Ti components of exhaust systems are lighter, so you get the improved HP plus less weight to drag around. My Mugen system on my S2000 is 24.4 pounds lighter than stock.

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What about true dual exhaust system, aren't they going to have the biggest gain and huge sound?
Is any company making a true dual exhaust system?

 

A true dual system is not optimal on the Z due to the tight clearance where the Y pipe fitting would have to go, the Y pipe would restrict the exhaust flow. There was a Japanese company that prototyped this system with little improvement in performance numbers.