Balance tube size

Walt Farar has done that.  See his posts on VAF.  His name on VAF is seagull.

Thanks.  I hadn't seen that thread.  https://vansairforce.net/community/showthread.php?t=198160  I send a PM to ask about the size.  I'm guessing it's about 1".  His results are pretty typical of what I've read. 

Rusty

Since I am a nerdy engineer I will throw in my 2 cents to further muddy up the water. It can fairly easily be calculated what the airflow rate is thru the standard, 1/2" tube I believe, at at low pressure (vacuum) differential. Even a nominally accurate carb sync should be able to keep that differential at or below about 1/2 psi; at least I am able to do this with my carb sync gauges thru the whole rpm range. At this differential the potential unbalanced airflow rate thru the standard tube is quite large; much larger than the required unbalance at 1/2 psi even at 3000 rpm.

I won't argue with those who claim to see definite smoothness benefits with the big 1" xover tubes, but my own personal opinion is that it is not because of the increased airflow capacity of the big tube. I believe it is because of the larger intake volume; kind of like tuning your intake and exhaust manifold for increased power. All engines, whether dual or single carbs, have resonance points at certain rpms where roughness is observed, but much of it can be tuned out. I'll bet the same results could be achieved on the Rotax engines by experimenting with added volume chambers added to the intake manifolds; they would not have to be crossovers at all. Take it FWIW, cheers.

If your carbs are synced at each annual or if there was an issue then using the larger tube won’t make any difference and certainly one you could feel. When you want something the happen bad enough then it happens regardless of the reality.

It's pretty hard to believe that all the reports I've ready were just from wishful thinkers.  That RV-12 guy bought another set of manifolds because he wasn't convinced it would help. 

I do agree that this is likely more about volume, than flow.  I read another comment that said not to think of this as flow from one intake to the other, but rather a column of air that sort of shifts a little back and forth.  If you think of the movement of air through the intakes being disrupted by opening and closing intake valves, you can easily imagine the pressure pulses that must exist.  I'm betting the larger volume balance tube helps smooth those out. 

Of course my original question remains- If there's a downside to increasing the balance tube size, what is it?

Cheers,

Rusty