I would also recommend upgrading the exhaust system to a full 3" mandrel bent system, including the downpipe. If you scroll down the attached link, past the manifold discussion, you will find dyno results on a stock 280zxt which showed a 24 hp increase over the stock exhaust system. No other modifications were needed!

I was lucky enough to actually purchase this system during one of Jeff's group buys several years ago. Made a big difference in the performance.

BTW, last I heard, Jeff's 280zxt was putting out over 700hp to the wheels!

http://www.angelfire.com/extreme/280zxt/page11.html

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Phil
1981 280zxt - Now in the care my son!
2014 370Z Sports Touring Roadster