Tech Article: A/R ratios and you

[GT40FIED]

So a lot of people throw around the term "A/R ratio" in reference to turbochargers these days as it pertains to what turbo setup will work best with what application. A lot of people just scratch their heads and nod along with the person referencing A/R ratios, so I figure it may be time that we all understand what an A/R ratio is and what it means to your turbo.

The A/R ratio of a turbo is simply a measurement of dimensions within the turbo. The exhaust and compressor sides of a turbo both use scroll designs. For example, the exhaust side's scroll is where exhaust gasses enter and are directed at the turbine. It's essentially a smooth chamber with a notch cut all the way around it to direct gasses towards the turbine. It gets smaller in diameter as it approaches the turbine, hence the name "scroll". This way it can help pressurize gases to make them leave the slot at a faster rate.

If we understand that, A/R ratio should be relatively easy. An A/R ratio is a measure of a cross-section of the scroll's "tube" (represented by "A") and the distance from the center of the "tube" to the turbine shaft (represented as "R"). So basically the A/R ratio refers to the size and shape of the scroll that is cast into the housing. The values apart from each other are pretty meaningless and normally R does not change much between different housings. However, by dividing R into A, you get the A/R ratio. This may all sound a little confusing, but in the end all it really determines is how restrictive the housing will be versus how fast the turbo will spool.

A lower A/R ratio (smaller scroll size, A) will result in a more restrictive housing. That being said, the restriction can be helpful since this allows gasses to expand more rapidly thus helping the turbo spool faster since the expansion of these gasses is what causes the turbo to spin. As a trade off, as engine rpm and output increases, the restriction becomes more of a problem than a friend as it will start to build up backpressure on the engine which hurts performance. A generally accepted rule of thumb for figuring out when you have too much backpressure is when pressure in the exhaust manifold is more than half of the pressure in the cylinder (not the easiest thing for the shadetree mechanic to test, but that's the standard).

The compressor side also features a scroll design, but with the opposite purpose. The air leaving the compressor will have a lot of speed, but not much pressure. In an opposite design compared to the exhaust side, the compressor's scroll starts small and gets bigger. This essentially builds up the air and allows it to create pressure. This type of design is used to convert speed energy of air coming off the turbine into pressure energy which is much more useful to the engine for power production.

All of this sounds very scientific but what it means is pretty simple. A turbo of a given size and identical trims (compressor/turbine design) will act differently given two different A/R ratios. For example, a Garrett T3 45 trim has 3 main A/R ratios, 0.48 A/R, 0.63 A/R, and 0.82 A/R. Obviously the biggest difference would be between the 0.48 and 0.82 A/R models. The 0.48 A/R housing will provide a much quicker spooling turbo for fans of people who hate turbo lag but are willing to sacrifice some top end power for the quicker spool. If you're the more patient type who doesn't mind a bit opf turbo lag in the quest for big top end boost, then the 0.82 A/R housing is probably more up your alley. Then again, you can split the difference with the 0.63 A/R housing and call it a day. It's all about what you want from your motor/turbo setup.

[thermal]

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