[wolfrpi]

probably no gain - the stock pulley is apparently very light to begin with. most of the gains are a resultant of reducing rotational inertia and because the smaller pulley is only marginally lighter the results will probably be minimal.

The reduction in drag is apprently not where the real power comes from (marginal). Check out unorthodox racing's homepage for details and comparisons of the original 951 pulley and their version. May be interesting to run a comparo though and see the results.