in your expierience do find to upgrade the OEM F23 I/M to an H23A1?
just for a little extra responsiveness? also is it possible to retain OBD-2 when doing so?
I've answered this question before, maybe on another forum, but I can't find my response. In short, no.
First off, it is not a direct swap. The port shapes are quite different and there's no good way to match them cleanly and reliably other than making a custom adapter. The results I experienced doing this weren't exactly thrilling. With the F23 manifold, I was drawing vacuum at high rpms indicating a restriction somewhere (you'll have to take this with a grain of salt and consider that I was already at ~176whp at that point. So I'm not saying manifold itself is an immediate restriction) but the car still pulled well up to redline.
After the swap, the car just didn't have the same momentum at high rpm's. Personally, I just don't think the runner design of the H23 IM complements the flow characteristics of the F23 head.
If you are looking for better responsiveness, a larger throttle body is the best way to go, but don't expect much in the way of power gains from just that. It's still up to the engine itself to draw in the air, a larger throttle body just increases the responsiveness of the throttle by allowing more air in at lower throttle positions. It's the same affect as increasing the pivot ratio of the TB by making the rotor smaller, the throttle has to open less to achieve the same air flow at any given position.
As for OBD2, AEM EMS is the only easily available solution to tune while staying OBD2, but I personally don't consider it cost effective unless you're boosted or have an entire swap with enough modifications to warrant NOT using the matching ECU for the swap. Either way, OBD1 is still a far better option in my book unless you're legitimately making over 300hp/liter and require some of AEM's proprietary (rebadged) technology, and its not all that difficult to swap back to OBD2 if done correctly.