What is the best place to put a O2 sensor on an EFI motor with twin turbos? Before the turbo or after? The exhaust is a dry system. I found a special plate that goes between the manifold and the riser that is 1" thick and is drilled and tapped for the O2 sensor. The plate is made out of aluminum.

Put the O2 in the downpipe of the turbo....not in front of the turbo.

OK, I am not familiar with Turbo systems. On my NA engine I have O2 bungs welded in the risers. I would think the AF readings would be more accurate before the Turbo. Again I am not a Turbo expert.

why is it better behind the turbo? Is in front of the turbo too hot for a wideband O2 or is it because of the damage that could happen to the turbo if part of the sensor breaks off?

O2 sensors don't like that much heat and don't like pressurization. I will get with my turbo guru if you'd like a more detailed explanation.