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I just picked up a used turbo, how do I verify which trim wheel it has and how do I measure the size of it?

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turbo6speed said:
I just picked up a used turbo, how do I verify which trim wheel it has and how do I measure the size of it?
Aside from it being stamped on the case (which it sometimes is) you'll have to do a bit of research.

Some of it can be determined by measurements.

Here's a writeup I did sometime back about turbo nomenclature that should help get you started. If you come up with the measurements described, we might be able to figure out what you got your hands on.


A/R describes a geometric property of all compressor and turbine housings. Increasing compressor A/R optimizes the performance for low boost applications. Changing turbine A/R has many effects. By going to a larger turbine A/R, the turbo comes up on boost at a higher engine speed, the flow capacity of the turbine is increased and less flow is wastegated, there is less engine backpressure, and engine volumetric efficiency is increased resulting in more overall power.

Illustration of A/R:

A (area) = Area of Exhaust Inlet
R (radius) = distance from the center of the turbine to center of the exhaust inlet
A divided by R = A/R ratio.

CHRA - center housing rotating assembly
The CHRA includes a complete turbocharger minus the compressor and turbine housings.

Illustration CHRA:

Clipped Turbine Wheels
When an angle is machined on the turbine wheel exducer (outlet side), the wheel is said to be ‘clipped’. Clipping causes a minor increase in the wheel’s flow capability, however, it dramatically lowers the turbo efficiency. This reduction causes the turbo to come up on boost at a later engine speed (increased turbo lag). High performance applications should never use a clipped turbine wheel.

Illustration Clipped vs Non-Clipped:

A free floating turbocharger has no wastegate device. This turbocharger can’t control its own boost levels. For performance applications, the user must install an external wastegate.

Illustration Free-Float Turbo:

On-Center Turbine Housings
On-center turbine housings refer to an outdated style of turbine housing with a centered turbine inlet pad. The inlet pad is centered on the turbo’s axis of rotation instead of being tangentially located. Using an on-center housing will significantly lower the turbine’s efficiency. This results in increased turbo lag, more backpressure, lower engine volumetric efficiency, and less overall engine power. No Garrett OEM’s use on-center housings.

Trim is an area ratio used to describe both turbine and compressor wheels. Trim is calculated using the inducer and exducer diameters. As trim is increased, the wheel can support more air/gas flow. Use these formulas when calculating trim:

Compressor/Exhaust Wheel Inducer and Exducer measurements.
Look in the front of your turbo. You are looking at the compressor turbine, the inducer part of of turbine is at the end of the shaft. The blades that you see there extend into a larger diameter at the back of the turbine. This is the exducer stage. The inducer on the compressor is responsible for generating the vacuum at the compressor housing inlet that pulls air into the turbo. The air is then pushed along the fins towards the exducer stage, which is a larger diameter, and gets shot towards the ouside of the compressor housing.

Here's a pic:

The exhaust turbine also has an inducer and exducer, but the exhaust turbine has the opposite function of the compressor turbine so the two are switched. The exhaust gasses are directed towards the outside of turbine through a nozzle. This is the inducer stage because it is the part of the turbine that collects the gasses. As the energy from the gasses is transferred into the turbine, the gasses slow down and exit the turbine through the exducer stage.

For example, looking at a T61 ,the spec for that turbo is as follows:

Compressor Turbine:
Inducer: 60.5 mm
Exducer: 90.0 mm

Exhaust Turbine
Inducer: 64.4 mm
Exducer: 74.2 mm

If you have a turbo with a pair of wheels that have measurements that match these, you are 99% positive that you are holding a T-61.

By measuring your A/R, your inducer & exducer (of both the compressor and turbine) and providing any markings on the compressor housing, the turbine housing and/or the CHRA, we might be ablet to figure it out.

Pics help too as many turbos are pretty unique and are easy to spot on sight.

18 Posts
This is a good post! :bigthumb: . Makes me feel more educated about getting into turbo applications.

2,329 Posts
^^ nice info! i think i read that somewhere else on the forums... is it stickied?

469 Posts
Good read, i dont think there is any stickies on how turbos work or how to tell differences, atleast in the MKIV forums.
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