Author Topic: Dummies guide to setting up a VNT for an R5 GTT.  (Read 7042 times)


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Dummies guide to setting up a VNT for an R5 GTT.
« on: 15:48, Mon 26 October 2009 »
Dummies guide to setting up a VNT (Variable Nozzle Turbo) for an R5 GTT.

Disclaimer - viewtopic.php?f=14&t=21

When adjusting / increasing the boost levels make sure you are measuring your AFR's, also if you are running over 14psi I would have the TDC sensor set to fully retard in order to prevent det.
Once you have fettled a VNT turbo to fit the R5 GTT you will then need to set it for usage on your 5. (This takes some time and is not always achieved first time around so please be patient when doing this).

Things you need to know when you first use the VNT on your car to give you an idea of what and where you need to adjust in order to set it up.

    What rpm is boost coming in at?
    What are your afr's? ( with a VNT I would look for WOT fueling of 11.8)
    What is the boost level you are getting?
    Do you have an after market boost gauge plumbed in?
    Is boost read @ manifold or carburetor top?

Vane Control

In order to control the flow of the VNT vanes you can do this by adjusting the VNT vane control mechanism.  The VNT vane control mechanism grub screw is located on the side of the VNT (See Picture below). This can be adjusted to allow the VNT to provide full boost at different rpm’s.  To do this you can screw the grub screw in to bring the boost in later in the rev range and if you unscrew it you can bring it in earlier. Remember when you do this you may need to adjust the actuator length.
Note - It doesn’t take much adjustment of the grub screw to make a difference so be careful and patient.

VNT Vane Control mechanism and Grub Screw


The VNT turbo’s from factory are setup to run a vacuum actuator.  This works by pulling the vanes closed (Restricting flow / boost).  Where as a conventional boost operated actuator would push the wastegate open to limit boost.  To use a conventional boost operated actuator on the VNT you will need to adjust the length of the actuator rod.  This will restrict the amount the vanes are open thus creating a flow limit but retaining the VNT boost effect.  Remember you do not want the vanes to be allowed to fully open as this will create overboosting problems.  To stop this adjust the actuator rod length so that when it is fully extended (boost pressure pushing it open) the vanes are not quite fully open this will then create a flow restriction. (This way will cause a couple of psi spiking).

There are a couple of other methods that I have not tried as yet but in theory should work:

1) Mount the actuator under the turbo so that it will then push the vanes closed,
2) Drill and tap the base of the single port actuator so that it will then work as a vacuum operated actuator.  (See picture borrowed from philr5t project thread).

Phil’s VNT with actuator modified for vacuum usage (untested at present as car off road when writing this).

Pic 1 Above shows vanes closed (No boost or very little boost),

Pic 2 Shows vanes fully open (full boost).

The VNT turbo runs an a vacuum actuator which works in the opposite way to a conventional turbo actuator the vacuum actuator will pull the vanes of the VNT closed thus creating a boost limit.

With a conventional boost actuator you need the actuator to push the vanes control mechanism closed.  (To set the boost operated actuator up you need to use the actuator rod length as a setting to allow how far the vanes open, if the vanes open to much you will get over boost.   If you shorten the actuator rod so that it restricts the vanes from fully opening you are then controlling the flow of exhaust gasses of the vanes thus creating a boost limit.  (Due to this setup you will get a couple of psi boost spike)

Remember the VNT is a very different turbo to a conventional turbo with the way it brings the boost in.