Turbo Wheel Trims 101 – What’s in a Number

Turbo Wheel Trims 101 – What’s in a Number


So you are looking to turbo your car and maybe even bolt on a turbo kit. The problem is you hear all this jargon about ‘turbo lag‘ and turbo wheel trims. You hear your friends and others around you say things like “60 trim” turbocharger, and you’ve got no clue what that means. Luckily for you, this guide will help you interpret turbo wheel trims and understand what these numbers mean.

While it sounds good to spout out numbers and terms, it’s not always a case of bigger being better when it comes to turbochargers. Nowhere else is this true than when it comes to turbo wheel trims. To make things clear the wheel trim of your turbo is the measurement of the inducer and exducer of both the compressor and turbine wheels.


This relationship between the size of the turbo wheel trims plays a huge role in how your turbocharger responds. By measuring the tips of the inducer and exducer, the area ratio is determined. This is what is commonly known as your wheel trim, and it’s an important fact of shopping for a turbo.

Common terms of turbo wheel trims

Inducer – The inner portion of the turbocharger wheel, where air enters from. As the first wheel to introduce air that’s entering axially into your turbo, the inducer wheel plays a big role in turbo response.

Exducer – Whereas the inducer wheel is where the air enters your turbocharger, the larger part of the turbine or compressor wheel is known as the exducer. This is where the air exits and into the compressor or exhaust housing.

turbo wheel trims 3What is turbo A/R? – Simply put the turbo A/R refers to the area / radius, or the measurement of the turbo housings. There is of course a compressor housing A/R and a exhaust housing A/R, both of which are important when it comes to selecting the right turbo.

More accurately the turbo A/R is the cross sectional measurement of the inlet, divided by the radius of the turbo centerline to the centroid of that area.

Based on what A/R figures and numbers you are talking about, the turbo will react differently depending on your application. Let’s take a look at the different ways in which turbocharger A/R can change your power curve.

Exhaust housing A/R – The most important factor to buying a turbo, the exhaust housing will play a direct role in how your turbo responds. Because the size or turbine A/R plays the biggest role in flow capacity, it means the larger the A/R the more time it will take for your turbo to create boost. This may increase the amount of turbo lag you experience, but it will also allow your engine to operate far more efficiently at high RPM.

Smaller A/R exhaust housings will increase turbine power because the exhaust flow will enter the wheel more tangentially. This provides faster turbo response but increases backpressure and limits how effectively the engine can breathe. Turbochargers with smaller exhaust A/R ratios will be limited in the amount of peak power they can produce, especially at high RPM.

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Compressor housing A/R – The performance of your turbocharger won’t be affected as much by the A/R of your compressor or cold side.

Sometimes turbo manufacturers will increase the compressor A/R to help low boost performance. The A/R ratio of your compressor doesn’t play quite as big of a role as the exhaust housing, but it’s an important factor nonetheless.

Learning how to pick a turbo is a careful blend of the A/R ratios to provide performance and response that you are happy with. Picking out the right turbo wheel trims for your application must involve the right A/R ratio that makes sensor for your application.

When shopping for the right turbocharger, or picking the best turbo the size of your turbo wheel trims is paramount. This ratio goes hand in hand with the A/R of your housings to make up the turbo you need. Go too big and you’ll have a sluggish vehicle, go too small and you might limit your power potential.


How to calculate turbo wheel trims

Your turbocharger inducer is usually smaller than the exducer. So in order to calculate your turbo wheel trims, you’ll need a very simple calculation that’s shown below. For example you can use a HKS turbo as a solid example for the turbo wheel trim calculation shown.

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For our example this HKS turbocharger has been measured already, with the following measurements.

Inducer diameter = 53.1mm
Exducer diameter = 71.0mm

By putting 53.1mm over 71.0 mm in our equation, you come up with a 56 trim turbocharger. The values of your turbo wheel trims gives you the ability of the airflow capacity of your turbocharger.

When shopping for your turbocharger, don’t be fooled into buying the bigger is better myth. Just because the turbo wheel trims are larger, it doesn’t always mean that the turbocharger will create more boost.

Have any questions about this turbo wheel trims guide? Leave us a comment below and let us know!