944 Turbo - holes in power band?

[Spongebob]

I have an issue with my 1986 Turbo. Up until a couple of weeks ago the car has been driving beautifully but recently it has developed a major issue under hard acceleration.
Car starts on the button and under 'normal' driving behaves beautifully. It pulls cleanly and has been 100% reliable. The issue that has recently come up is under full throttle acceleration. Revs pick up to about 3k rpm and then it's as if you have driven in to a wall. It will then judder and then pop (sounds to be in the exhaust) and then clear itself letting out huge clouds of white smoke which rapidly clear and then it revs cleanly. It does this in every gear. It feels like a huge hole in the power band between 3 and 4k revs.
More gentle use of the throttle allows it to rev cleanly and freely to the red line in all gears and there is no evidence of smoking or hesitation under throttle. It's literally under full boost we have the issue and first happened when flooring the throttle to overtake an HGV.
I can't see anything obvious loose or missing under the bonnet, it is using no oil or fluids and as i say off full throttle drives fantastically. It is certainly a new issue as up to a couple of weeks ago it was a super fast beast on all throttle positions.
The car is fitted with Augment ECU, electronic boost control, up-rated fuel regulator and dual port wastegate. Until the recent problem it has behaved beautifully and is a very fast car.
Any ideas welcomed?

 

[Eldavo]

I'm assuming from the mods that the original cycling valve has been bypassed, otherwise it sounds very much like the over boost protection. Back when I was only running a boost enhancer mine would do this on cold days as it would briefly overboost leading to the KLR cutting fuel.

 

[PSH]

I would say overboost although my mind is a little fuzzy these days, however...possible vacuum leak...also check you BOV....if it's original it's likely to be toast....back many years ago in my first 5 years of owning and modifying my 951 I developed similar symptoms , a Bailey motorsport BOV cured these problems.

Pete

 

[Neil Haughey]

Definitely sounds like it, will happen on pretty much any turbocharged car with similar mods and in similar conditions. My old SAAB 900 would do the same thing in the winter or late at night when really cold.

 

[Spongebob]

I got in touch with Tom at Augment. He's going to put it back on the dyno and run some diagnostics. I initially thought vacuum leak but I can't see any pipes that are loose or disconnected. I did not think it was anything to do with ambient conditions as i used it before Christmas on cold days and it was fine. Yesterday in Oxfordshire was pretty mild and it was playing up badly.

The fact that it has just started suggests failure of something.
Oh well. More expense!

 

[PSH]

as I said check the BOV..if the diaphragm has failed (very possible) you could have such issues ..

 

[Spongebob]

so I took the car up to Tom last night and all the way to Gloucester it continued to do the same thing. Tom initially went through everything he'd done to the car to check nothing had fallen off or visibly broken. He removed the electronic boost controller, inspected it and put it back. He also checked the BOV would still hold a vacuum which it did so no issue there. He then ran it up on the dyno with his diagnostics plugged in and all was as it should be with boost dialled in at about 1.2 bar. Car ran beautifully but a rolling road doesn't give the same challenges and pressures as a real road so he dialed in a greater resistance to the rollers and had his dad lie in the back to try and maximise traction as the car was spinning the back wheels on the rollers on full chat. Again no problem, no smoke but really awesome glow under the bonnet as the headers glowed orange.
We then road tested the car giving it maximum beans up a few local hills. Performance was back to where it was and not the slightest hint of any problems.
Lots of head scratching we came to the conclusion that either some dirt or debris must have been trapped somewhere and dislodged or the process of taking bits off and back on again cured a leaking seal.
I drove back home using every roundabout and traffic light as an excuse to test the car in anger and it didn't miss a beat. Used nearly half a tank of super unleaded for the evening and I think I triggered a speed camera on the A40 just outside Gloucester but at least I have the huge grin this car creates back on my face.
Thanks all for the advice and suggestions.