When discussing the spool-up and high-end flow characteristics of a given turbocharger, the term “wheel trim” may pop up. If you’re in the market for a turbocharger upgrade, comparing new units, or are planning your own backyard DIY turbo kit, you’re bound to run across wheel trim.
It’s a real term and its mission is to establish the relative size of a compressor (cold/intake side) or turbine wheel (hot/exhaust side). In simpler dummy-like terms, a turbo wheel is the blade-like spinning fan inside a turbocharger. The turbine wheel is spun by exhaust gases and is connected to the compressor wheel, which compresses (duh – Ed.) intake air for the engine.The often-overlooked aspect of this term is that it is used to compare wheels within the same turbo family. So the trim size can be, and often is, mistakenly used for comparison between turbo families.
Some turbo manufacturers, such as Turbonetics, have inserted extra letter designations into their turbo line-up. Turbonetics’ GT-K turbo series has P-trim and S-trim designations and its T3/T4 lineup has Stage I, Stage II, and Stage III trims. Such designations camouflage the numeric value of wheel trim and often add to the confusion that surrounds the term.
TRIM MATH
First, understanding the mathematical calculation that creates the wheel trim number will help us crystallize the meaning of the term.
Trim = Inducer diameter divided by outer diameter, squared, and multiplied by 100
or
(in / out) 2 x 100
So a compressor (cold/intake side) wheel with an inducer measurement of 51.7mm and an outer/exducer of 76mm is calculated like this: 51.7 divided by 76 = .6802631. Then .6802631 squared = .4627578. Multiply that by 100 and you get 46.27578 or 46-Trim. Easy, right?
Plugging in numbers is meaningless though unless one knows the turbo family’s entire range of wheel trims. In this case, the 46-trim wheel is from the T04E family. There are four wheel trims offered for the T04E – 46, 50, 57 and 60. The bigger the trim, the more the wheel will flow but it does so at a slight loss in efficiency. It should be noted that this comparison is within the T04E family and the compressor housing A/R ratio needs to remain constant.
The difference in flow has to do with the blade speed but issues such as the blade contour, swept angle, and surface area of the blades also impact the overall flow characteristics of the wheel in question.
When it comes to compressor wheel selection, lean towards a large trim rating in a smaller wheel instead of going to a larger wheel. Larger wheels are heavier and rotational weight equals lag.
TURBO TUNING
Basically, wheel trim is a fine tuning element once you have determined the proper turbo family for your application. Turbo proprietors often rate turbos by horsepower or flow (in cfm).
We find the cfm rating much better because it is easier to quantify. Will a 600hp turbo make 600hp with both a 2.0-liter four cylinder and a big block V8? We don’t think so. The use of a compressor map to pinpoint the flow characteristics is always the best way to go.The turbine side (hot/exhaust side) of the wheel trim equation is a much murkier proposition. The calculations are non-issues because the manufacturers have kept their turbine wheel maps under wraps, making them harder to find than satisfied GM shareholders. Experience from the manufacturer, tuner, or fellow enthusiasts is the best bet. Luckily, wheel trim is not as critical to turbo performance as the compressor wheel selection.
A/R RATIO
In fact, on the hot side, the A/R ratio of the turbine housing is far more important as it dictates spool-up time, backpressure, maximum flow capabilities, and the part throttle responsiveness of the entire unit. The turbine housing can be swapped for a bigger or smaller size to optimize hot-side performance, whereas the turbine wheel is not interchangeable.
Area/Radius (A/R) Ratio is a numeric reflection of the size of a given housing gleaned from an equation that divides the area at a point in the housing by the point’s radius as measured from the shaft of the wheel.
The area of A/R is the area at a given point within the housing, where the compressed air moves from the inlet to the nozzle. The radius of A/R is multiple points measured from the shaft center to positions on the housing.
The formula takes all the area measurements and divides them by their corresponding radius measurements.
A1/R1=A2/R2=A3/R3=A4/R4 and so on
or
Area/Radius=Constant
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 will be 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 backpressure 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 so you can make informed decisions, especially when you upgrade to a custom off-the-grid turbo or find yourself as the only person on Earth looking to turbocharge a Camry the right way.
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