for what your trying to acomplish.

In a performance Rpm enviroment, OB is the proven way to go. JMHO Id at least go 51% in your case. Its not going to hurt a thing and most likely help you.

Well, I've done a bit(?) more Googlin' on the subject of balancing / over-balancing and even under-balancing. David Vizard seems to have gone from one recommendation to another lately, since his latest tech stuff mentions how Richard Maskin told him how they (Dart) found some quantifiable improvements from under-balancing some engines. Apparently DV's latest approach is to have his BB (doesn't specify how big) cranks underbalanced 1.5%.

So... short of a fully-funded research program using multiple engine configurations and a wide variety of design parameters, I'm back to the idea of going for a traditional 50% (and don't see a need to get all worked up over exactly how accurate that is). IMO, doing anything else falls into that broad category of "It depends..."

One problem is that the "50% factor" has no mathematical or theoretical basis - it's "what works".
It fails to account for the fact that almost all of the rod's small end is not pure reciprocating weight, that material above the pin center is traveling opposite the rod beam, that the difference in forces between long and short rods is completely absent, and that other very large forces (cylinder pressure and vacuum) are completely ignored.
De-constructing over and/or under-balancing to justify an observed result is not science, it's rationalizing.

Read my article here: http://victorylibrary.com/mopar/crank-bal.htm