I have a 93/94 wrx jdm and my up pipe has been blowing for some time now so I have decided to replace it.
While I have bits off the car I thought I would port the headers while im at it.
Now I have noticed that the lambda sensor is in the drivers side header just where all 4 ports come together but the down pipe I will be fitting has a hole for the sensor. which hole would be the best the one in the headers or the one in the down pipe?

 

Olly Clarke from Roger Clarke Motorsport advised me to leave the Sensor where Subaru intended it.

On my model, in the header.

 

Although later on Subaru realised that many sensors failed in that location so they moved it to the downpipe where temperatures are not so extreme.

Your lambda sensor will last longer in the downpipe.

 

Well my new stainless up pipe has just arrived and that has a hole for the lambda to so now I have a choice of 3 places but the up pipe has a second hole that is smaller that the one for the lambda but im not sure what that’s for

 

I think its for Exhaust Gas Temperature sensor EGT or Cat on Fire as used to be called.