Ya know, discussing velocity stacks is kinda like discussing which oil or radio is best! While we know there can be benefits from using one, especially on piston ported engines, their performance is not always predictable.

I've seen just about every case imaginable regarding stacks. Sometimes adding a stack will cure or cause a problem, sometimes removing one will do the same. Likewise, sometimes a slant forward will resolve or create a problem and in other cases, facing the slant to the rear will do the same. Sometimes you need a straight cut stack, sometimes a flared one. Sometimes you just have to get rid of the stack completely when you think you need one, etc. You get the picture.

The point of all this is we're dealing with strange airflow and pressure here and nothing is 100% predictable. You just have to be willing to experiment to see what works .... and what works in all airplane attitudes. And that's all I have to say about that.