ok, im looking at two differant turbos, one being shall we say - large turbo escort - T34 - compressor A/R 0.60, i then look at another T3 variant but this time with a MUCH larger looking compressor with an A/R of 0.50 - so what is this all about then >? this is obviously a larger volume compressor but where and what meaning does the A/R figure have ?

please enlighten me -

 

the A/R figure is kind of complicated to explain and i'm no expert but from my (limited) knowledge..

if you look at the where the housing begins, at it's inlet ( outlet for compressor housing) but where it's first at a 'cylinder shaped point' and measure the area at that point ( say you pushed a coin in as far as it could go, then measure the area of that coin ) now measure from the centre point of that coin to the centre point of the shaft, this is the 'R' (radius ) . divide this distance by the area of the coin and you get a your ratio;

so say the coin is 5cm in diameter, do piR2 to get area ( pi X radius squared ) coin = 19.635 cm
distance between centre of coin and shaft = 9cm
9 divided by 19.635 = A/R of 0.45

if you do this same process throughout the housing the ratio remains the same, because as the housing gets smaller, you are closer to the shaft

so A/R stands for 'Area of inlet cross section vs Radius

excuse the shoddy pic



if it resizes , the text at the top says area of inlet cross section and is pointing at the 'coin', the two thick lines are where the radius is measured from

that is MY understanding anyway, i accept i'm probably wrong in a few areas

 

One has a bigger compressoe wheel relative to it's inlet throat.

 

so not many people know then ?

but that still doesnt explain a smaller A/R ratio being a much larger compressor ??

 

It just means that the area of the inlet must be big. A/R ratios on compressor housings don't tell u a lot anyway, not like exhaust housings. The housing can be bored for a bigger wheel too...

 

so how would you decide , looking at the compressor alone and its ratio what output its good for ?

 

You don't as such, compressor maps make choosing the compressoer side of a turbo easy. It's the exhaust housing that's difficult.

 

so from a particular compressor housing without a part number to tie it to specific unit you have no idea of its output ?

sounds very strange to me, surely any given housing can only flow a certain amount of air, and the A/R must be relative to its size ?

 

 

yeah the A/R is relative to housing size. you really need to spec the whole turbo i reckon , for your needs, including compressor wheel trim