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To get a "reasonable" increase in volume and performance, I spent a considerable amount of time and effort and found:
1) Decent equal length headers are a must - Top Speed parts in Dallas, US, work very well and are a bargain price.
2) Cat bypass pipes - again from Top Speed but illegal in Australia and you need to install a boss for the O2 sensor.
3) Retain the rear muffler - heavy but no drone - quiet untill 3,500 rpm then quite loud.
I've had the above system fitted for 2 years now and my wife still doesn't know its fitted as its still very quiet at crusing speeds.....
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2001 Boxster S (triple black). Sleeping easier with LN Engineering/Flat 6 IMS upgrade, low temp thermostat & underspeed pulley.
2001 MV Agusta F4.
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