I am Teh Edumacator.
Join Date: May 2005
Location: ATL, London, and Barcelona
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I actually think that stock M5 headers are definitely not as efficient as they could be. The exhaust headers on my E38 are Dinan headers. You can view them on dinanbmw.com.
I'm thinking 3" piping should be fine for your exhaust.
Deleting resonators will not cause a CEL, but it will make for a drone inside the car. Deleting the cats probably would cause a CEL.
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1999 E39 540i/6-VF Stage 1 S/C, Dinan exhaust, Dinan strut+shock braces, Dinan ltwt flywheel + clutch, Dinan Brembo BBK, Dinan monoball, H&R Springs, Koni Yellows, M5 sways, Dinan forged 18" wheels, &c.
1995 E38 740i. Highly modded. Faster than you.
Sold 2-28-07: 1997 E36 M3/4, 8-30-05 RIP 1997 528i
Lindsay fan club member.