I'm trying to choose the right turbo for my setup, and I'm getting a bit confused with all the compressor maps and A/R ratios and such. The search function told me more about what A/R was, but not how it is mathamatically relevent in turbo selection. Do you multiply your intended CFM at a certain point by A/R on the compressor map?

 

The A/R ratio affects the compressor map quite dramatically.

If you go up in A/R you will improve your potential power, but it will take more impeller rpms to generate it. So for a larger A/R the map will 'stretch' up and right, losing some of the bottom left on the map.

The islands will also become a little narrower, as larger A/R ratios reduce the efficiency.

I hope this makes sense to you.

 

Cool that made some since to me.

So do you have some compressor maps that will show the differances between them and how they will affect a cars drivability? I also would like to know what the "ideal" compressor map look like for good response and top end.

 

There is no ideal. It's very specific to each car's setup. One turbo on a streetport won't work as well on a bridge port.

- Steiner

 

Ok that makes since!

 

If you check out the Garrett catalogue, you can see the compressor maps for some of their turbos. Some of them show the maps for different A/R's.

Check out:

Garrett Catalogue

 

Just in case anyone was wondering, they're talking about compressor housing A/R, not turbine housing A/R. I can see where people could get confused because when someone is talking about A/R, they're usually talking about the turbine housing A/R.

 

A/R is the Area/Radius of the compressor or turbine housing, but it usually refers to the turbine housing in posts on this forum. The housing is simply a nozzle that is wound up like a snail. The AR number indicates the taper of the nozzle by describing the ratio between the Area of a given cross-section of the nozzle vs. the distance (Radius) of its center point from the centerline of the turbine shaft. If that sounds confusing, then just look at it like a fire hose nozzle, where a larger AR will have less restriction and less velocity, while a smaller AR will have more restriction and more velocity.

The compressor map should state the A/R of the compressor housing if different housings are offered. The turbine A/R has nothing to do with the compressor map, and vice versa.

The math involved in choosing a turbine A/R is extremely difficult, and will depend on your application. It is usually best to give your requirements to the turbo manufacturer or your local turbo guru, and have them choose the proper product. Believe it or not, this is what most of the pros do.