The least bends in the exhaust system, the better, especially for turbocharged motors. I run a straight through single exit Magnaflow Muffler, but it's 3" inlet/outlet. The sound is great and not too loud.
I think a 4" would probably be too loud for my liking, but hey, each to his own 
125shifter, that IS nasty! Very nice 
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Merv
'89 911 Turbo Cab
Protomotive MAP ECU, Twin Plugged Heads, GT2-EVO CAMs, 3.3L fully finned P&C's, ARP fasteners, C2T head gaskets, Titanium Retainers, Turbo spec valves, springs & guides, 964 splash valves, GT35R BB turbo, GSF Stainless Headers, Magnaflow Exhaust, Full bay Intercooler, TiAL 46mm w/gate, TiAL 50mm BOV, Apexi AVC-R EBC, SPEC Stage3+ Clutch kit, Crane CDI Ignition
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