Vehicle technology has continued to improve to reduce emission levels. As a result, engine and turbo technology has increased in complexity, and the settings and control of advancements, such as the Variable Nozzles, have become more critical to the correct operation of the turbo. This is now presenting new challenges to turbo repairers, who must use an Air Flow Rig on some turbos.
What is a Variable Nozzle Turbo (VNT)? The variable nozzle (also referred to as a variable geometry) is designed to change the exhaust gas inlet area to adjust the turbo boost. For low speed response, the nozzle vanes move to the ‘closed vane’ position to reduce the nozzle area. This increases gas speed through the turbo giving improved response at low engine speeds, similar to squeezing the end of a hosepipe to make the jet of water more powerful. At higher engine speeds, the actuator the nozzle vanes are fully open, to maximize the exhaust gas flow and reduce boost.
Vane Setting Accuracy When setting up a new/rebuilt turbo, vane positions are set using accurate air flow equipment, which ensures that the ‘minimum vane opening’ position is set to allow a specific mass of air flow through the vanes. If the vanes are too closed, this can cause choking of the engine and overspeeding of the turbine. If it is set too large, the turbo will have too much ‘lag’ and not respond as well as it should.
Traditionally, turbo repair workshops did not use an air flow rigs to correctly set the flow. The actuator position was set based upon an accurate measured position of the actuator arm. This produced acceptable results and allowed the repairers to keep on repairing. In reality, this method of setting the vanes can produce quite large inaccuracies in the flow of air. On older turbo repairs, the variable nozzle position had to be a long way out before the performance was unacceptably affected or for the ECU to flag a problem. From an OEM perspective, this is not acceptable and is the reason for their lack of support of repairing.
The need for accurate air flow setting of turbos was well understood by reputable repairers, and hence some quality repairers developed their own air flow equipment to accurately set their turbos, resulting in a reduction in warranties and the ability to build on their reputation as a quality repairer.
Today’s Turbos Many premium brand vehicles have moved to electronic actuation, which gives positional feedback to the ECU. Some more advanced turbo controllers now sit within the CANbus, talking directly to the injection system and air mass sensors, to respond more quickly to engine demands. For these turbos, the settings are either correct and accepted by the ECU - or not, which results in warning lights, limp home mode or refusal to start.
Making the Right Choice Traditionally, in the turbocharger aftermarket, the customer had a choice between a new OE turbo and a remanufactured turbo. What we now have is three tiers, a new OE turbo, a high quality remanufactured turbo repaired using quality parts and the correct equipment, and a poor quality repaired turbo, using inferior quality low cost parts. There will always be a market for all three options depending upon the vehicle owner’s requirements.
It is important that garages understand the different levels of quality for repaired turbos and therefore a different level of associated cost. When outsourcing your turbo repairs, it is crucial to consider the real cost of replacing a turbo and to educate your customer about the different options and associated risks, such as a turbo failure that damages other engine parts. Many turbo specialists already have a flow rig and are repairing turbos to an excellent standard.
To request further details about Melett, including information about where you can find your local quality turbo repair specialist, email:
turbo@melett.com.
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