Parallel will give you more speed / mechanical power, but will also run a lot hotter.
My suggestion is to try it in series and if it works for you leave it there.
If you find you need more speed, connect it in parallel.
Dont forget to set the correct currents.
If it is the FL86STH156 or SY85STH156 it is 3A in series and 6A in parallel.

Quote Originally Posted by Robin Hewitt View Post
Me again

Parallel or series? Would it make any noticeable difference to the top speed or just generate unnecessary heat?

Robin