Need some advice, twin turbo project
 

I'm in the process of building a twin turbo 4.6L Northstar engine for my Miata, and I'm getting close to the point where it's time to pick turbo's! Only problem, is I'm just a newb in the turbo world, so that's why I came here for the expert opinion. I'm shooting for 650 flywheel horsepower, with a redline of 7000 rpm. It'll be mostly road race so I'd like it to have a broad mid-range powerband. Any suggestions? Help is appreciated!!!!

should be a fun build. i would go with two gt30's or two gt35's or scrap the twin turbo all together and go with one t70.

The gt30's compressor map looks pretty nice. The only thing I was concerned with about with it was that at full boost I'd be right in the middle of the efficiency island. Would that mean I'm looking at a lousy low- mid area of the powerband?

two gt30's would start boosting at ~2000-3000 rpm. two gt35's would start boosting at ~3000-4000 rpm. i would go with one t70. its efficient at high rpm and it will spool at ~2500 rpm. we had a guy here put a t70 on a 4.3 v6 s10 and it spooled at ~2500 rpm. it was a very efficient package. very little lag and more than enough airflow for high engine rpm power without over-reving the turbo. either of the turbo setups above will be great at mid range to high rpm power.

garrett offers these turbo's in a variety of exhaust air ratio's. if you want lower boost levels then get a .63 a/r exhaust housing. if you want high rpm power with more lag then get a larger a/r such as .82 a/r.
two gt30's should be more than enough airflow so all you have to do is decide on which exhaust housing you want to use.

Hearing about the 4.3 has me curious now. The single t70 seems like a nice way to go, especially looking from a cost perspective...

the t70 is a definite price difference considering you will only need one and there will be less fabrication and supplies needed. shop around and see what exhaust ratios you can find it. im currently building a cobalt 2.2 turbo and im using a gt35. i like high rpm power so im ok with lag. i think that t70 will work out great for your application.

Any specific turbine and compressor a/r's to watch out for? those ecotec's are nasty if you build them right!

go with a 70 a/r compressor side and 80 a/r or larger turbine side. make sure you get the exhaust side with the bigger t4 housing as the t3 exhaust housing is smaller and will spool up quick but choke out at top end.

lol...

heres my expert opinion. screw the turbos and build a stroker