A/R ratio explained

[bales]

I was wondering whether someone who knew their onions about turbo's could explain a bit about A/R ratio's and the generalised effects they have on turbo characteristics.

So far what I understand is that the A/R ratio is the ratio of the area of the throat intake to the radius of the scroll.

Am I right in thinking that this applies to both impeller and turbine so you have two A/R ratio's, one for the compressor side and one for the exhasust side. In this case which one does the figures generally refer to as you only ever generally see on number for the A/R ratio.

Also the area bit, it simply says the throat intake in all the descriptions that I can find, is this the area at the flange, or is it the cross sectional area within the volute literally at the tangential entrance to the inner housing.

I guess that the overall ratio effects the speed of spool and the overall flow at higher revs, with too small or too big having a detrimental effect on one or the other. But how do you calculate this in reference to your own engine, is it simply a matter of finding the max airflow that you require for your power, and then get a housing and scroll that can flow the max amount at higher revs whilst maintaining good gas speed at lower revs.

And is this the same for both impeller and turbine or do they require differing characteristics.

Any help greatly appreciated, I am hoping to turbocharge my 205 over winter and I like to have a proper grasp of what i am doing and why. Too many people just bolt things on and expect huge power or alternatively complain about turbo's being laggy and not having good characteristics but I personally think it is because people bolt whatever they can and dont take into account these things.

Whether or not you can actually be this choosy with A/R ratios and housings and stuff I dont know, but I would like to get a set-up that is as close to optimum that I can realistically get. I want approx 220bhp at about 1 bar that I think is relatively realistic, and pretty quick also in a sub 900kg car!

Cheers

Alex

(oh I am intending to use a T3 turbo, I can't afford a GT series at the mo (either GT25 or 28) as they are still pretty expensive)

Edited by bales on Saturday 23 December 22:25

[Matthew-TMM]

www.turbobygarrett.com/turbobygarrett/tech_center/turbo_tech101.html

If you work your way through that and the following 2 pages you should have picked up on a few things, the first page is very basic though. Basically there are two A/Rs but the one for the compressor doesn't have a huge impact so you don't have different options for the compressor side.