Turbo Calibrating - Setting VNT Turbos

turbo stop screw

How do I adjust Turbo VNT stop screw?

Modern Turbochargers use a VNT type design, also known as a variable geometry. The turbo shaft speed is controlled by a nozzle ring and vanes which direct air to the turbo Turbine Wheel. As turbo shaft speed reaches optimum speed, the turbo vanes open and allow exhaust gases to pass around the Turbine Wheel and not through the Turbo Blades.

If too much air is directed to the Turbine Wheel, the Turbocharger will over speed. This will pump too much air into the engine and cause running issues and possible damage. Many turbocharged vehicle's ECU can see too much air flow and will put the engine into limp mode. As engines consume less air at lesser RPM, a way of restricting the air at low RPM is required. This is done via a locked off grub screw or as it is more commonly known, a Turbo VNT Stop Screw. If the Turbo VNT Stop Screw is unscrewed too far, you will (if you are lucky) find the turbo seems to not be boosting, chances are the engine's ECU has entered limp mode as it can sense the turbo is over boosting. If you are unlucky, the ECU will not see this and you can destroy the turbo and the engine.

If the Turbo VNT Stop Screw is screwed in too far, the turbo will not begin boosting until a higher RPM. This will put excessive load on the engine, cause over fuelling, sticking vanes and can result in turbo failure and again engine damage.

set turbo vnt stop screw

Setting a Turbo VNT Stop Screw is a dangerous job without a dedicated flow bench and isn't something we would recommend. Simply matching the Turbo VNT Stop Screw against the original WILL NOT guarantee a success, after all, the new Turbo CHRA will not be an exact replica of the original and, as such, you won't get the same result. Also, the VNT unison ring and nozzle ring and turbo vanes may have worn, so the increased play will result in incorrect turbo air flow. Garrett say that the VNT Turbo cannot be repaired as the VNT mechanism cannot be correctly calibrated to ensure the airflow is correct, BUT it can now, and with Turbo Rebuild being the first company in the UK to have this particular machine, we are the leaders in not only adjusting Turbo VNT Stop Screws, but full turbo calibration.

Turbo Rebuild have a state of the art turbo calibration machine or VNT flow bench. Our VNT calibration is more than a tool to just set the Turbo VNT Stop Screw. Our VNT calibration bench will test and calibrate not only the Minimum Flow, but will also test and calibrate Turbo Maximum Flow. This will make sure that not only is your new Turbo or CHRA operating at the correct engine RPM, it is also never over boosting.

adjusting turbo stop screw

So your turbo is calibrated, but what about the Turbo Actuator? Over time, the internal spring in your Turbo Actuator can become weak, the diaphragm could stretch or the arm could even stick. Not only does our Turbo Flow Rig calibrate the Turbo VNT stop screw, Minimum Turbo Flow AND Maximum Flow, it will also calibrate your Actuator!

Whilst testing, our calibration machine not only tests for vacuum leaks, it also checks for arm operation. Finally, at the testing stage, it confirms two arm positions at set RPM to make sure the turbo will always be in the right place at the right time!

set turbo actuator

Turbo Rebuild have successfully calibrated 1000s of VNT Turbochargers without issue, and with over 1000 turbo settings stored on file, we are confident we can calibrate most VNT Turbos. We can calibrate and set VNT Stop Screws on not only Vacuum-style Actuators, but also Electronic Turbo Actuators. With Electronic Turbo Actuators, our calibration machine will also test the Electronic Actuator to ensure you do not have a faulty Electronic Actuator. If you have, that's not a problem, we should be able to fix that too!

Replacement Turbo VNT may get your turbo working, but it wont:

  1. Ensure turbo spools at the correct RPM
  2. Ensure you dont over boost your turbo
  3. Ensure the actuator is correctly calibrated

Many customers of ours call and say how their turbo has never performed so well... Turbo Rebuild WILL restore ALL of the boost, not just some!

How much to calibrate a VNT Turbo?

We can offer a complete Turbo Calibration service that includes:

  1. Visual inspection of Actuator
  2. Manual inspection of VNT arm movement
  3. Test Actuator for leaks
  4. Test Actuator for full arm travel
  5. Test and calibrate Turbo for Minimum Flow (setting VNT stop screw)
  6. Test and calibrate Turbo for Maximum Flow
  7. Test and calibrate Turbo Actuator High Boost Position
  8. Test and calibrate Turbo Actuator Low Boost Position
  9. Calibration certificate
  10. Return postage


The cost for this service is £80.00 plus VAT



To arrange your VNT Turbo Calibration