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This thread is going to hold images of our exhaust project for our 545i.

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If you notice, the factory squeezes the pipe down after the dual resonators before it enters into the catalytic converter.

In fact, it's squeezed down almost 3/4" of an inch.

The factory system is 2.125" OD, where the tubing enters the catalytic converter is roughly 1.480" OD.

We are planning on replacing the section from the back of the resonators to just behind the muffler where it is already 3".

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Step 1:

Saws-all.

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Step 2:

Deburr & beat it back to round.

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Step 3:

Cut your collector. Any chop saw will work. You need (2) 45 degree 2.25" OD bends.

We are using aluminized steel. Stainless would be preferable.

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Step 4:

Weld & Expand.

Any muffler shop can weld & expand your collector. Mig, tig, stick ... Just no jbweld.

We are expanding the end to 3" so it mates up with the factory exhaust attached to the muffler.

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Step 5:

Raise the collector into place. Measure the distance from the back of the collector to resonator.

This is our straight tube length required x 2.

Ellicit

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STEP 6:

Tack together. Expand the 2.25" tubing to 2.25" slip fit. Slap it on the car.

Notice one resonator sits back further then the other stock.

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Step 7:

Fit a 3" 45 degree bend between the collector and the factory 3" exhaust. You gotta cut it back to around a 30 degree angle and add straight.

Once you got it fit, mark it with a nice pink pen, take it off and weld it up.