a good Explination of A/R?
 

Search was useless. Im pretty sure it has something to do with the size of the turbine and the whole circle or something like that. But how do you know what kind of A/R you need for any given application and how do different A/R's work. Like what benefits do a big A/R have over a smaller. Any info would be helpful.

A/R is a ratio. Area/Radius

The lower the ratio, the higher the exhaust velocity is through the turbine. Higher velocity help spool the turbo, and thus small A/R ratio turbos will build boost earlier. This increase of velocity comes as an expense - small A/R ratios are more restrictive, so top end might suffer.

In and of itself the A/R ratio is not a good for comparison between two vastly different turbos. This is due to the fact that it is a ratio. Larger tubines have larger wheels. This means the radius of the snail (for lack of a better word) will also be higher - thus at the same A/R the larger turbine will be less restrictive than the small turbine.

Expirence is about the only way to know really what A/R to use with which turbo. I'm sure if you had an engineering degree and access to the complete specifications for both the compressor and the turbine it would be a simple matter of choosing what compromises you wish to make, but that not being the case, about the only thing you really can do is ask for others experiance/expertise.

nice :)

My question is, how much suffering on the high end would you have by changing from a 1.00 A/R on a T04R to say a .84 or a .91 A/R on the same turbo?? I'm thinking that a 1.00 or a 1.15 would just take too long to spool off of a 13B, even though I'm going to have a half bp job done. Any advice with the A/R on this one?? Thanks in advance.

Zach

Whats good for comparison then besides just plain experience, compressor maps?