Hi there! I've read through this entire thread, but my head is spinning with numbers. I want to do +3 in the rear like you did, Did you have all the CEL and gear indicator issues? Did you need a bigger chain? I race and ride track exclusively on my 400. I noticed Norton has the different front/rear sprocket an chain kits that come with a flash. I was hoping to just be able to do an aluminum Vortex 44T on the rear without changing the front or the chain. Is this possible to do?
is correct from what I've heard. But I've also read that people got the CEL with just a +2. YMMV, but unlikely with +2. I was replacing the sprocket and chain together, so it wasn't an issue with me. Unfortunately I don't have enough experience to say if you'll need a new chain or not. Although depending on how much it was stretched, maybe you can get away with it until your next chain swap.
Regarding the CEL and gear issue, I didn't want to bother with the chance of running into it and clearing the codes, so I just preemptively shorted the wire. The safety feature is something that'll probably never catch me off guard due to the way I start my bike, so I didn't care about disabling that feature.
If you're going to end up doing mods to net more HP, I would probably stick with the stock gearing or just go +1 or 2 max in the rear. Don't want to hit max speed with spare power
With +3, top speed is 118 according to the calculator. Fastest I've hit was 123 indicated on the speedo.