2006 Jetta, does the turbo make boost even with VNT vanes fully open?

Trying to figure out the problem with the car which has a freshly rebuilt turbo. Currently for testing purposes the vacuum hose to the VNT actuator is disconnected which means the vanes are fully open. Does this mean that no boost can be made? If so, why does it feel like it is? It has plenty of power when I step on it and can feel the boost kick in.

Turbochargers in general shouldn't really make boost at idle conditions. If it did, the exhaust plumbing would be overly restrictive and the turbocharger sizing would be much too small to support higher power levels.

When the vanes are fully opened, the exhaust is allowed to flow freely to the turbine. Thus, as soon as extra power is generated by the engine, in any amount, the exhaust will increase in heat and volume. Boost will then be generated when the turbine speeds up. At low power levels, it may seem that the time delay is excessive. Usually referred to as 'turbo lag', it is one of the big factors in judging turbocharged systems, particularly compared to supercharged systems.

The vanes used as control of boost as opposed to using a wastegate has the advantage of retaining some back pressure in the exhaust manifold to increase response as well as being able to control the boost level with a much better level of granularity. To achieve the same level of control with a wastegate would require an array of small wastegates, which would be a large disadvantage both in terms of installation (size, weight) as well as cost. Computer control of the vanes is both cost effective and more efficient.

Sorry to wander afield of the original question, but maybe it will help someone else.

Cheers,

PH

So it's normal to have some boost even when the vanes are fully open? This thing is going to be quick once the vanes are connected and it makes some real boost.

You would be overboosting at low rpm with the vanes fully open.

Someone correct me if I'm wrong.
The turbo sits with vanes fully open. When you start the car the N75 applies full vacuum, closing the vanes. So your car with no connect to the actuator will be full boost, as far as the turbo is concerned. So yes, it would feel good under acceleration and high speed.

You are wrong.
By closing the vanes you normally do not close them up entirely. The vanes narrow the exhaust path (as your thumb would over a garden hose) and what that does is increase the velocity of the exhaust and aim it at the outside edge of the turbine (hot side) wheel. Max boost position.

IF they close too far, as they happen to do on a great many BRM cars, then the vanes are choking the engine off so that it can't spool the turbo and create boost. When that happens exhaust manifold pressure will get so high that it eventually forces the vanes open against the wishes of the ECU map and that is when you get the surge of boost, black smoke and power. The actuator will not allow the vanes to open until the MAP sensor reads that there us enough boost to satisfy the request. So the actuator is fighting to keep the vanes closed to create the boost that cant be made with the vanes shut down too far.

To add to what Jetaah said, this is why so many people do the band aid "zip tie mod" on the BRM BW turbochargers. To limit the vanes from going too far. This happens when the turbo is worn as well as because the stop is worn down.

Rebuilding these seems to always result in a condition worse than before. A NEW turbo will fix it.

The stop is a grub screw/jam nut affair on the cast iron exhaust manifold housing, and they love to break off if you try and mess with them. They are set by BW at the factory. So messing with the cool side (the aluminum housing compressor and center section) won't help.

As I mentioned to the OP in his other thread regarding this subject:
Here is a post by Jeff from Rocketchip tuning regarding the BRM's turbo
https://forums.tdiclub.com/showpost....45&postcount=3
Some will over boost with no vacuum attached because of the manufacturing defect.

If the hose is attached to the VNT actuator the car runs lousy. Engine bogs down at lower rpm, lots of black smoke, knocking noise, smells like the cat is overheating. I could barely make it up a hill before my house in 2nd gear. If I get to 2200 rpm it usually clears out and gets its power back. So what should I do. Spent $450 on the rebuild. Adjust the actuator? Then I have to take the turbo back out which is a big pain.

Did you try the ziptie mod? ...just to verify that is your problem?


It is such a common problem that I have made a more durable fix for this that accomplishes the same as a zip tie fix.




The collar limits the travel of the actuator rod. This can all be done from the top side with the turbo mounted.


