I just found out something interesting. Excuse me if it's already old knowledge:

Recently fitted a boost gauge to my 16VT, referenced to the inlet manifold, and was wasn't impressed with the erratic boost. 1.2bar dropping to 0.8ish (as it's supposed to), But then boost dropped off significantly above 5000rpm. As though the turbo wasn't man enough for the job.

Fitted a manual boost controller (mbc,prv - whatever you wanna call it) and that made worse. Boost was all over the place, depending on what gear I was in, how quickly the throttle was opened, etc. And boost still dropping to 0.6bar above 5000rpm.

So I removed the compressor outlet pipe from the mbc and swapped it for the pipe from the dump-valve. So now the mbc reads the inlet manifold pressure instead of the compressor outlet pressure.

And Presto. Steady 0.9bar all the way from 3000rpm-redline - in every gear - all the time.

any pics of compressor outlet pipe from mbc as this sounds interesting

Yea I'm intrigued too!

Me too!

Now put a Boost Gauge on the Turbo side of the throttle body and you will probably see a big spike of about 20psi when you snap the throttle shut and well OTT boost on part throttle, Wont be long untill you need a new turbo,

Its like that for a reson as the waste gate now only reacts to the boost in the Inlet manifold not the actual boost the turbo is having to make

If you want to get smooth boost you need to Have a take off for the Wasegate as close to the Throttle body as posible but NOT on the Manifold side it must be on the I/C side even then it can cause issues if the i/c is restrictive

The drop off at high rpm is caused by poor flow through the I/C ie say there is 10PSI on the inlet side of the I/C there may only be 9PSI on the outlet side due to the restriction at high flow rates