VNT control idea

[DonGTI]

Boost in the brake's vacum system... who thinks that's unpleasant?

[burnt_servo]

lots of different vehicles use the same vacume source as used by the brake booster .....  
anything added on would have to be engineered in such a way to prevent vacume from bleeding off of the bosster ( like a one way valve put in the line going to the  booster that some cars already use ) .

if failure of the break booster causes failure of the brake system ,   there is something seriously wrong .

[DonGTI]

ummm i shamefully admitt that in a hurry to drive my car after an aaz conversion i was't too fussy regarding which vacum huse went to where... they all looked alike, i just wated to see how it goes, after all i was used to driving with booster off when my vacum pump dies before conversion...

At 60 mph i was nearing a roundabout, started applying brakes... and lo and behold the pedal was extremely stiff! panicked, engine braked real hard and managed to stop it... limped back home to garage... at low dpeed it was not doing it... looked over all hoses and decided to re-test it.  Again at low speeds it was normal, but feel was funny when engine revved.  Did another run and much the same happened.

Took the whole vacum system pipe out and started fitting all things and tracing them... then i got to a particularly long pipe which went to a fitting on the intake manifold! apparently someone had a boost gauge hoked up to this engine and he left the pipe there... and i was ummm stupid for not paying propper attention, should have known better

[MrDave]

Put a check valve on the line, of course, so you keep vacuum for the brakes.

The boost-valve wouldn't start to bleed boost pressure in till you get close to the setpoint (ie: 18lbs).

In theory, the VNT would deliver maximum possible boost for a given driving situation (ie: cruising) and if the boost got to 18lbs, would hold that pressure.

-Dave

[OM617]

Using just a pressure actuator will kill economy because the vanes will always be restricting the exhaust to try and reach the maximum boost pressure no matter the engine load.

Mercedes has a great vacuum system that has variable vacuum valve that alters the system vacuum based on throttle position. It could be possible to adapt a similar system to your VW, that would eliminate the linkage mess. The turbo will also build boost instantly to the altered pressure rather than be dependent on throttle position to determine how much the vanes close on acceleration.

See what I did with mine:
http://vwdiesel.net/phpBB/viewtopic.php?t=10365&start=62
I often get a 1:1 or better pressure ratio driving around town under 2500rpm.