Thread: Turbo GT2256V VNT - How to adjust boost?

Here is a link that describes both types of VGT turbo's

Variable turbines - BTN Turbo, the turbocharger people

and on the same website for a wastegate

Wastegates - BTN Turbo, the turbocharger people

Hope this helps.

Rob

Originally Posted by robm

My understanding of a wastegate is that it opens at maximum boost to limit the boost to that value by by-passing some of the exhaust gas. If the boost falls then the wastegate closes.

With a VNT it is similar in that the vanes must open to limit the boost otherwise as the engine speed increases the turbo speed would also increase and there would be no limit other than a meltdown.
In the photo of the VNT the lever is against the stop and as you said the vanes are closed. As boost increases, the diaphragm in the capsule pushes the rod down and opens the vanes.
The vanes opening is like the wastegate by-passing gas around the turbo.

You waste gate understanding is ok.

Waste gates are used for turbos that have a small turbine that spins the compressor faster (for more boost) at lower speeds. As engine speed increases the flow of exhaust gas becomes too great for the small turbine so the waste gate opens so this extra exhaust gas can bypass the turbine. Without the waste gate the exhaust pressure in the manifold will be too high, reducing engine power, and the turbo can overspeed leading to turbo failure.

VNT turbos have a larger turbine housing that what would be fitted to a wastegate turbo. The vanes change the angle of the flow to the turbine impeller. Vanes open or close is a poor description of what is happening.

Originally Posted by robm

I've started tweeking the injection pump.
I screwed the boost compensator stop screw in 1/2 turn.
It gave a noticeable reduction of the off-idle hesitation
It also improved the low rpm (1000 to 1200 rpm) smoothness and I can now drive at 1100 rpm in 5th gear and pull away smoothly.
I therefore gave another 1/2 turn to see if I could further reduce/eliminate the off-idle hesitation.
It made a smaller diference than the first 1/2 turn.
There is still a very slight hesitation.
I suspect that the pre-load on the compensator spring is too high and that the turbo boost is not pushing down the diapragm early enough even thogh I have boost even at tickover when the engine is warm.
This weekend I will try turning the control cone and backing off the stop screw to let the boost give a more linear effect.

The stop screw on the top of the compensator adjusts the fuel rate when there is no boost pressure. Once boost pressure starts to push the diaphram down against the spring the screw position has no affect. You will get smoke if the screw is wound down too far.

IMHO the cone position should be the 1st adjustment. This is because it changes the fuel rate across the full range of boost pressure, from no boost through to maximum boost.

So if you adjusted the screw on top, or the the spring underneath the diaphram before rotating the control cone, those previous adjustments will be affected and will require re-adjustment to get the tune right.

IMHO you should have an egt gauge before touching any adjustment (other than stop screw on top of compensator).

Also you need to test at full load. If you are making high egt at medium load it will go dangerous when you put more load in the back or climb a steeper hill.