i operated a internal wastegate with around 2 bar of manifold pressure on a vf34/vf35, i then tried a external waste gate (38mm turbosmart) and upped the manifold pressure to 2.3/2.4 bar, it was still more then enough to cope with whatever manifold pressure you plan to operate. remember that your header-up pipe diameter is not much larger then 45-50mm anyhow, so to slot a 38mm hole into the up pipe for a waste gate and still experience boost creep- there must be problems with the gate or boost control.

as a side note, there was little to no improvement in installing an external gate.

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