I have run up to a 16/8 Apc on my st90 w/Davis head on 81" GP Cub ARF on floats. I would think the new 18/6 wide blade 3D prop would work on the st90/Davis diesel head. I think Zinger makes an 18/6 wood prop, which would look better.

I made the mistake Davis warns about: running too rich and overcompressed. This carbons things up real fast. The ring sticks, compression is lost and it won't run.

So make sure it is lean enough.