XR's gears have straight cut teeth that are thicker. The problem is the hardening on the gears wears faster and then when a tooth breaks it takes out the entire upper. XZ's gears don't seem to have the same issues.

what does the XR lower provide than, other than a stronger 1.25” prop shaft? Just better at retaining the prop in the event something is hit?

the upper shaft is already the 1.25 in all X’s? Anything different in the diesel X, other than a sticker (1.5 ratio)?

Nothing in the drive is designed with taking a hit in mind. At 500+ hp you will start to twist the splines of a standard 1” propshaft. I think an Xz would be your best choice, stronger input retainer, steel tower, helical cut upper gears with the stronger propshaft in the lower.

Thx to all for the great input! You’ve confirmed what the majority of my drive rebuilders told me. Many said if an XR gear fails they’d rather not reuse the housings, much more damage than the helicals typically do.

I’m going for the XZ’s.

Here's link with the specs on all Bravo type drives https://www.go-fast.com/mercruiser_b...terndrives.htm