Just doing some research. I'm running a stock 4EFTE and I would like to know the power difference between the CT9 and Ct12 as I am looking for a slight upgrade for my CT9 without really changing supporting hardware like manifold.
They're not really an upgrade in my experience if you are referring to the EP71 CT12?
Years ago when I was building hybrid ct9s for customers the best combo I found as using a CT9 from a Previa van as the front cover of the turbo was larger so had more meat to spin out on the lathe to accommodate bigger compressor wheels. With the stock 4efte CT9 i'd have to weld around the snout to create enough material to machine into. Anyway, point is that in stock form the Previa CT9 might be worth a try. Ultimately your better off going to a TD04 or something which is proven to be a worth while upgrade.
i've had a ct12 on my 4e before going full forged/ gtx 2867 gen2 etc...the only difference i had compared to the ct9 was that it held boost a bit better on high rpms(didn't loose so much boost compared to the ct9)other than that only minor hp gain(it was 5 or 8 don't remember exactly...)