Right now I have xr drives with 2 inch imco shorties and my 42 fountain is running mid to low 90s. I'm swapping over to b max uppers with imco 2 inch shorties. How much mph if any will I loose swapping to bmax

I would think none. It's the same gear set.

Xr 1.5 bmax 1.36 but goin to smaller props due to gear ratio change

upper gear is much bigger 5" if I m correct so the b max will eat more hp then the xr and the ratio change could also hurth a little bit .but its much better not to worry to blow a drive !

2-4mph from what I hear....

it may just be a lateral move. Going to the smaller prop on a taller ratio is going to benefit from having a more efficient prop in the water. What you lose in parasitic loss you may gain back in less prop slip. (I have nothing to back any of this up . . . . just trying to be an optimistic fellow boater lol)

I agree with that theory, Slippy what pitch prop were you running with the 1.50 ratio?

slippy what year Fountain?

Going from a 1.5 to a 1.36 normally requires about 3" less pitch. I would go down 4" switching to the BMax.

Really doesn't matter the strength increase is worth the money!