67' 318 crank in a 69' 340

gregsdart said
One thing to do to figure out a combo is come up with some solid numbers on how far off (overbalance?) the bobweight can be. A one percent OB would be what, 20 grams? That allows some fudge room. I am not a balancing expert so don't go by my 1 percent idea. But there must be someone on this board that has an idea of what can be done.

The man is a genius LOL. Consulting the magic spreadsheet here, and change the balance factor to .49 instead of the standard .50, and the bobweight for the KB167 piston with the heavy 340 rod becomes 2140 grams which is now slightly under the standard 318 bobweight. So looking at it the other way, that 2160 gram bobweight represents approximately a balance factor of .507 applied to a piston/rod set that would have 2147 grams bobweight with a balance factor of .50 even. (Did I make that confusing enough?)

So your thoughts and number are pretty much spot on, GD.

The .50 factor minimizes the peak magnitude of vibrations for a 90 degree V8 engine. (See the middle pair of diagrams on page 8 of this article: https://www.tonyfoale.com/Articles/EngineBalance/EngineBalance.pdf) Using that .507 factor, one would expect some increase in some vibrations at some RPM's. I don't know what they are, but the above is really stating the obvious: it is gonna be a bit imbalanced compared to normal simply because it it not optimally balanced... like everybody knows, duh! But whether that would be bad is the question; I bet there are lots of uses where it would be a non-issue, like a budget drag car. I just cannot say if it would or would not have an annoying level of added vibration at your main cruising RPM, for instance.