here is a comparison of size and shape of the eurosport tuning 3in S/S downpipe...on top...and a stock replacement B21ET downpipe...nominal diameter of 2.25in.

The EST pipe actually fit in rather well, despite the apparent difference in shape. There were no clearance problems. When we changed over from the old style exhaust manifold to a 90+ manifold a couple of weeks later, I had to weld in a bung for the oxygen sensor.

The B21ET pipe fits in exactly like the B21FT pipe that has the catalytic converter. While the 21FT pipe has a converter welded in [starting at just about where the flange is resting in the image above], and uses an extra mounting bracket on the block that is bolted to the inboard side of the converter; both the ET and FT stock pipes utilize the welded on bracket [shown on the ET pipe right after the second bend of the "S"] to attach the pipe to the flexible mount attached to the bellhousing bolts that was common on B21s in the '70s and 80s. That flex mount locates and supports the downpipe, and is necessary to reduce the load stress on the wastegate housing of the turbo.

The EST pipe does not have that type of support bracket. It comes with a band clamp and bracket setup to locate and support the 3in pipe to a bellhousing bolt....almost identical to the design found on B230FT downpipes in 700s.