So I'm just wondering- and I'm not sure exactly where to put this post-

On 4 cylinder accords at least, I see there are two potential locations for the upstream o2 sensor, up near the header/manifold pipe or down just before the cat converter. Is there an advantage or disadvantage to either location?
Aside from accessibility.
Id assume one gets more air flowing through but the other gets hotter air and would not reveal any leaks before then.

 

I can imagine several things for the car designer to consider.

Close to the head gives a faster warm-up. It's also easier to remove/replace the sensor, but car designers don't seem to worry about that too much.

Close to the head gives a more responsive measurement. The measurement isn't delayed by the time it takes for the exhaust gas to get there. That becomes more important on newer cars with tighter emmissions requirements.

On a car with 4-into-2-into-1 exhaust the sensor is located downstream of the point where all exhaust is finally joined together in 1 pipe. I suppose if there's at least one version of the car with 4-2-1 exhaust they would want all cars to have the same wiring harness.

 

I didnt think of a faster warm up. I suppose thats better for winter conditions too. Car goes into closed loop faster- saves gas right?

So if I were to get a new header, would it be of benefit to get one that has the sensor hole at the main pipe? Its where the 2nd and 3rd cylinder exhaust pipes meet.

Just splice and solder the wires to reach or somehow get the harness connection and sensor closer to the front?

 

Yes, you'll want to place the sensor just a bit downstream of where ALL cylinders are finally mixed together. Then make the wires reach. You can probably find male & female plugs from a junkyard, make yourself an "extension wire" for the O2 sensor. Tie that wire back so it doesn't drag against the hot exhaust pipe...

 

On 4th gen accords the lx/dx have the sensor next to the head while the ex/se have the sensor downstrean, the only reason I can think of is that the ex/se's have a lil more power and perhaps a higher redline(just a guess haven't looked it up) which should translate into more airflow. Putting it downstream a lil more allows for less chance for error in the air/fuel mixture.

 

EX has a 4-into-2-into-1 exhaust so they had to locate the sensor down where all the exhaust joins together in 1 pipe.

Closer to the head would be a more accurate, more responsive measurement. But it would only be measuring exhaust from one cylinder.

 

Ill have to see what options I have then. The DC header has the hole for the sensor in two places.