duwem,
That is the high pressure turbo exhaust bypass. The little turbo cannot handle all the exhaust energy and would boost out of control without it. Ideally I'd use a small non-vnt turbo without a wastegate and use the big wastegate to control the boost pressures but I have the VNT and can be more flexible controlling it.
Control of the low pressure(big) turbo is completely separate from the control of the high pressure turbo, the LP turbo takes care of itself with the stock wastegate, I'll set it between 16 and 18psi. The bypass wastegate is also a safety device of sorts, EMP:IMP is usually around 1 and its pretty consistent except for some situations(spool and high RPM), so that it will be set up to open when exhaust differential pressures are too high and it will keep the turbo from overboosting individually.
Andy,
I'm 100% sure I don't need a compressor bypass
, they're very well matched and the LP can't handle the pressure ratios needed to use a bypass.
The turbine bypass was the smallest one I could find! Its 38mm.