I put some NGK racing plugs in ( they retailed for 42 bucks a pop)
Well it seems that my idle is rougher and i get a check engine light sometimes at an idle. could it be the plugs? Im gonna go and get a set of bosch platinum 4+'s or should I

Were the plugs hotter or colder than the stock plugs? That might be your answer.

they run colder. Do you know the gap specs? and im an going to get Bosch 4+

moved to get a better response (and access to a larger knowledge base)

"racing" plug are usually a colder range. Is it's too cold you may misfire at low rpm and/or when the engine is cold.

platinum plugs actually burn hotter so they are not good for poweradders (boost/nitrous) when dentonation might be a problem

stay with oem heat range or 1 colder at most if it's more or less stock.

I have a 1.9 with a D/A supercharger. Will this be a problem? I put the Plat 4+ in and it idles better. Should i just go and get some new plugs? again :(

Same thing here, I replaced my Bosch Platinum 4 with stock NGK from the dealer and now I get a slightly rough idle. What can I do to solve that?

I put the bosch in adn it is runnign great
seems to have more power too. The power increase may be true, reason is that two Z3's with identical mods D/A, exhuast, check valve and CAI, the one with the 4+'s had about 10 hp more, again im not sure if teh plugs did anything but it seems weird that it would increase that much tot eh RWHP

I've found that plugs are really peculiar on BMW's. I have the +4's on my 325i, and the ///M3. I tried the NGK which were recommended, but had a very noticeable miss at idle. So I try the +4's and no miss (smooth idle). I've also heard from a lot of people who are mostly split 50/50 on each brand of plug.

So, My theory, it probably relates to where you live, temperature, humidty, etc. you just have to try diff ones which can get expensive.