Turbo Underkill??
 

Well, we're installing the turbo and he (my builder) is playing what I think too conservative... you tell me. We're Using a T3 with a .43 AR housing and a 45 compressor trim... maybe good for, what, 6psi? We can change out the compressors (he has up to .60 trim on hand) but that means more tuning and more tuning after change outs... why not start with something mainstream (bigger) and limit boost? Sure, spool will be quick but what kind of power can it put out? Not sure what I'll be seeing at the track but I'm not expecting much... not in the 225 area anyway. Correct me if I'm wrong... which I can expect around here!
BTW, this is a track only gutted and caged '91 miata

Stock 1.6L
MSPNP with LC1 WB
Turbosmart boost controller (for quick manual manipulation)
TiAL external WG
Turbosmart Bypass valve
7MGTE 430cc injectors with 255pump
Mtuned reroute kit
Godspeed Radiator
3" all the way out
25row oil cooler
OBX bar/plate intercooler
ACT prolite wheel and 3puck Exedy
Open 4.1 diff (changing to torsen or Giken soon I hope)
Running Kuhmo V710s

If it were me I'd look at a 50 or 60 trim compressor. Not much difference in spool time and more headroom in the future. (you say you don't want it now, but you always want more later.) I'd stick with the same .48 a/r turbine. A 50 trim t3 will get you to the 225hp area and still have great response.

$.02

For the track I see no reason to not get a bigger turbo.

My little hair dryer would really piss me off on the track. It's great around town...but peters off at higher rpms...where you'll spend most of your time.

That turbo is very similar to my IHI-3991 compressor, and almost identical turbine trim. Good to about 12psi on our 1.6Ls...and can get you up to the 225whp area.

Trust me...you will want more...I've got an upgrade compressor (TBird supercoupe) on the shelf.

Defiantly bigger for the track I could not stand the power dropping off on my last turbo, it felt like you were lifting off the throttle mid corner.

Bigger Turbo for the track for sure. Note that when you get on track with a 150+ whp miata and an open diff, it will suck.

If 160 whp is all you'll ever want, that's fine, but it just seems a shame that you have a lot of quality components going into the build all of which could support a much much higher hp setup than the turbo and psi you are looking at using are going to be capable of producing. For nice parts like that, you've spent probably 1/3 of what some spend for just 160 whp with cheaper parts. The whole reason for the quality parts instead of cheap band aids is higher capabilities..

I say always leave yourself headroom, within reason.

You can go with a bigger turbo that will still spool just as nicely.

You have left nice headroom with supporting components, and it will be as easy as adding a manual boost controller to aid in acclimating to boost, and then later adding more power if you install a larger trim compressor.

I'm running a .60/.48 and it responds pretty well given that I'm running a cat and 2.25" exhaust. It will hit 9psi by 3k rpm. I posted my spool data in the spool thread.

Stpped by the shop again to turn some bolts and I mis-spoke on the sizing...
It's a .42 AR housing with a 50 compressor trim. I'm in the process of aquiring a 1.8 (and LSD) and will forge build the thing. I fully expect to kill my stock 1.6 at some point, but I want to be at LEAST capable of doing it... if that makes sense. Guess we'll see.

He says it's no big deal to change out housings and compressors, and this is a starting point. On the flip side, I don't want to have downtime swapping housings and compressors...

If it's only for the track I would recommend a bigger compressor definitely, and maybe even a bigger turbine. The .48 turbine will spool pretty well, and the next size up, 0.63, will spool slower but will lower your underhood temperatures and overall be easier on you're engine on the track (less backpressure). I use a t3 60 trim w/ the .48 turbine on the track (1.8L though) and it works great for me, but I'm hoping to change to a bigger turbine soon.