There are a million threads on this subject already. Please search.
Stillen Gen 3 seems to be the consensus. However, post-MAF tubes are a much better bang for your buck as they offer decent gains for significantly less $$$.
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2011 370Z 6MT Sport Gun Metallic | ARC | CJM | Ecutek | FI | Fujimura | R2C | SPL | Stillen | TWM | Z1 | ZSpeed | |