How does using a VTG help in reducing turbo lag

  • Context: Automotive 
  • Thread starter Thread starter monty37
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Discussion Overview

The discussion revolves around the role of variable geometry turbochargers (VTGs) in reducing turbo lag, particularly at low speeds, and whether it is possible to eliminate lag entirely. Participants explore the mechanics of VTGs, their efficiency across different engine speeds, and the nuances of control mechanisms involved.

Discussion Character

  • Technical explanation
  • Debate/contested

Main Points Raised

  • Some participants inquire about how VTGs specifically help reduce turbo lag at low speeds and whether lag can be completely eliminated.
  • One participant asserts that turbo lag cannot be entirely eliminated due to the time required to spin up a turbine.
  • There is a suggestion that while VTGs may reduce the perception of lag at low speeds, their effectiveness is questioned, particularly in high-speed scenarios.
  • Another participant discusses the concept of "fine grained" geometry control in VTGs, suggesting that the degree of variability in geometry affects performance and lag.
  • Participants mention that VTGs are designed to have a low boost threshold and are efficient at higher engine speeds, contrasting them with traditional turbochargers.
  • There is a discussion about the control mechanisms of VTGs, with some participants suggesting that the electronic control unit (ECU) plays a significant role, while others note that many systems may be purely mechanical.

Areas of Agreement / Disagreement

Participants express differing views on the effectiveness of VTGs in reducing turbo lag and whether lag can be completely eliminated. There is no consensus on the extent of their effectiveness at low versus high speeds.

Contextual Notes

Participants reference the complexity of control mechanisms in VTGs, including the potential limitations of electrically operated systems versus purely mechanical ones. The discussion highlights the variability in performance based on the design and control of the turbocharger.

monty37
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In what way does the VTG help reduce turbo lag at low speeds? and can the lag be eliminated completely?
 
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So one can still feel the difference when driving at low speeds ...and when shifting to high speeds..but they are really effective only at high speeds na?
 
You might not be able to "feel" it. If the geometry control is fine grained enough, the point when the turbo starts adding power might not be noticeable, but it would be measurable. I'm not sure what you mean by the latter half of your post.
 
The link says "VGTs have a minimal amount of lag, have a low boost threshold, and are very efficient at higher engine speeds" so that's what I asked. And may I know what you mean by geometry control being fine grained?
 
monty37 said:
The link says "VGTs have a minimal amount of lag, have a low boost threshold, and are very efficient at higher engine speeds" so that's what I asked. And may I know what you mean by geometry control being fine grained?


Low boost threshold : this means they start making boost at low RPM.

Minimal lag: Same thing.

Very efficient at high engine speed: Traditional (non-variable) turbos are trade-offs and only work well at one end of the RPM range or the other. Variable turbos work well at both ends.

By fine grained I mean the degree with which the geometry can vary in relation to engine RPM. The purely membrane/mechanical ones should are infinitely variable in theory. Electric and hydraulically operated vanes will be subject to the granularity provided by the control mechanism.

An electrically operated VGT that only has two states (open and closed) would function exactly like a non-variable turbocharger. I'm not familiar enough with the systems to say if the electrically operated ones are digitially controlled, but I would bet they are, and this means there are a finite number of "steps" the vanes can be put through between fully open and (nearly) fully closed.

The more steps you have, the more fine grained the control, and the better the turbo is going to operate over the full RPM range.
 
The control has mainly got to do with the ECU then right?
 
monty37 said:
The control has mainly got to do with the ECU then right?

I would assume so on the electrically operated ones. I believe most are purely mechanical though, operated via intake manifold vacuum.
 

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