What is the reasoning behind the placement of the idler pully on a blower set up? I see some on the inside of the belt and others on the outside?

Not really sure what you are asking. Ther are number of different types of blowers and belts. And typically idlers are for adjusting the tension on the belt.

I have no idea how they decide which way to go. However, in general, idlers on the outside cause the belt to wrap further around the pulleys giving better grip for the belt and idlers in the inside cause less deflection of the belt giving (slightly) less resistance or drag. Does that help?

I don't think it matters . My 8-71's are inside the belt pushing out but my belt is short so it only pushes out a very little bit . I have seen them both ways . If it is inside ,the ideler probably wears the belts teeth a little quicker .If it's outside, then it rubs the outside if the belt and makes it look scuffed up .Just my opinion .

Thanks for the help guys I kinda figured there was no right or wrong way as I have seen them both ways from the same engine builder.

Kinda like the question of what determines the rotation of the center drive on a triple engine application.

The obvious is for belt tension. the next issue is getting belt wrap.
which is surface area of belt to pulley. so design configuration dictates position of idler

As the engine revs the belt will have a tendency to 'grow'/ stretch. If the idler is on the inside pushing out it will 'help' it grow and loosen and unwrap. With the idler on the outside pushing in it will tighten and increase wrap. I believe outside pushing in is the best if your configuration will allow it.

When the belt pushes in, which is the best way for the wrapping reason, it's because there's room to do it.

Alot of the time when the belt is pushed out, it's cause there's something in the way.

DAVE