external wastegates on o2

[Quammen]

i dont get how an external wastegate could relieve some of the exhaust gas from going into the turbo if its after the turbo on the o2 housing?

 

[Blk_99gst]

i dont get how an external wastegate could relieve some of the exhaust gas from going into the turbo if its after the turbo on the o2 housing?

I have an o2 sensor housing with the external dump tube. Here's how it works. We know all the exhaust gases go into the top of the turbo, and then there are 2 outs that the exhaust gases can take coming out of the turbo. There is the turbine side, and then the wastegate side. And to match that, the o2 housing has the main section for removing gases from the turbine side, and then it has a whole for the wastegate side, which then gets re-routed into the turbine side right before the downpipe.

When the turbo spools up, all the gases are used to spin the turbine, so the wastegate is closed (no gases escaping through it) and all the gases come out the turbine outlet of the turbo into the turbine section of the o2 sensor. When the turbo hits your desired boost, the wastegate flapper opens, giving the exhaust gases an easier means of escape, so they go out the wastegate side and do not spin the turbine up anymore which would give you more than your desired boost. So the excess gases are coming out of the wastegate outlet of the turbo, into the wastegate inlet of the o2 housing. So depending on what o2 housing you have, they wastegate gases can either be re-routed into the main turbine exhaust stream again, OR you can just have a tube dropping down to the ground to release them right there (dump tube).

If you choose to dump, then the gases from the wastegate don't have to flow through the entire exhaust system and out the tailpipe, which does 2 things. A) gives more room for the turbine only gases to get out. and B) You'll experience higher top end because the excess gases that aren't spooling the turbo can get out of the engine faster; it's easier to push them out of the dump tube rather than them traveling all the way down the exhaust piping.

Hope that clears things up a bit.

EDIT: Not sure if your question was concerning internally gated turbos with an external dump o2 housing (what I described above). If you are talking about externally gated turbos, then it's the same prinicple. The external gate gives the exhaust gases another way out, bypassing the turbo, when the desired psi is reached. Where the external gate is located can be in different areas; on the exhaust manifold, o2 sensor housing. Either way it's just bleeding off the excess gases that the turbo doesn't need.