They are good for a stock 660 with a pipe, but if you plan on building your motor in the future you might need two of them :lol: ,...but I don't think they are compatible in sets of two like the fcr's...I'm sure anything is possible...but at this point it would make more sense to get the fcr's for a bunch of reasons.
I bought one and spent a day and a half trying to get it tuned in with no luck. So I sold it and put the stock carb back on, which works great. Its jetted damn near perfect. Personally I wouldn't spend the money on the FCR's unless you plan on doing a motor build. If you take your time and adjust/jet your stock carbs you should see a very noticeable power gain.