Like the title says, I've been trying to figure out what the A/R stands for. I'm looking at buying two GT28R turbos and they say they have a .64 A/R and I have no idea what that means. Thanks for the help. I'll stop asking all these dumb questions soon, I promise.

Area/Radius. The area of the outlet of the turbine, divided by the distance from the centre of the outlet to the turbine axis. The bigger the A/R, the more the turbine can flow, but the later it will make boost.

Can we get a diagram?

I can see if I can dig one up . . .