I have a 2008 Sportster Nightster... Whats the largest engine I would be able to put into it?

If there is none, What are your suggestions so I can get more power out of it?

Also, I have a wierd sound when I accelerate. Sounds like a whistle/ styrofoam rubbing against eachother. any ideas?

 

That depends. How much money do you have? With enough money you could fit pretty much anything into it with the proper custom fabrication and cutting into the frame to stretch it.

If you're asking what's the largest you can go without getting into ridiculous sums of money, that seems to be about 88 CI, though that is still very spendy. Even the 1250 kits aren't cheap, though they're not horribly expensive either.

Head work, cams, and upgraded valve springs to handle the cams, assuming that you've already done the basics like a free-er flowing air cleaner and exhaust, along with a proper EFI tune.

No idea. My instinct would be to check that the belt isn't rubbing somewhere.

 

Hi from the UK and welcome to HDF. You need to more clear about what you want of your bike, to better answer your question. Staying with Sportster engines you can readily convert it to 1250cc, which is a popular mod and works very well. While doing that you can consider porting the heads, changing cams and I recommend you check out the Hammer Performance website for more details. Using a proven mix of parts from the likes of Hammer you can get around 100/100 HP/TQ, which is enough for many of us! Bigger than that requires more extensive and expensive mods to the engine.