The 8 speed chain is slightly wider so you may experience some slight chinging noises on the rear cassette,and precise indexing can be tricky,if you've changed the chain only,it will almost certainly slip in 9th cog(smallest)under hard pedal pressure,nothing to do with 8\9 speed chain,9 speed cassettes just don't like new chains at all,best changed as a pair,90% of people that say there new chain doesnt slip in 9th gear,simply aren't producing enough power to make it slip