The topic of bicoolers causing turbo lag came up in a discussion, and for my own part I put this question to rest shortly after obtaining a bicooler.
I filled it with water, measured the true volume of the core at 248 cubic inches, and did some quick math based on an estimate for the volumetric flow rate of the engine at 2000 RPM to guess that even if the core was a void, containing no air at all (not the case), that it would take 0.15 seconds to completely fill the additional volume.
As mentioned, this bicooler core isn’t a void, there is always air inside, so pressurizing it happens quickly.
A while back I measured boost onset with the IS20 and stock IC to get some baseline values. I also took similar readings once the bicooler was installed but in looking for the “after” data realized that I’d never posted it to the website and it was still only in an Excel file.
Intent on remedying that oversight, here’s the chart comparing boost onset of an IS20 turbo, APR stage 2 tune, with and without the bicooler connected to the stock GTI intercooler:
When posting the baseline data I explained the reason for measuring boost onset in this way.
There is no indication from this data that there is a difference in boost onset time when adding a Majesty bicooler to the stock GTI intercooler using an IS20 turbocharger.