CobraMarty

Yep, the GM M90 is a very 'long' SC indeed.

Johnny808

The pictures below show the attached 3800 SC motor throttle body and a Jeep throttle body I pulled off some late 90s XJ. In case any of you were wondering how a throttle position sensor (TPS) worked, it uses the butterfly valve position to send a signal to your ECU. That being said, both the 4.0 and 3.8 TPS units have the same 3-pin output port, with a different size connector. Any guesses that they'd work with the harnesses spliced to it? Same goes for the idle air control (IAC) sensor with the spring plunger. While I wasn't able to recover the one from the Jeep TB, the inlet port is identically-shaped and should send the same reference voltage. Anyone got any idea? Given the size difference and air restriction (background in fluids/laminar/turbulent flow) I'm led to believe that the throttle body isn't just a minor restriction on airflow; it's huge. Keeping the 3.8 throttle body and adapting the throttle cables to fit would be an enormous benefit. Insight appreciated.

aducarab

Personally, I don't see why you don't just buy the kit from CobraMarty. They have it all setup for you already. You just install...tuning is handled through the split second, and you keep the stock injectors and use a 7th injector. I've had their kit for the last year now on my jeep and I haven't had any issues. 5-6 psi is plenty. Why reinvent the wheel??

Johnny808

With all due and welcome respect to CM's supercharger kit, I'm a broke college kid who got my XJ in a trade for a $1400 truck with $600 wheels lol. By that logic, spending more than that on a supercharger for a vehicle with 290k miles doesn't nearly make as much sense. Rather, I'm doing it for the fun and challenge of fabrication and hopefully an inexpensive power adder to what's been the most reliable vehicle I've ever owned.

CobraMarty

Thanks for the plug.

That's like a 'Trifactor Oxymoron'


The Jeep 4.0L I6 is a great platform to learn skills on. Reliable, robust, simple. Supercharging is great fun but can cause destruction very fast. Just remember supercharging effectively raises the compression ratio. So you will need more fuel and less timing advance. These are the things that really need to be addressed, not just the mounting of the SC.

4-5 pounds of boost will add 25% more power and will require 25% more fuel and 4-5 degree less timing advance and some form of intercooling.

Johnny808

I'm working on the mounting currently because it keeps me busy while I research the injector and timing issue. I don't know how much wiggle room the crank and cam position sensors have to adjust distributorless ignition, or whether there's a spark knock sensor in there smart enough to knock it down a few degrees. They're definitely issues I want to hone in on.

I'm thinking that six oversized injectors that I can get from a junkyard 5.0 or similar will work for the application; timing is next. These injectors, if properly chosen, should support boost and will hopefully manage to keep the engine fed well.

I've attached some snapshots of injectors and their fuel output values, adjusted for an 01 Jeep fuel system PSI (49). The "pass" parameters require that the injectors not exceed 6 PSI more than the fuel system and not get less than 49 PSI, to prevent them being held open longer than they should be. Feedback and insight greatly appreciated.

CobraMarty

Unfortunately No knock sensor on the 4.0L
30-31 lb/hr injectors will what you are looking for.
You could retard the timing with a MSD BTM.
You don't want boost going to the brake booster or pcv or EVAP or HVAC.
If stock MAP sensor sees >1-2 pounds boost, you will get a CEL.
You need to look at SC and crank pulleys and figure out how much boost your combination will be making.
Need to control the fuel. Could use a AEM FIC or Split Second product.

Photojared

FI is probably not for you then. You're going to have to get some sort of tuning. That will cost quite a lot. Don't skimp on cost when doing any sort of power adders, it's a recipe for disaster

TwistedWrench

I get what you're trying to do here, and I applaud you for doing so. I hope it all works out, but from my own personal experience, the one thing you won't do is save money. You won't have to shell it all out in one chunk, but bit by bit it's gonna cost a good bit more than a RTR kit. Nothing wrong with doing it that way, it's just not really a "cheap" plan.

That said, address the head issue sooner rather than later. The 00-01 are notorious, and at 290K the pin is pulled. Add boost to the equation? It's seriously not a question of if, just when. If you get this all ironed out and working, you really want to pull it all back apart again? Head, gasket, hardware, timing set, cooling system, oil pump. Then supercharger.

Good luck man, I'll keep an eye on this one.

Johnny808

Just made it to 292,000 this afternoon. Also checked cylinder head through the valve cover. Miley is a January 2001 car without the TUPY head. If it hasn't cracked yet, is there still a good chance it will, supercharger aside? I'd rather remedy an almost-problem first and would happily swap to an 0630 head. And some aftermarket head studs and a stronger head gasket. And I suppose I'll take a harder look for cross-hatching in the cylinders and excessive blowby at the rings. So far there are no signs of any sort pointing to a leaky/cracked cylinder head, but I'm sure that could change if I supercharge it.

Photojared

With any high mileage cars FI will greatly increase the chance for damage (or exasturbate existing issues). Being that you're a "broke college kid" I'm assuming this is your only car. I'd just make it run as good as possible until you have another option.

Johnny808

Funny enough, this is the third vehicle I've got thanks to some interesting circumstances...but I absolutely agree, I'd love to have it running properly through its lifetime. I feel I've got the knowledge and wrench-turning experience necessary to make this a solid project build, as murky as it may seem right now.

New question: I've found a 1995 Cherokee head on craigslist I can pick up for $60. I imagine there'll need to be some modification to fit coilpacks, as the 95 had a distributor, but otherwise, will this thing fit? It's cheap, it's got half as many miles, and if it's not in the batch of bad castings, I'll pick it up, clean it, maybe give it a port & polish, lap the valves, etc.

Casting marks are the following:
NH3
95
7120
AT
E23

Insight appreciated!