A/R Turbo Housing Ratio Explained

turbo exploded view

a/r turbo

Illustration of compressor housing showing A/R characteristic

What is a/r Turbo Housing Ratio

The A/R turbo ratio (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 center line to the center of that area. It is a term defining the properties of the compressor or turbine housing’s curved scroll. Compressors are relatively insensitive to A/R changes, but turbine performance is greatly affected by changing the A/R. Proper A/R is determined by application, engine size, and, ultimately, real-world tuning. The A/R in a relationship that is obtained when dividing the interior area of the turbine where the inner walls are found, through the turbine housing radio from the center to the tongue. (See the illustration)

  • A/R turbo housing values are expressed as .35, .47, .68, .84, 1.00, 1.15, etc.
  • A small A/R indicates a small interior volume in the small turbine and a large A/R indicates a greater volume.

At a minimum A/R the motor’s response is produced at small revolutions per minute, but at high revolutions you would not achieve an adequate caudal. You should always find a compromise between achieving the lowest response possible and having enough caudal at high revolutions. Likewise, a middle of the road turbo usually makes a fair compromise.


A1/R1=A2/R2=A3/R3=A4/R4 and so on



Turbo Parameters

ar turbo housing ratio

A/R turbo measurements throughout turbo

The A/R turbo 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 back pressure. Likewise, this will reduce the engine’s ability to “breathe” effectively at a high RPM, adversely affecting peak engine power.

Conversely, using a larger A/R turbo housing will lower exhaust gas velocity, and delay boost rise. The flow in a larger A/R turbo housing enters the wheel in a more radial fashion. This will increase the wheel’s effective flow capacity, resulting in lower back pressure 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 power band characteristic.

A/R Turbo Example

a/r turbo housingHere’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 turbo ratio of 0.63
  2. Engine #2 has a turbine housing with an A/R turbo ratio 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 back pressure, 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 back pressure 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 since Engine #2 will be operating at high engine speeds most of the time.

ar turbine sizing

Before you go buying the largest A/R turbo turbine housing you can let’s break it down more. You go too big and you actually lose flow because you lose the scavenging effect. Go too small and you bottleneck the exhaust and loose power. The turbo A/R size will be the same. If you go too large for your size motor the turbo will be lazy and slow reacting. Go too small and the turbo will be choked up in the upper RPM power band.


While compressor housings have A/R ratios the measurement is most important on the turbine side of the turbo. The size of the turbine housing is critical in determining the responsiveness and/or top-end flow of the turbocharger. A responsive unit will have a lower A/R [.63, .78, etc] and will spool-up quickly, but will sacrifice maximum flow and possibly cause back pressure in the system. Conversely, a big A/R housing [.93, 1.12, etc] will sacrifice responsiveness in order to flow more volume at the top end. It is always best to go with tried and true turbo kits and choices that have proven track records in regards to power output and spool-up characteristics. But it is still good to know the down-and-dirty details. This makes it so you can make informed decisions, especially when you upgrade to a custom off-the-grid turbo or find yourself with your truck torn down and a turbo in hand. You can use one of the flow calculators for your engine found at Garrett websites, for example.

If you have anymore questions about turbos or a/r turbo ratios give us a call at 866-737-4966.

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  • Kenneth Kraus says:

    I am trying to find new Turbos for my Diesel Engine MD855KUH

    My turbo,
    TURBO Spec: MX34 8607
    Serial # RHC7 24 179C
    Part # 24100-1830A

    Compressor Wheel 79.5mm
    Exhaust side 94mm
    A/R 38

    Is there ANY other Turbo that I can purchase that will fit onto my Engine?
    I keep getting quotes of $1700 for a new turbo. Is there anything cheaper?

    I am replacing the compressor wheel now, but I need to buy 2 new turbos for my two Boat engines soon, I’m not sure how long these will last
    and I cannot have the boat down again for months waiting for new turbos.

    Engine: MD855KUH (Hino block Yamaha engine)

    Can you please help? .

    Thank you so much for any help you can give.

  • tbreddiesel says:

    Crosses over to a Garrett 465824-0003. I do not have anything available, hopefully this Garrett number will help you in your search. Good luck!

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