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1995 GMC 7.4 liter, 4l80E, 4.11:1 full floating rear end with 7427 computer. Doug thorley try-wye headers and K&N filter charger intake
replaced throttle body and manifold with edelbrock MPFI conversion kit, newly rebuilt bottom end, put a comp cams extreme roller lifter and rockers with a mild 264 grind with flattop pistons (9.2:1 comp.) with edelbrock 60499 performer heads. Major motor changes and major tuning changes occurring.
john
Should run great when you are done. Just need to jumper the injector current sense resistors in the PCM and put the Memcal into PFI mode with a jumper. Then dial in the tune.Quote:
Originally Posted by 95GMC_sierra
Hmmm current sense resistors... Hmmm I now have four high impedance injectors in parallel for each formerly low impedance throttle body injector... 4-16 ohm injectors with a circuit load of 4 ohms total. The original injector was 1.5 ohms total. So I am decreasing the injector driver current/increasing load impedance from 1.5 ohms to 4 ohms. So the current draw is 1/3 approximately of the former circuit... So the recommendation is to short out the injector driver current sense resistor(s) in the PCM. This would be to turn the injectors on more reliably upon initial transition from +12 Vedic to ground? Where can I find an assembly drawing so I can find the resistors on the PCM board to solder a jumper across them?
Memcal in PFI. Hmmm. Not sure how to do that, I might understand a CPI mode a bit better. Help me with this one a bit more kindly
much appreciated
john
http://i243.photobucket.com/albums/f...IMemcalMod.jpgQuote:
Originally Posted by 95GMC_sierra
http://i243.photobucket.com/albums/f...bum/Before.jpg
http://i243.photobucket.com/albums/f...lbum/After.jpg
Superb fast355. Do you understand the theory behind the changes to the circuit and what the jumper for PFI enabling does to the batch fired circuit? Help me understand the reason for the changes....
John
It has to do with the TBI injector driver and basically it prevents the driver from entering the "hold" mode. It has to do with the shape of the waveform of electricity reaching the injector during switching. I never had much luck running in PFI mode without the 3 jumpers in place.Quote:
Originally Posted by 95GMC_sierra
Tom aka Haulin@$$ could tell you more about this then I can.
Thank you so much, your experience and results are proof in themselves. I do need to understand the PFI and jumper changes and intentions better to feel comfortable with them. I am working with a chip tuner (what a good guy too) on this and am currently exchanging TTS data master live drive data files electronically and doing this chip swapping via snail mail. I am seeking a way to skip the snail mail and exchange data and burn memcals myself and MAYBE emulate real time to change parameters to and while I figure out why the popping through the exhaust persists and I suspect limits the validity of the narrowband data recorded from the truck recorded by TTS data master. I am thinking the moates APU1 and the adapter cable to read/write/emulate real time for expediting the tune and eventually learning it myself
This jumper fix recommended could very well be the issue and I will implement it to test the results. Jumpering the current sense resistor will increase the voltage across the injectors by the voltage drop the sense resistor had at load current, this I understand and it makes sense but this current sense resistor also is the current limiter device for the circuit which would protect the current sinking driver chip from over current failure if one of the injectors shorted. That's a schematic and chip spec level question for sure but I would like to know that and the difference the PFI jumper makes. What a great website and I appreciate your comments and welcoming immensely
john