I have an 2000 ssei and want to change the pully on the supercharger to a 3.4in and was wondering if i need a new belt or if the tensioners will make the differents,,,,and what kind of HP can i expect...

i put on a size smaller belt, and i have a 3.5 inch modular. i probly could have gotten away with the same belt, but i was told that would have too much slack and wear out your tensioner spring. btw, when i got the next size smaller gatorback, it was twice as much.

Yes you will need to go one size smaller on the belt.
Make sure you have freed up enough intake and exhaust restrictions to allow knock retard free power from that 3.4" but you should be fine.

30 hp and 50 ft-lbs of torque. Torque is what you feel in the seat of your pants, so this is a big boost..... so to speak. You should gain 2 psi boost.

my intake and exaust are stock will that be a bad idea to do

It could be.
My first recommendation for you is to switch to a 180 coolant stat.
Next is to purchase an INTENSE fenderwell induction kit.

Paul here, he knows whats up.

i myself have a 03ssei, with a custom intake. (for now)

soon to come 180* stat, 3.4-3.5 pulley, and magnaflow muffs.

thatll be all for a while.

I would suggest going with a 3.5" pulley. The 3.4" will most likly give you a lot of KR. I was getting around 7* at the shift point, and about 2* under WOT. Upgrading the PCM and installing a high performance shift kit knocked that down to 2* at the shift point, and zero at WOT. This is with a 3.4"

I switched to a 3.4 MPS last summer but never checked for KR. No other mods. How long before KR can cause real damage?

Do I continuously need to monitor KR? Or will my 'yet to be insalled' Intense PCM take care of KR automatically?
Anyone?

KR is Knock Retard. It is what the computer (PCM) does when the knock sensor detects a Knock or Ping.

So KR does not cause the damage, the knocking will and the KR is what prevents the damage.

Unfortunately the result of KR is decreased power output. About 3 hp per degree of KR.