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Old 01-22-2006, 01:19 AM
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Injection Qestions

All right ladies & germs, Bless me with your knowledge.
I have a 1989 Short box Chevy 1/2 ton that I'm going to redo. It's a 305 5spd. I have a 350 turbo to put in, and have a couple of 350's and a 454 lying around.
These are my questions: Aside from the actual injector itself, how does the ECU know if it's a smallblock or a big block?

If I install a 350, I'm told I can use the 305 TBI if i put in the 350 injector and chip. I was also told that if I tweak up the ohms resistance on the TPS and add 4 degrees base timing i would see some improvement.

If you use a BB chip in the SB ECU with all the same sensors and the BB injector, why won't it work? I'm told that the ECU's weren't all that sophisticated for that year.

Do those Holley replacement TBI's work better than OEM?

Go ahead and fill this with all the tricks you got! I can hardly wait!!!


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Old 01-22-2006, 03:04 AM
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Mark Goulette
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the tbi systems for the big blocks and the small blocks, although very similar, are programmed different. i also think, but i'm not sure on this either, that the big block throttle body was slightly bigger than the small blocks. the big blocks used a different timing and fuel map than the small blocks did, and that's all in the ecm. changing the chip won't compensate for the difference in engine size. i've never heard of increasing the resistance on the tps either, as the settings it needs to be at is basically the same for all the systems. all that does is tell the ecm where the throttle is at. some of them, if i remember correctly, are non-adjustable anyway. the 305 system should work alright on the 350 though, but i wouldn't expect optimum performance from it. we've got a customer here that has a 383 in his suburban that still uses the stock tpi setup from the previous 350. runs pretty well, but just seems flat, or down on power.........not sure if that's just how it is, or if the computer isn't able to compensate enough for the larger engine. changine the base timing won't matter either, but can adversely affect performance, as the computer has a programmed timing map it uses. set the base timing at 4 degrees, and the computer will just put it back to where it is programmed to have it. the distributor doesn't control timing whatsoever, but if the base timing is too far off, the computer will have trouble getting the timing right, and it can cause a runability problem. best bet for you to keep costs down would be to put in the 350. not sure about smog testing or requirements in your area though, as those could come into play with this as well. if you're dead set on the big block, i'd recommend you find a donor vehicle and get the engine wiring harness and ecm from it for your swap. there are a few differences in the wire harness for a big block and a small block, and you'll need a good schematic to make sure that all the connections go where they are supposed too. as for the early holley tbi injection setups, well, they didn't work that well. i installed one on my buddies 68 mustang, and ended up going back to carb as i could never get it dialed in. get the idle set right, and it wouldn't start. get it so it would start, and it won't idle or rev up...............when i finally got it running decently, it would just shut off without the carb went back on...............................
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