Question about overbalancing

[My427stang]

To the guys with some time on a balancer

Assuming a crank/balancer/flywheel that was balanced with a heavier combo, when would lightening things up be too much to keep your existing balance? 

Keep in mind I am not asking about my combo, this is academic... I am rebalancing, but I was playing with numbers and it's a significant difference.  I know balancing is as much art as science, and higher RPM builders often overbalance using a slightly heavier bobweight, but when is overbalancing too much?

Using a 50% balance factor for reciprocating weight,, a $ 427 crank, factory rods and old TRW pistons, we came up with a bob weight for my 427 of 2340 according to my notes.

I am building a little long rod 396 using the same crank, etc, but the combo is a lot lighter with Diamond 4.08 pistons, and with the same 50% factor comes up to a bob weight of 2190 grams, or 150 grams difference

That would equate to about a 56% recip weight balance factor.  Thoughts?

Thoughts?

[Joe-JDC]

You might check with Ted Eaton since he does overbalance engines, and I would not answer for him.  I balanced engines when I got my Automotive degree, but we simply did neutral balance back then.  He has several articles on balancing on his site.   http://www.eatonbalancing.com        Joe-JDC

[CaptCobrajet]

I've tried it over, under, and right-on.  The general consensus is 1 to 2% over or under......so we're only talking about 22 grams......your bobweight being 2190........either side of hanging the calculated value on there.  Just like balancing a tire/wheel, it is done at a chosen rpm.  There are places they will have resonance that will come and go through the RPM range.  The over/under ideas stem from the idea that the balancer can't simulate a "10,000 rpm, etc." condition, so we fudge it in theory to compensate.  It is a WAG pretty much.  I have never seen detrimental problems, or noticed the shake from 10, 20, even 30 grams one way or the other.  I think the 150 g difference you have would make it shake right off idle, and on the way back down.  If you rev it, it will go through the shake, and then shake again when it comes back down.  Much like putting a 428 flywheel on a 390 or vise versa.  It will shake again at some point, more violently if you turn it there, which may or may not occur.  If it's really bad, one will shake all the time.  Sometimes they will shake more and more often, depending on damper diameter, stroke, overall assembly weight, etc.  If the machine shows it "out" more than 20g, we always fix it.  I have changed pistons, rods, etc. on a known balanced crank up to 25 grams bobweight myself, and it is undetectable.  I can't tell you more than that because I have never tried it past that.

[Barry_R]

I will agree with Blair on this one.  I have been way off by changing out pistons in an engine and could not tell the difference at all.  I asked the guy that owns CWT balancers about it once and he said something akin to "I have Cup guys that go over 10, I have Cup guys that go under 10, and with a 90 gram difference from one to the other they are neck and neck across the stripe at Daytona"...

[scott foxwell]

IMO none of the above is a reason to NOT do it right. 

[Barry_R]

IMO none of the above is a reason to NOT do it right. 

Of course.

But balancing seems to be one of those places where it's not quite as easy to define "right"...

[My427stang]

As I said in the beginning, absolutely not trying to get out spinning the crank, it's not like it's going to be a tough one to balance anyway

Good discussion, and about what I have seen playing on a big old school Stewart Warner balancer and in general swapping parts over the years.  Small differences are pretty much undetectable, especially at the street RPM level.  Just wondered if anyone tried the extremes

Thanks!

[Joe-JDC]

Extreme is installing a 428 flywheel/flexplate on your R code that you just bought used, and start driving it around home only to find out that the shortblock is a 390.  BTDT, and it is a wake-up to what a balance is needed for.  Never assume, and always balance the individual parts as close as possible.  I always want the big ends of a set of rods to match in weight, and the pin ends to match.  I also want the pistons to be balanced individually and then with their respective pins, rings, and clips.  "Anything worth doing is worth doing right the first time."  Joe-JDC

[My427stang]

Extreme is installing a 428 flywheel/flexplate on your R code that you just bought used, and start driving it around home only to find out that the shortblock is a 390.  BTDT, and it is a wake-up to what a balance is needed for.  Never assume, and always balance the individual parts as close as possible.  I always want the big ends of a set of rods to match in weight, and the pin ends to match.  I also want the pistons to be balanced individually and then with their respective pins, rings, and clips.  "Anything worth doing is worth doing right the first time."  Joe-JDC

I bet it did shake!

I did a 460 as a teenager and made a 400 flywheel fit it in our shop truck, MAN did it vibrate. Even then we could have bought a setup from L&L but I was going to do it low budget.  Even tried spinning the flywheel on a wheel balancer,  never got there

This discussion is of course a little different, all parts for this discussion are matched to each other, flywheel and balancer remain zero, just using a lot more bobweight on all 4 throws when they spin it than normally calculated (rotating plus 57% of reciprocating instead of rotating plus 50%)

I am sure Blair nailed it pointing out out that essentially, every balance is for a certain RPM range and when way out, you will have a harmonic happening somewhere. Of course someone came up with the original math for good reason LOL

[Falcon67]

The good thing is these days is that if you go from say a 4.03 bore to a 4.125 and you're already in the custom piston space, they can just about dial in a new set of pistons to match the old so you don't have to rebalance the whole deal.  If I can afford one of Tim Meyers new CGI large bore 351C blocks, I'll be counting on that LOL.