By using the 800 pipe it won't make near the horsepower as the stock 900 pipe but it can be used. Other things to consider is the timing is different on the 900 so you will have to buy the new coil pack for a 900. B)
I have done 2 800-900 bigbore kits using wiseco 86 mm piston kit. The stock 800 pipe will work fine but an aftermarket pipe or 900 pipe will make more power. The jetting seemed to stay the same but careful how you recut heads and leave plenty of squish (70-80 thou). I had detonation problems (minor ) with tighter than this. I have run over 2000 miles with coated pistons and they looked great still. also be careful with exhaust valve clearance. The mountain cat CDI box works best or run a 2 degree key for best performance. Good luck
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