Have some issue with sudden drop of boost after new GT2860RS on duty......here what's happen....manual boost controller set at 1 bar(waiting for Flea map to be sent, and can't afford too much), spools fine from 2500rpm and 1 bar reached at 3100 - 3300rpm.....third gear....when in 4600-4900rpm suddenly boost instantly drops to -0.5(vacuum), and it seems that ecu cuts ignition or fueling.....or like wastegate wide opens without any reason. 2 different boost controllers tried - same result.
Can MAF reach it's upper limit, ecu unable to find appropriate map or fueling, and thus cutting the ignition or fueling. Did some onboard diagnostics with FiatEcuscan, but this soft is very slow, and i can only see that spark advance change values when that happens.....
So is the MAF reach it's limit, or there's problem with boost controller or wastegate?

If you are getting a high vacuum while your foot is to the floor then I'd guess you are getting a constriction in the intake somewhere. Colapsing intake hose?

Checked and double checked all hoses for leaks....BOV as well.....boost drop is so huge and instant, that i must have hose disconnected to cause it.

I don't mean a leak. With foot to floor no boost should be just less than zero. If you mean -0.5 bar then something is blocking the intake. Most likely cause is a weak hose going to turbo collapsing due to the suction. Dirty air filter wouldnt help.

Could be the wastegate. I had different symptoms on my car as mine would not boost at all until over 4000rpm. Checked all the hoses, disconnected the EBV but finally worked out that the wastegate actuator had come loose from the gate itself. A simple fix but caused JBT much sucking of teeth during the diagnosis!

A wastegate that's stuck open wouldn't cause vacuum, it would seriously restrict boost (probably no not much more than 0 bar) but definitely not vacuum

A boost hose coming off would cause all sorts of other running problems - in fact any hose off after the turbo would probably stop the car altogether

My guess is an issue with the turbo - either the shaft is seized, or one of the wheels (compressor or turbine) has loosened on the shaft, meaning there's effectively no turbo

Costly guess - it's brand new - 3-4 weeks used GT2860rs....will continue to investigate....

Dear friend, i promise that if i ever come in UK again(saw you live in Suffolk), i'll do my best to find you, and buy you at least few cans of your best beer money can buy.....
Here you can see how stupid i'm...
Have 500mm silicone hose between the turbo and MAF, and it's too weak to hold that suction. Problem now solved......can't wait for Flea's map to arrive, and see the real power of my beast....
Thank you very much.....

Just out of interest... how is flea mapping your car when its in Bulgaria

... im intregued

Glad its sorted. Depending on how weak the hose was it may be that your air filter is a constraint near maximum power.

Well, with experience i guess.....told him all of my car's specs, and for sure he did it many times on other coupes locally....have wideband controller with sensor+gauge, and can monitor AFR(have probe and option to mount additional sensor just to be sure fuel mix OK and accurate).....
Don't get me wrong, our bulgarian tuners and mappers are all best in our country, nobody can do maps better than them, and everyone "knows" that all other mappers didnt have a clue about spark advance......no matter that will cost me more money, i prefer to bet someone with more experience,no matter where in the world he is......have awful troubles when mapped both of my Alfa 156's locally and don't want to do mistakes anymore......

I'm with KN used in Alfa 156 GTA, maybe must find less restrictive one....

ahhh fantastic best of luck!