OK guys, this is my first post here, and I don't even have an Explorer, but I am researching them because I'm thinking about buying a cheap 4x4 4D one in a few months. I had to chime in here, though. All of you interested in turbocharging should check out
TurboFord .
Just a few points, though. I have an '86 Mustang SVO. It's a 2.3 turbo with air-to-air intercooler (from the factory), rated at 205 HP (5 more than the 5.0 GT in '86!). Ialso have an '89 Mustang LX with the NA 2.3 and POS A4LD (I understand a lot of your pain!). I would never put any more than about 5 PSI of boost through the NA motor. The NA 2.3 had 9:1 CR and cast pistons. The 2.3 T is totally different. That's why people usually don't just slap a turbo on a NA 2.3 because one could easily blow it up. The 2.3Ts had 8:1 CR and FORGED pistons to handle the 16-17 PSI of boost (for the SVO). A low-boost turbo kit would probably be OK for the 4.0 since the CR is about 9:1 instead of higher. You could add an intercooler from another car for very cheap (<$100 if you know where to look) to reduce the chances of detonation. Then maybe get an adjustable fuel pressure regulator or FMU and a MSD 6BTM to handle any fuel and timing issues if they arose.
If you are serious about a 4.0 turbo (which this thread has really got me thinking about!), here's my recommendations. If you happen to be thinking about a rebuild on your 4.0, do it with dished forged pistons for strength and to lower the CR to around 8-8.5:1. Make up some custom turbo exhaust manifolds for a twin-turbo setup and use small turbos. From what I can tell, the 4.0 really isn't a revver, so you might as well get the boost as quickly as possible. For the cheapie route, get two IHI turbos from an '87-'88 T-Bird TurboCoupe or '87-'88 turbo Cougar XR-7. You'll probably see full boost by 2500 RPMs (how does 18 PSI sound?). If you want a little more power and airflow, get a pair of .48 A/R Garrett T3s from an '85.5-'86 SVO, and that'll move your full boost up to about 3000 RPMs but with more airflow and power. Run one turbo from each bank, not a sequential setup (like Mazda RX-7 TTs). You'll probably have to upgrade your MAF meter (70-80 mm) and injectors (30-36 lb/hr)as well if you want 15-18 PSI. Then you might want to think about getting rid of the 4R44 (A4LD on steroids) for a seriously built one or swapping to a C4 trans if you have an auto.
A list of donor turbo Fords (the good, blow-through EFI ones):
Mustang: '83-'84 Turbo GT, '84-'86 SVO
T-Bird TurboCoupe/Mercury Cougar XR-7 Turbo: '83-'88
Merkur XR4Ti: '84-'89
I am not an expert, admittedly, but I've had plenty of experience wrenching on my SVO and learning everything I can about it. If you are serious, look around TurboFord, there are some really knowledgable people there. Good luck, and keep everyone posted on your progress. I may not be around here often, but we'll see!