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A/R is a ratio.
A = area of throat at intake
R = radius of scroll in housing
Basically a smaller A/R ratio makes the turbo spool quicker, but becomes more of an exhaust restriction at higher RPM's, limiting top end power. A larger A/R ratio promotes top end power, at the expense of low-end boost response. A .63 A/R T3 housing gives you a nice mix of spool and top end power for a stock honda, a .48 is too small, and a .82 is too big for most mild motors.
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