Ok I feel it's at a good point in this thread for me to weigh in on this, because I'm very old-school when it comes to turbocharging and tend to DIS-agree with the popular opinions in this thread. Before I got into the S12 I came from DSM's at a time when there was no EMS, and MS was considered a pro-level modification. Most of the guys with DSM's were running with SAFC-II's (including myself), and while some of them were running the AFC's as Apex'i intended, many had come up with new tricks and modifications with AFC's to make them work a bit differently and more advantageous for higher boosting. But the real point here is that even something that works as simple as an AFC can be tweaked for success. Before I came to DSM's, I came from oldschool 80's turbocharged Chryslers (2.2 and 2.5L SOHC turbocharged engines), and those guys had literally NO aftermarket. They employed the use of pressure valves, pieces of garden hose, JB-weld, tap screws, and old cold start injectors from ancient Saabs in junkyards... and many of those guys used that kind of hodge-podge approach to force upwards of 30psi out of factory T3's and run 12's and even 11's in the quarter mile, with full interiors in heavier cars than ours. And virtually NO datalogging, just using NARROW-band O2 meters, Hardware-store cobbled manual boost controllers, plug reading (VERY old school), and a lot of patience.
SO... my point is the same as it was when I first started posting on the old Classic 200SX email group, before I even came up with the idea for "Club-S12".... you can spend a lot of money, or you can be clever like the hill billies were, and patient, and surprise people.
I feel that I've adequately made my point, so let me kinda put that in perspective of an S12... specifically a CA18ET platform. Coincidentally, it's what I started with, and what I was obsessed with even before I finally found my first S12. It's also what I helped one other former member with, and he took his car into low 12's with very little. So here is what I think.
You can use an AFC on your car. Successfully. There's nothing "wrong" about it. Someone else on here did so many years back, using an SR20DET MAF, a T25G, big front mount IC, and bigger injectors (although I can't for the life of me remember what injectors, but that's trivial this post). He ran a 12.1something in the 1/4 mile before he blew a head gasket on the next run. While he had no dyno numbers, you can figure out he was pretty fast. Him and I figured that with ARP head studs and a cometic head gasket, he could hold the head on for even more power. He also was running an UN-ported head, so while it's true that SOHC CA heads have poor flowing ports, even poor flowing ports can be boosted. Also, port geometry becomes less of a concern in higher boost, and overall port size is more important... so the design of the port is compensated for if you were to port it open more.
Him and I were also figuring on switching to a T28, which should have been plenty...we had seen a guy in the UK slap a T3 with adapter plate onto the factory CA18ET turbo exhaust manifold, and with supporting mods run 300hp to the wheels. Of course he had a CA18DET bottom end, but again... we figured some ARP main studs and rod studs would cover the bases there, too. Shot-peening and cryo-treating the rods would also add some more strength, but it might be around the same price to provide a single rod to Eagle or Pauter and have them make you a custom set of forged rods which would be well beyond anything you'd need.
So while there are limitations to the CA18ET, there are ways to make it handle power. And there are ways to work with simple things like AFC's, and even to run cheap home-made extra fuel injection and pressure switches. And yes, you don't need a wide-band O2 meter, because the resolution of a narrow-band meter is plenty IF you have the conservative (and I think prudent) tuning ethic of running a little extra fuel for safety sake... one light bar's worth of fuel mixture on a narrow band is the perfect size fuel cushion IMO.
NOW... I currently have a KA24DE, and I think for the money and results, a KA24DE swapped in, followed a bit later by a bolt-on turbo setup... that would be more cost-effective than tuning up a CA18ET for good power levels. But I don't think the CA18ET tuned up is a bad idea, and I'm more than happy to offer ideas for you. I think that because I came from a couple other turbocar backgrounds (especially the old turbo Chryslers), I'm in a unique position to offer a very different perspective than other people. I've mentioned a few techniques in this post, so if you want more information, just ask and I will explain, and point you to links for other pages with details and diagrams/photos... Your present modifications are a great start, and just need some more brainstorming to make it work with an AFC or any other simple, no-fuss method.
