fluttering noise when lifting thottle
 

86 Turbo, when im driving it hard through the gears, i hear a sort of fluttering noise when i lift the thottle between shifts. I do have a cracked header, but this sounds as if its coming from elsewhere, I checked turbo piping, all seems ok. any ideas?

No problem...just BOV (Blow off valve) releasing the pressure....

It should be that way.

I didn't know there was a stock blow off valve, where is it??

It's a black plastic valve connected to the intake just before the throttle body. 1 vaccum connection on top, 2 1" (or so) hose connections. They are often bad. Mine was bad when it made the fluttering noise. You probably have an aftermarket intake, or you normally wouldn't hear the noise. You should not be able to blow through the 3 openings. The 930 valve is marginly better using steel internals.
DanD

stock intake, how can i check to see if the valve is bad???

Sergio

You can use a Mity Vac and test the vaccum connection. It should hold a vaccum. Or just blow through the 2 openings.
DanD

I don't know much about 951s, but if they use a Vane-type airflow meter, like on 944 NAs, it could be on the way out. The lower edge of the housing will wear slightly (you won't be able to see it) and the vane catches on it. It makes a fluttering noise. Sooner or later it leads to rough running and poor fuel consumption. I had the exact problem with my old BMW 535i. Took a while to diagnose, but ran beautifully after it was done.

Good Luck! SmileWavy

I just wanted to clarify that these are not called BOVs. They are Compressor Bypass Valves. Maybe they do the same or simular thing. I'm not sure.

I'm not even sure if this is his problem, but it was on mine.

Here is a picture.

CBV pict

As the text in the url says, the real name is Air Cut-off valves, but that is not that important ;)
What's more important is that these things have to be in order to have a smooth turbo-acceleration. Try the test as described on the page and if it needs replacement, do it asap, and you'll notice a nice increase in turbo-efficiency.