Perhaps an ignorant question-

Most, if not all dyno curves using the stock twins/manifold show breakup after 6K- 6.5K RPMs. Some have implicated the internal WG, which is probably correct. It is possible that the WG is flapping, but more likely, the poor design of the TT manifold is such that the WG spits exhaust directly in front of the turbine exhaust, creating turbulence.

Okay, on to the question...

Would it be possible to remove the internal wastegate and weld a flange to the stock manifold (say, somewhere in the center) and bolt on an external WG?

Gene

 

The stock ports are what cause power to fall off after 6500 RPM's.

 

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Nope. Compared to a ported motor, yes. On a NA motor, yes. But remember, you're grealy enhancing VE with turbos. I've seen stock ported cars exhibit power rolloff after 6,500, but a simple switch from sequential to single will remove this power reduction after 6,500K.

Of course, this could also be due to running the stock twins beyond their maximum efficienty range.

Gene