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Turbocharger Area Ratio

A/R
The other consideration is the A/R. It determines when the turbine starts to spool. The turbine housing A/R is the cross sectional area of the turbine housing divided by the distance from the center of that cross section to the center of the wheel. This makes sense if you look at the graphic.

If you take for example, the area in A1 and divide it by R1, you will have found the A/R for this turbine housing. Each cross section and radius have the same proportions so the A/R will be found by using any cross section/radius.

Common turbine housing A/R's are .58, .69, .81, .84, .96 and 1.00. The turbine will start to spool sooner with a .58 A/R, and later with a 1.00 A/R. Lag will be a problem if the A/R is too large, but if it's too small, the turbo will run out of steam and be nothing more than a restriction.

Trim
Trim for a compressor or turbine wheel is the squared ratio of the smaller diameter divided by the larger diameter, multiplied by 100. Turbocharger wheels have a large and small diameter. On the compressor, the small diameter is the inducer and the large diameter is the exducer. This is the reverse for the turbine wheel.

Compressor trim = inducer2 x 100
                exducer2



Turbine trim = exducer2 x 100
               inducer2

For compressor wheels, given a constant exducer size, the larger the trim, the better the wheel flows. It also means the wheel has slightly lower efficiency. For turbine wheels, given a constant inducer, a larger trim means the turbine has better flow with less back pressure. It also means that less energy is recovered from the engine's exhaust slowing spool time.

Remote Turbocharging Considerations

Special considerations in A/R need to be taken into account for remote mount turbo systems. Since heat, acoustic pulses and exhaust gas pressure supply the energy to spin the turbine, and these pressures are reduced as the distance increases from the source, adjustments in A/R need to be made.

Keeping the turbine A/R at .84 or smaller will keep the turbine spool time to a minimum. Anything larger than that will not spool quickly, and the infamous turbo-lag will be very apparent.

Garrett offers ball-bearing cartridges that help keep friction to a minimum. This helps the turbo to spool much quicker. These turbos tend to be a bit pricey, but may be worth it if you are not on a budget. Look at the Garrett GT series with the ball bearing cartridge.