Because it is a ratio of the AREA of the two wheel diameters and the formula for the area of a circle is pi x diameter squared. Since pi is in both the numerator and denominator of the equation it cancels out (pi/pi = 1) and you are left with the inducer diameter squared divided by the exducer diameter squared....

As close as I can measure it, this turbo has an inducer diameter right at 46mm and exducer just a hair under 60mm - that makes it a .60 trim. So it is a T3 with a 60 AR .60 trim cold side and a 63 AR hot side. Seems like the right one to me.

It turns out this unit came out of a 2.3L T-bird turbo coupe, not an SVO Mustang. I'm assuming that since the 4.3L will produce almost twice the exhaust volume of the 2.3L that this turbo - with an exhaust scroll sized for the 2.3L - ought to spool up at significantly lower RPMs on the 4.3L. I'm also thinking that the sligtly lower backpressure and EGTs of the .63 AR of the hot side and .60 AR of the cold side are also going to be good things - again, since I'll be pushing nearly twice the volume of exhaust gas through it than what it would normally see on a 2.3L

BTW, KarTek, did I interpret your axial versus radial play comments correctly?

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1984 300 Coupe TurboDiesel
Silver blue paint over navy blue interior
2nd owner & 2nd engine in an otherwise
99% original unmolested car
~210k miles on the clock

1986 Ford F250 4x4 Supercab
Charcoal & blue two tone paint over burgundy interior
Banks turbo, DRW, ZF-5 & SMF conversion
152k on the clock - actual mileage unknown