[eschaider]

For a carbureted engine you can put it fairly far down the exhaust system because it is only telling you what the AFR is. It is not telling an ECU that is making fueling decisions and changes in real time.

That said, even on carbureted engine, you still want to keep it away from the end of the exhaust path because of the back flow of ambient air. The ambient air will produce a fake lean reading that you will chase with larger jetting to correct. All the time you will be making the engine run richer than it should because of the ambient air the sensor is sensing.

On EFI engines you want the sensor to be within 12 to 18 inches of the exhaust valve. The reason is time delay. Further down the exhaust, the event the sensor is reading about has occurred so far back in time the the fueling correction no longer has any meaning except perhaps in a stationary power plant engine that continuously runs at the same rpm.

In an automotive application where load and engine speed are continuously changing so to is fueling demand. To properly monitor and respond to changes in load and rpm the sensor needs to be 12 to 18 inches from the exhaust valve. The easiest way to do this is weld an O2 sensor bung into one tube. The problem is it only monitors a single cylinder.

The easiest fix is to make a rectangular spacer that fits between the header and the side pipes with the center hollowed out to allow all four cylinders to mix up their exhaust pulses. Place the O2 probe so it's sensor is in this chamber and you will sense the entire bank in real time soon enough to make fueling changes that will make a difference. Be sure to correct the header / side pipe dimensions to correctly reposition the side pipes outside the body.

The absolute fix is to use an O2 sensor on each exhaust tube close to the port like the OEM manufacturer does during their EPA fuel system calibration. This is expensive and unnecessary for our use.

Sensors in the collectors or anywhere that is not 12 to 18 inches from the valve will give false reports to an EFI system and forever keep your engine out of tune. There is a reason the OEM's place the O2 probes so close to the exhaust port.


Ed