I would see if the place that rebuilt your turbo could do something ...change the stop screw position. It can be done with the turbo in place but you might have to move a couple of things to be able to reach it.

I have a feeling that the only thing that was rebuilt on your turbo was the rotating assembly and nothing to do with the vanes. I would venture to say that they did no adjustment of the lever stop screw and now it maybe difficult to loosen the jam nut without the risk of breaking the casting as Oilhammer mentioned. I have broken a couple and vowed to not try that again unless I had the turbo on a bench with a torch on hand. The rebuilder couldn't have tested it with any kind of serious equipment if they gave it back to you like that.


AND...if your car runs reasonably well without the vacuum attached, you probably have one of those faulty BRM turbos that does not fully open the vanes when the lever is in the no-boost position...all the more reason to suspect that the vanes are in turn closing down too far with the vacuum applied.



You did not answer if you car has a tune on it.

Suppose I could try that. Looks like it was gone through pretty thoroughly though.


Before dropping it off I took it apart myself.

Apparently it has a Rocketchip stage 1F chip tune.

atikovi said:

If the hose is attached to the VNT actuator the car runs lousy. Engine bogs down at lower rpm, lots of black smoke, knocking noise, smells like the cat is overheating. I could barely make it up a hill before my house in 2nd gear. If I get to 2200 rpm it usually clears out and gets its power back. So what should I do. Spent $450 on the rebuild. Adjust the actuator? Then I have to take the turbo back out which is a big pain.

Click to expand...

You WASTED $450 on the rebuild, which was halfway to a NEW turbo that would have not had this problem:

https://www.idparts.com/borg-warner-turbocharger-a5-brm-p-1436.html

Seriously, I get a half dozen "rebuilt" TDI turbochargers in here a year, and they all end up either getting a new one, or....

atikovi said:

Apparently it has a Rocketchip stage 1F chip tune.

Click to expand...

That will make the symptoms worse.
I had BRM cars Rocketchipped that had this issue and had to send the ECU back to restore the OEM program. It made the car undrivable just like yours.
With the OEM program it still overboosted at cruising speed but not enough to throw into limp mode.

With the OEM map restored, the car would settle in at 8-12 PSI boost on the highway when it should have been closer to 4PSI. That made the owners oblivious to what was going on with the boost but still had the bog characteristic upon acceleration from a standing stop that might have been treatable with a zip-tie mod.

An adjusted stop screw or the zip-tie mod does not fix the entire scenario because your actuator is now topping out at the new 'stop' at a lower vacuum quantity. And, the full open position now reads differently (higher%) for the actuator position sensor.
Shortening the actuator stroke with the zip tie now makes the lever meet the stop at 14-15"hg rather than 17-18"hg. If you adjust the preload of the actuator rod to meet the 17"hg figure at the stop you have extended the rod and fudge the actuator's position sensor figure.

You can do that to a point but too much deviation and it will throw a code because the position sensor figures do not make sense in the ECU. That is not where the actuator should be under the conditions. It is a balancing act. An actuator sensor delete in the ECU programming would work here.

If you love the car and have long term plans to keep it you are better off ditching the turbo and plugging in a new Garrett.

No I don't love it, I bought it to sell. The body and interior are very clean, the air blows ice cold, it steers and handles tight and it's a one owner car who took good care of it. Transmission rebuilt with new clutch not long ago, new cam 100K ago, recent tires and brakes. If it didn't have this turbo issue I wouldn't hesitate doing the timing belt thinking somebody will take it beyond 400K with 500K being doable. Not buying a new turbo, it drives just fine now with the hose off. I might try the zip tie mod but wonder if I get much for it the way it is, and if so, dump it.

Still haven't done anything with it, just been driving it and the AVERAGE mpg on the gauge is 54 mpg. Steady highway mileage is even more. Is this normal? I know diesels get high mileage but never thought it would be this good, or is it because the waste gate is full open? If so, might leave it the way it is.