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ORIGINAL: sirlimpzalot

520 Conversions are intended for racing, to save weight since the 520 chain is lighter than your stock chain..less weight on the drivetrain will equal more HP available to the back wheel....not HUGE horsepower gains, but a gain none the less ( maybe 1 HP??) The drawback to 520 chains is the life span isnt always as long as a stock ( heavier duty) chain.
Make sure that ALWAYS replace both sprockets and chain at the same time, since the items wear together and if you introduce a new sprocket to a old chain, you will only be getting a fraction of the life you could have got!!
As far as gearing goes; -1 is equivilent to adding 3 or 4 teeth to the rear sprocket, so eith way the end result is very similiar...one thing to think about though is that the sharper any chain "bends" ( smaller countershaft sprocket is what I am talking about here) the higher the heat ( friction) will be. So this is where lubing your chain becomes very important..and if you go up in the back you will require a longer chain that stock, and a shorter one by going smaller in the front