The computer is tuned using software that is provided with the computer. There are loads of parameters to adjust, but the major item is a fueling table that has different load and rpm points on it. THe computer consults the table for where the engine is currently running and injects that amount of fuel. The extras add acceleration enrichment (accel pump), etc. that are needed for a real application.
Why not the multi-port? Seems like an obvious question doesn't it? Well, in my case, I ran across an unsolvable problem (2 years of screwing around). At certain load points, the engine would stumble for no reason, but if you moved the throttle at all, it would clear up and run on. I tried richening, I tried leaning, but no adjustments ever seemed to help. I'd already been over the manifolds to make sure they weren't leaking and that all injectors were working properly. Further investigation of the spark plugs and exhaust ports in the head showed that cyls 1 & 6 were running lean, but 2,3,4,5 were all rich.
I've since come to the conclusion that one of the following is happening:
1) air travelling down the manifold runs on the outer wall around the corner into 1,2,5,6 with the result that 1 & 6 are being ram charged to a degree, leaning these cylinders.
2) reversion is happening in the simamesed intake ports such that the air/fuel mixture is not ending up equally in both cylinders at certain rpm points.
3) the multi-point injection events are timed such that injectors 1 & 2 inject closely to when intake 2 opens, thus 2 gets more of the fuel charge.
I have further opinions on these 3 ideas, but the bottom line was that how could any of these problems be addressed?
So, for kicks, I grabbed a TBI off a 3.1L GM Lumina van for $25. I stuck it on where I had had the progressive 4 bbl air throttle body. I then changed the injector wiring to fire the TBI, not the MPFI. A bit of computer reprogramming later, and bingo, the engine was running again. Pretty quickly I noticed that the areas where I used to stumble most frequently were gone. This makes some sense since the original carb had never stumbled like the MPFI did. Also, I didn't lose my great EFI starts, so I'm pretty much sold on the TBI idea for these engines.
I have one minor problem to fix (keep this in mind if you do a TBI swap). The injectors that come from the smaller TBI unit (model 220, regardless of injector size) with 1 3/8" bores (S10 trucks, all the 60 degree V6 stuff) are rated at 33 lbs/hr each. This isn't quite enough for my otherwise stock 250. When I was running MPFI, I was using 19 lb/hr injectors, or about 120 lbs/hr total. Now I'm at about half that. So, I can't floor it above about 2500 rpm or it'll lean out since the injectors have gone static. I'm going to swap in a set from either a 4.3L or a 5.7L (350). You could just swipe the TBI off a larger engine in the first place, but at my yard the small V6s are way more common than the V8s. Also, I'm not convinced that the smaller bores of the V6 are a problem. I think they still have enough area for the inline. The 4.3/5.7 use 1-11/16" bores and the 454 uses 2" if my memory serves.
Anyhoo, that's enough of that for this post.
Greg
