Turbo A/R's, what does this specify?
 

So the current turbo on my car came from a Nissan SR20 and the specs are as follows:
Compressor: T-25, 60 trim 56mm BCI-1 compressor.
Turbine: T-25, 62 trim 53.8mm 0.64 A/R turbine housing

I just bought this turbo:
Chinacharger T25/T28 Hybrid BEGi
for a great deal used from a member on here, and the turbine says: .86 A/R, I measured the compressor wheel on it and it is actually smaller than the SR20's compressor wheel. The specs are these:
Compressor wheel: roughly 46mm
Turbine wheel: roughly 48

I measured these by eying them up with a micrometer while the turbo was completely intact, so it's an estimate. My question is: What does A/R stand for and signify? How does a bigger turbo end up having smaller wheels? I understand that there is a ratio difference between the comp and turbine wheels, so help me understand:
If I were to bore out the compressor on my SR20 turbo and install a 60mm wheel, then the turbo should take a bit longer to spool, but it should also become more efficient and make more power, right?

A/r is area/radius, or something like that (when in doubt google it) but basically its the size of the HOUSING not the blades.

Larger a/r's = larger housing = little more lag but more top end flow (or ability to flow more than a smaller a/r all else being equal)

You should be looking for a gt28 sized turbo, IIRC the compressor on my 2871 is 71mm wheel, .60 a/r and the turbine side is 54mm and a .64 a/r

On the turbine side, it's the ratio of the area of the inlet side to the radius measured at that area. Turbine A/R is a very large part of what makes a turbo react a certain way or reach a certain power level. Compressor A/R means absolutely nothing.

Common T25 A/Rs are .49, .64, and .86. I don't think you can get the 2554/2560 in anything but a .64. 2860RS through 3071 are available in a .86.
Common T3 A/Rs are .48, .63, .78, and a few larger as well. Most folks run a .48 or a .63. .48 T3 is similar to a .64 T25, .63 T3 is similar to a .86 T25.

Twin-scroll A/Rs are a different ball-game - I forget whether TS turbos act larger or smaller than they measure.

both

Remove the damn compressor housing and measure again. The 46mm number is the inducer blade size. A GT2554 is measured at 42.0mm/54.3mm. you need to know both the inducer and exducer size of the wheel.

But the A/R of the turbine makes a lot of sense why they spool so damn slow. Same thing goes for the turbine, you measured the exducer, not the inducer size. Your SR20 T25 will have an exducer size of roughly 42mm.

Turbine Trim = ex^2 / in^2

62 = x^2 / 53.8^2

62 = 1800 / 2894.4


FWIW, this is what tyson ran on his 1.6L with that same "2854" chinacharger:

Ok, now I have a better understanding of what A/R's mean and how wheel ratios affect how a turbo spools, but back to my last question:

If I were to bore out the compressor on my SR20 turbo and install a 60mm wheel where a 56mm wheel once was, then the turbo should take a bit longer to spool, but it should also become more efficient and make more power, right?

spool probably wont change that much. but power output will.

gpopshop charges like $225 to do this. they bolt a 60 trim T3 wheel onto it. then you'll end up with a turbo more like a 2560 than a 2554.

http://www.boostedmiata.com/random/t3_vs_t25_wheel.jpg

If you have a larger compressor wheel in a internally enlarged housing and we just make the MASSIVE assumption that the charteristics of the housing are the same, just with more flow and that the larger wheel is just as efficient a design and that these things remain true when you combine the tow, then yes. You see this a lot with semi-custom turbos like the 'sleeper 16G' that Forced performance used to do a lot of way back in the day. They basically made a 20G that fit into a 16G footprint. Works pretty well. Spool is usually more effected by hotside changes anyway, so you might get a bit higher boost threshold in a given gear and get a bit more lag, but not catastophically so.

The issue that one can run into there is that you might start getting a lot more flow on the intake side than a small AR, small overall turbine housing can really deal with, which means the back pressure will limit your power gains. A big EWG that takes the gases right off the collector will help with this a lot though.

his turbo right now is sized like a 2554, if he puts that wheel in place it's sized like a 2560, the end. we all know how each spool and handle power. no need to think more about it.

But fwiw, the compressor wheel has a lot to do with spool (wheel ratio aka "speed match" between compressor and turbine.) and since the wheel is larger and heavier, the bigger the wheel the more effect it will have on spool...

look at the spool difference between a 2854 and a 2871, both use the same hotside. takes more energy to spin the larger wheel fast enough to compressor air.

^Good point. That hotside will handle what he is doing flow-wise. Did not think the inertia of the compressor wheel would be that large a actor, but the dynos/boost plots dont lie.

If you mean what I think you mean by "boring out" the compressor, the machining would have to be fairly precise as the tolerances aren't huge, and the back side of the housing is radiused to fit the curvature of the wheel fins. I don't think it's a DIY job and by the time you pay somebody to do it you might as well just buy a good used wheel & housing, maybe a new wheel if the old one is fux0red. It shouldn't be too expensive. A 60mm 62 trim wheel in a .60 A/R housing are very common parts used on a lot of turbos.