Thread: Turbo GT2256V VNT - How to adjust boost?

Originally Posted by robm

Hi,
Since reading the quote about adjusting max boost by shortening the rod I have been on a fast-track learning curve about turbo diesel engines and I agree with you that adjusting the rod will only adjust the start of boost but not max boost. To get more boost I need to get more fuel in, more power out with resulting higher exhaust gas temperature to transfer more power into the turbo to compress the inlet air more i.e. get more boost.


Rob

Shortening the rod on the actuator will increase the boost rate (come on earlier) but you may end up running to higher boost when cruising.

Max boost is set by the stop screw on the actuator.
This is critical and a little change makes a big difference.
The stop screw sets the minimum that the vanes can be closed.
like putting your finger over the end of a hose.
Lowering the stop screw will increase your maximum boost but......
be careful and use a boost gauge if you choose to change the setting.
VNT's are usually setup by the manufacturer using a bit of high tech gear so if you change anything mark the factory setpoints on the rod and stop screw so you can restore them if things get pear shaped.

Cheers,
Paul.

Adjusting the rod on the actuator should also adjust your boost levels.

I suspect that your engine has a pressure driven dash pot to control the vanes. The spring in the dashpot will compress as the turbo builds more boost and the actuator will move the vanes to a more open position. This increases the exhaust passages created by the vanes just before the turbine. The bigger the opening the slower the gas speed. By adjusting the length of the control rod you will cause the vanes to close down a little bit more than previously across the whole of the pressure range not just as boost starts to build. In order to get the vanes to open up to the same amount as previously you will have to build more boost as you will have to compress the dashpot spring further to make up for the change in actuator rod length.

harro beat me to it.

Originally Posted by harro

Max boost is set by the stop screw on the actuator.
This is critical and a little change makes a big difference.
The stop screw sets the minimum that the vanes can be closed.
like putting your finger over the end of a hose.

I disagree but I'm no turbo expert.

I understand as Rob has. More fuel = more boost, it will keeping making boost if you can keep giving it fuel. Vanes are there to get the turbo spooling earlier and delivering torque lower.

I control my VNT with one of these Manual Boost Controller

Originally Posted by harro

Shortening the rod on the actuator will increase the boost rate (come on earlier) but you may end up running to higher boost when cruising.

Max boost is set by the stop screw on the actuator.
This is critical and a little change makes a big difference.
The stop screw sets the minimum that the vanes can be closed.
like putting your finger over the end of a hose.
Lowering the stop screw will increase your maximum boost but......
be careful and use a boost gauge if you choose to change the setting.
VNT's are usually setup by the manufacturer using a bit of high tech gear so if you change anything mark the factory setpoints on the rod and stop screw so you can restore them if things get pear shaped.

Cheers,
Paul.

Where is the stop screw?
Do you have a drawing or photo to show it?
Cheers
Rob

Originally Posted by slug_burner

Adjusting the rod on the actuator should also adjust your boost levels.

I suspect that your engine has a pressure driven dash pot to control the vanes. The spring in the dashpot will compress as the turbo builds more boost and the actuator will move the vanes to a more open position. This increases the exhaust passages created by the vanes just before the turbine. The bigger the opening the slower the gas speed. By adjusting the length of the control rod you will cause the vanes to close down a little bit more than previously across the whole of the pressure range not just as boost starts to build. In order to get the vanes to open up to the same amount as previously you will have to build more boost as you will have to compress the dashpot spring further to make up for the change in actuator rod length.

harro beat me to it.

What you describe suggests that by changing the rod length I will move my torque curve up or down the rpm axis.
I think that where it is at the moment is OK.
But I was looking to increase the max boost to the specified value.

Originally Posted by robm

Where is the stop screw?
Do you have a drawing or photo to show it?
Cheers
Rob

Not familiar with your particular VN turbo but the rod coming from the actuator should be connected to a lever on the turbo.
At rest this connection should be sitting on an adjustable screw usually with a locknut so it cannot turn by itself.
Lowering this 'stop' will allow the vanes to close further increasing the 'maximum boost'.
As I have already said BE CAREFUL if you adjust this!


Here you go, I found this, probably not your particular turbo but enough to get the idea. Stop screw with yellow paint
Cheers,
Paul.

Originally Posted by harro

Not familiar with your particular VN turbo but the rod coming from the actuator should be connected to a lever on the turbo.
At rest this connection should be sitting on an adjustable screw usually with a locknut so it cannot turn by itself.
Lowering this 'stop' will allow the vanes to close further increasing the 'maximum boost'.
As I have already said BE CAREFUL if you adjust this!


Here you go, I found this, probably not your particular turbo but enough to get the idea. Stop screw with yellow paint
Cheers,
Paul.

Hi Paul,
thanks for the photo.

You are right about the stop screw setting the closed position of the vanes.
But I have also found other information about VNT's.

The following are my conclusions.

The closed position does not give the maximum boost. It only determines how low in the revs the turbo starts to give boost.
Maximum boost is obtained when the vanes are fully open.
Reducing the length of the rod could increase the maximum boost if there is enough travel of the diaphragm in the control capsule and if by reducing the rod length the lever is still sitting on the end stop with the turbo at rest. If it isn't then the turbo vanes will be slightly more open and boost will start to be produced at a higher speed.
If there isn't enough travel of the diapragm to fully open the vanes, after the rod length has been reduced, then this would restrict the exhaust gas flow at high revs and strangle the engine as well as probably overheat the turbo.

I am obviously answering my own questions now!

To be safe the only way to adjust the turbo is to take it off and apply an adjustable accurately measured pressure source to the boost capsule and see at what pressure the vanes start to open and at what pressure they are fully open and check that they do fully open.
Then changes can be made to the rod length to see if it is possible to increase the boost (by needing more pressure to open the vanes) and check that the vanes do fully open at the desired max boost.

I will do that when I take the turbo off to fit an egt to the exhaust manifold.

Cheers
Rob

Originally Posted by robm

Hi Paul,
thanks for the photo.

You are right about the stop screw setting the closed position of the vanes.
But I have also found other information about VNT's.

The following are my conclusions.

The closed position does not give the maximum boost. It only determines how low in the revs the turbo starts to give boost.
Maximum boost is obtained when the vanes are fully open.
Reducing the length of the rod could increase the maximum boost if there is enough travel of the diaphragm in the control capsule and if by reducing the rod length the lever is still sitting on the end stop with the turbo at rest. If it isn't then the turbo vanes will be slightly more open and boost will start to be produced at a higher speed.
If there isn't enough travel of the diapragm to fully open the vanes, after the rod length has been reduced, then this would restrict the exhaust gas flow at high revs and strangle the engine as well as probably overheat the turbo.

I am obviously answering my own questions now!

To be safe the only way to adjust the turbo is to take it off and apply an adjustable accurately measured pressure source to the boost capsule and see at what pressure the vanes start to open and at what pressure they are fully open and check that they do fully open.
Then changes can be made to the rod length to see if it is possible to increase the boost (by needing more pressure to open the vanes) and check that the vanes do fully open at the desired max boost.

I will do that when I take the turbo off to fit an egt to the exhaust manifold.

Cheers
Rob

With the vanes fully open boost will be at it's least.
Similar to a wastegate turbo if the wastegate is fully open.

Cheers,
Paul.