Put a new(er) F23A1 in the car last week, replacing the F23A4. I eye-balled the A1's exhaust manifold to see if it would fit up with the downpipe from the A4 motor, and it didn't look very promising, so I skipped the idea.

Well, got bored tonight and figured I'd find out if it fits. After pulling the A4 manifold off the car, I found the following:

1. A4 manifold uses two brackets at joint with DP. A1 uses one on the right side.
2. Both A1 and A4 manifolds utilize three-bolt flange.
3. A4 manifold has studs in the manifold and uses nuts to connect with the DP. A1 manifold has threaded holes.

The greatest find... The four bolts removed from the A4's two brackets are the correct size for the threaded holes in the A1 manifold flange, and that leaves one left over to attach the A1's single bracket.

Other notes: The 02 sensors are indeed different, though I can't say whether the A4 sensor is a wideband as some suspect. I used the A1's O2 sensor and with the A4 ECU, it threw a CEL. No such problem with the A4's sensor for obvious reasons.

Any questions??

 

the A4 uses a wideband sensor

 

Well tomorrow I'll throw an autometer AF gauge on it and see if it dances.

 

if i were to switch from f23a4 to f23a1 i would have to change the ecu then? lol