One of the best mods you can do for the E46 M3 (while keeping it naturally aspirated) to get more horsepower is to swap out the headers. The stock headers have two catalytic converters, which restricts the airflow coming out the exhaust. A free flow aftermarket header will free up about 10-15 whp. My next mod, Agency Power headers. And while I’m under there, I decided to change out my transmission fluid with Royal Purple Synchromax and the differential with Royal Purple Max Gear 75w140.

For this install, I went back to Kevin because I did not have all the right tools to complete this DIY. If you want to try to tackle this install.

BMW E46 M3 Agency Power headers and Royal Purple Synchromax gear oil

Make sure to buy new hardware for the installation to make sure you don’t have any exhaust leaks. Here is what you should buy:

  • (18) Hex nuts. Part #11721437202
  • (6) Manifold gaskets. Part #11627830667
  • (2) Crush gasket rings. Part #11627830668
  • (2) Flange gaskets. Part #18307830674

BMW E46 M3 Agency Power headers and Royal Purple Synchromax gear oil

BMW E46 M3 bank 1 and bank 2
It’s very important that you wire up the O2 sensors back to the right banks or else you will get a Check Engine Light and your car will go into limp mode, meaning it tops out at 40mph or so.

BMW E46 M3 Stock headers
Agency Power headers (left) vs. stock headers (right).

BMW E46 M3 broken connecting rod
Kevin’s connecting rod shattered into pieces! What a nice shelf display.

BMW E46 M3 and E90 M3
Kevin’s Alpine White E90 M3 sitting in the rain.