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DanRHeller

New Member
Joined
Sep 29, 2007
Messages
16
I know little to nothing about turbos, and must learn ahah. I've been reading around a bit, but still have some questions. What is AR?
 
Area Ratio explained.

Here is the full story from Garretts website.

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


compressor_housing_showing_A-R_characteristic.gif


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.
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Here is the link to the article.
TurboByGarrett.com - Turbo Tech102

Now, how this relates to us with the Turbo Buicks is this.
The .63 A/R on the turbine housing is standard for stock setups all the way to low 11's. Once you want to go faster than low 11's, people typically upgrade to a .85 A/R on the turbine housing. But doing so will require a stall upgrade. Also, a .63 A/R ratio on one turbo with a small turbine wheel, can have a much different stall requirement than on a turbo with a large turbine wheel. Example:
TA-49 with a .63 A/R Garrett turbine housing, vs a PT70 GT-Q with a Precision .63 A/R turbine housing. This is an example of going from one extreme (TA-49 = 1st upgrade turbo) to the next extreme (PT70 GT-Q largest upgrade that is recommended for a 109 block production motor.)

How A/Rs affect compressor covers for us is this. Cooler charge temps when using a larger compressor cover that has a larger A/R.
But slightly slower spool up characteristics in the journal bearing turbos.
Example:
TO4E compressor cover that has a .60 A/R, vs a Precision H cover which has a .75 A/R.

In some cases, the TO4E cover is better suited to the turbo and combo, than a super huge H cover would be. Every car is different tho.
Hope some of this helps.


Patrick
 
It is so hard to learn about these when they all have crazy abreviations and names haha. I want to have a siccccccck weekend car, like...street monster, what turbo should I be looking at?


What size is are stock stall?(RPM wise)
 
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