Noob here with some questions. (Wastegate, Blow off)

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First, I did use the search feature, and looked at the noob sticky post.

I am a complete noob to forced induction. I have been reading rather extensively on the subject. I'm not mechanic but I do my own car work. IMO most mechanics charge way to much per hour. So I basically learned everything I know about cars by searching and experimenting.

So my question about the wastegate is what size you should get. On this site

GTM Motorsports*::*FORCED INDUCTION*::*GTM-370Z-(TUNER) TWIN TURBO KIT

^my planned stage 2 twin. It has options for the wastegate. Is this tunable(adjustable PSI)? If it is tunable is there a reason not to get the biggest one and tune it down to what you need? I would suppose larger ones could divert more exhaust away from the turbo. Is there a disadvantage somehow to having a larger one? If you guys could give me some info on this it would be great.

I have been reading on here for some time in preparation. I will be ordering my 370Z Nismo in January 2012 (hopefully '12 model by then) and hopefully ordering my turbo kit shortly after. I plan on making an entire build thread out of it like other with millions of pictures and videos My goal is 500WHP. Also plan on doing all the work myself (aside from tuning which I will go to a shop for). So as a noob I have my work cut out for me.

Thanks for all your help, and I hope to be a good addition to the community eventually.