Talk:Variable-geometry turbocharger

message: A/R describes a geometric characteristic of all compressor and turbine housings. It is defined as the inlet cross-sectional area divided by the radius from the turbo centerline to the centroid of that area.
    * Compressor A/R - Compressor performance is largely insensitive to changes in A/R, but generally larger A/R housings are used to optimize the performance for low boost applications, and smaller housings are used for high boost applications. Usually there are not A/R options available for compressor housings.
    * Turbine A/R - Turbine performance is greatly affected by changing the A/R of the housing. Turbine A/R is used to adjust the flow capacity of the turbine. Using a smaller A/R will increase the exhaust gas velocity into the turbine wheel, causing the wheel to spin faster at lower engine RPMs giving a quicker boost rise. This will also tend to increase exhaust backpressure and reduce the max power at high RPM. Conversely, using a larger A/R will lower exhaust gas velocity, and delay boost rise, but the lower backpressure will give better high RPM power. When deciding between A/R options, be realistic with the intended vehicle use and use the A/R to bias the performance toward the desired powerband.
Understanding housing sizing: A/R
A/R (Area/Radius) describes a geometric characteristic of all compressor and turbine housings. Technically, it is defined as:
the inlet (or, for compressor housings, the discharge) cross-sectional area divided by the radius from the turbo centerline to the centroid of that area (see Figure 2.).
compressor housing showing A/R characteristic
Figure 2. Illustration of compressor housing showing A/R characteristic
The A/R parameter has different effects on the compressor and turbine performance, as outlined below.
Compressor A/R - Compressor performance is comparatively insensitive to changes in A/R. Larger A/R housings are sometimes used to optimize performance of low boost applications, and smaller A/R are used for high boost applications. However, as this influence of A/R on compressor performance is minor, there are not A/R options available for compressor housings.
Turbine A/R - Turbine performance is greatly affected by changing the A/R of the housing, as it is used to adjust the flow capacity of the turbine. Using a smaller A/R will increase the exhaust gas velocity into the turbine wheel. This provides increased turbine power at lower engine speeds, resulting in a quicker boost rise. However, a small A/R also causes the flow to enter the wheel more tangentially, which reduces the ultimate flow capacity of the turbine wheel. This will tend to increase exhaust backpressure and hence reduce the engine's ability to "breathe" effectively at high RPM, adversely affecting peak engine power.
Conversely, using a larger A/R will lower exhaust gas velocity, and delay boost rise. The flow in a larger A/R housing enters the wheel in a more radial fashion, increasing the wheel's effective flow capacity, resulting in lower backpressure and better power at higher engine speeds.
When deciding between A/R options, be realistic with the intended vehicle use and use the A/R to bias the performance toward the desired powerband characteristic.
Here's a simplistic look at comparing turbine housing geometry with different applications. By comparing different turbine housing A/R, it is often possible to determine the intended use of the system.
Imagine two 3.5L engines both using GT30R turbochargers. The only difference between the two engines is a different turbine housing A/R; otherwise the two engines are identical:
1. Engine #1 has turbine housing with an A/R of 0.63
2. Engine #2 has a turbine housing with an A/R of 1.06.
What can we infer about the intended use and the turbocharger matching for each engine?
Engine#1: This engine is using a smaller A/R turbine housing (0.63) thus biased more towards low-end torque and optimal boost response. Many would describe this as being more "fun" to drive on the street, as normal daily driving habits tend to favor transient response. However, at higher engine speeds, this smaller A/R housing will result in high backpressure, which can result in a loss of top end power. This type of engine performance is desirable for street applications where the low speed boost response and transient conditions are more important than top end power.
Engine #2: This engine is using a larger A/R turbine housing (1.06) and is biased towards peak horsepower, while sacrificing transient response and torque at very low engine speeds. The larger A/R turbine housing will continue to minimize backpressure at high rpm, to the benefit of engine peak power. On the other hand, this will also raise the engine speed at which the turbo can provide boost, increasing time to boost. The performance of Engine #2 is more desirable for racing applications than Engine #1 where the engine will be operating at high engine speeds most of the time.   —Preceding unsigned comment added by 74.219.202.130 (talk) 21:56, 18 July 2008 (UTC)
thread_title:  A/R RATIO 

message: I found this article good and useful as I was searching for some information on something I had not heard of before (VGTs's). Only comment I have is that the term 'A/R ratio' is used repeatedly without any further explanation. I was not familar with this term (although I did know most other technical terms used in the article) and there was no link or explanation in the article. I searched Wikipedia for the term 'A/R ratio' & could not find it. After googling about and finally coming to the right page (the main Turbo page) I found a good explanation. I think therefore, that like other terms like 'wastegate' that are linked to an explanation, A/r ratio should be linked somewhere where it can be found. I am not sure how to do this - I can change the term to a hyperlink, but there is no article of sub-section on this to link it too. Can anybody help me on this? Or do people disagree & think the  article reader is expected to have read the main turbo section first.
Sorry about the long winded explanation.
-=<O^0>=- 02:34, 12 January 2007 (UTC)
thread_title:  A/R Ratio 

message: Can anyone confirm that the Japanese market Nissan Maxima from around 1986 used a variable A/R turbo?  I'm sure I've seen these turbos at the import wreckers.  They have a flap, similar to a 2 stroke power valve that ranges open or closed depending on load and rpm.  203.217.13.50 (talk) 06:55, 4 September 2008 (UTC)
thread_title:  Nissan Maxima? 

message: More information was added covering types of VGTs along with linked photos.  0zneckblock (talk) 03:33, 21 September 2023 (UTC)
thread_title:  Types of VGT