Flip gears: I did in my 901...possible in a 915?
 

Flipping gears in my 901 got me the best short box ever...and cheap.

I did the searches and cannot find any resources suggesting the same is possible on a 915. On the 901, you can flip 3rd and 5th interchangably.

Anyone have any first (or even second) hand knowledge if gears can be flipped on the 915 and if so...which ones?

great question

Please explain. I'm not familiar with "flipping gears." How would I do it in my 901, and what are the benefits?

It means to swap the position of different gear "pairs" on the input and pinion shaft.
So you can get different ratios at different gear positions at the shifter in the car.

You cannot flip gears in a 915. Because of the change in shift pattern for the 915, 5th gear has a different spline size on the fixed gear than 3rd and 4th do. The only interchangeable gears in a 915 are 3rd and 4th gear.

If you are looking for a "budget" short ratio gearbox for the 915, what some guys do is run the 3rd gear in the 4th position, and then "just" buy replacement 3rd and 5th gears. I say "just" because these days you're talking close to $1500 for 2 new 915 gears. However, this approach requires that you don't have the need to drive on the highway and don't drive any really long tracks. Top speed in 5th will be between 130 and 135mph.