Search for the threads around here, they are much more complete than any answer than you can get from a single post. I think it was Tony (from motordyne) has made great posts showing dynos of each and where each would shine.

It basically came down that 1/2" would be high end power and 5/16" would be more usable mid/low end power. Generally most people want the more usable mid range power offered by the 5/16".

 

http://www.motordyneengineering.com/

 

i ordered mines from b2auto on the forum i think? 214 shipped for iso thermal!

 

g35parts.com is a really good website to order from... and Big OGre i believe that if you want maximum hp gain go with the 1/2 inch its a little more $ but i think you get like almost 15-17 hp out of it to the rear wheels or somehting

 

im not really sure though

 

I ordered from b2autodesigns and got a very good price and they shipped fast. I'm extremely happy I went with the 5/16th; i want the low end/mid power and wanted to keep my engine cover

 

motordyne mrev for 05+ revup engine, not sure what year u have

 

Can anybody shed any light on the Z1 Motorsports spacer? Done some searching but really not much at all on this.