By true MPFI I mean sequential , the '7427 will not support this . The normal "TBI" mode is injectors fire every 2nd DRP , the PFI mode I run the '7427 in is every 4th DRP (The '7427 can also run CPI mode which is every DRP). What I experienced in running the PCM in TBI mode with the 8 injectors is that the PW went so low idle quality started to be affected.
I do not know the whole story behind the sense resistor bypass , I was told for the conversion to be 100% reliable it needed to be done , I have over 75K and 10+ years on my conversion on only the Netres mod .
TOM
1994 3500 Dually , 502 (509) , 264HR , Edelbrock MPFI , PFI '7427
1992 S-10 434 SBC/Tremec - '7427
1986 Monte Carlo SS
1984 S-10 , SAS, 496/700R4/205 , D44/14BFF -'7427
1980 Z-28 496/700R4
1979 Corvette 496/700R4
1977 Olds 98 Regency 403/700R4
Thanks much for the reply. Do not know what DRP stands for. Where can I find something about DRP and Netres mod? Please respond.
Rolly
Just came to me what DRP is. Must be Distributor Reference Pulse. Still unable to find something about Netres mod.
Nasty-Z:
Which mode, PFI, TBI, MPFI, should I run and how does the ECU get into these different modes? Is there something written about these different modes that can be read? Do I need to be in the Port Fuel Injection (PFI) mode and install the jumper in the Memcal? Do the resistors need to be shorted or removed? Please advise.
When I think of NetRes, I think of Limp Home Mode, where a set of fixed resistors in the Memcal will set the fuel delivered by the injectors at a fixed pulse width.
When I think of Cylinder Select, I think of the injector firing sequence based on Distributor Reference Pulse (DRP). Cylinder Select is a voltage, which will range from 0 VDC to 5 VDC. The Memcal sends the Cylinder Select voltage to the PCM to set up injector firing sequence.
It's my understanding the '7427 MPFI conversion requires a Memcal modification to set up the correct MPFI Cylinder Select voltage.
It's my understanding NetRes and the Current Sense Resistors of the '7427 are separate and unrelated components with different functions.
It's my understanding the Current Sense Resistor is modified to prevent the '7427 from defaulting to TBI Mode.
There is not a Factory Service Manual that I can reference to confirm my understanding of NetRes, Current Sense Resistor, Cylinder Select / DRP sequence. Most of my understanding is based on information I've read on the "Internet", which most of the time is accurate but not always.
dave w
Rolly ,
You do not need to do the mod if your truck is running good as is , I did mine as more of an experiment as to see if I could get it "better" .
Dave is correct , it really isn't a Netres mod although it is often called that by many , it is really a jumper on the Memcal to alter the injector firing strategy of the PCM (TBI/Batch/Bank). The Memcal mod is needed to tell the injectors when to fire in relation to DRP.
That being said , here : http://www.hptuners.com/forum/showth...s-modification
That is a good write up by Ken Canatta of HP tuners on the modifications , he is who I first worked with way back when I first did the conversion in the early 2000's. There are many different ways to do the PFI mod that I see referenced , some say you have to do the sense resistors some say not , I really have never had someone tell me one way is better than the other , I do know that you have to do the Memcal mod , as I said I have many miles on mine with nothing more than the Memcal mod mentioned in the link and it runs good , not great , but good .
Hope this helps.
TOM
Last edited by Nasty-Z; 01-22-2015 at 11:48 PM.
1994 3500 Dually , 502 (509) , 264HR , Edelbrock MPFI , PFI '7427
1992 S-10 434 SBC/Tremec - '7427
1986 Monte Carlo SS
1984 S-10 , SAS, 496/700R4/205 , D44/14BFF -'7427
1980 Z-28 496/700R4
1979 Corvette 496/700R4
1977 Olds 98 Regency 403/700R4
This thread, post number 6, is the instructions I followed when I converted my 7427 to mpfi. http://www.gearhead-efi.com/Fuel-Inj...Information-0D
I did the resistor mod on mine cause it was listed in there to do so.
79 Jeep Cherokee, AMC 401, T-18 manual trans, hydroboost, 16197427 MPFI system---the toy
93 Jeep YJ Wrangler, 4.0L, 5 speed, 8.8 rear, homebrew hub conversion and big brakes, hydroboost, 2.5in OME lift, 31x10.50's---the daily driver
99 Jeep WJ Grand Cherokee limited, 4.0L, auto, 2wd, leather and power everything, 99% stock---the long distance highway ride.
Gentlemen Dave W., Nasty-Z and JeepsAndGuns: Many thanks to all of you for all the assistance. I have two spare 16197427 ECUs. I will burn another BJKW 0E binary and install same into one of the spares. Then install it into the '94 BBC suburban and drive it to make certain the ECU is good. Then I will take it out and jumper the two resistors and apply the jumper to the MemCal. Then re-install into the Suburban and test drive it again to verify any changes to the performance of the ECU. Will also compare an un-mod'd versus mod'd ECU. If the mod'd ECU performs properly, I will proceed with fine tuning the binary to get the appropriate BLM values by altering the VE maps. Again, thanks for sharing the knowledge.
With the injectors firing every 2nd DRP (TBI mode) the Pulse width of the now 8 injectors (especially if you upgraded to the 29#/hr units) becomes very low. This is a result of the PCM commanding an injector pulse every 2nd DRP and essentially overfueling at idle and part throttle. Once the fueling is expanded to an injector pulse once every 4th DRP the injector pulse width is restored to a more tolerable level , resulting in a cleaner idle and an easier transition to tune from idle to off idle.
Pin 56 on the Memcal is the cylinder select pin , depending on what the voltage is at on pin 56 it controls what fuel mode is selected.
There is a chart somewhere on this site , I think by 1Project2Many , that describes what fueling mode is selected by what voltage is seen on pin 56. I will try to search it out if I get a minute.
Hope this helps.
TOM
1994 3500 Dually , 502 (509) , 264HR , Edelbrock MPFI , PFI '7427
1992 S-10 434 SBC/Tremec - '7427
1986 Monte Carlo SS
1984 S-10 , SAS, 496/700R4/205 , D44/14BFF -'7427
1980 Z-28 496/700R4
1979 Corvette 496/700R4
1977 Olds 98 Regency 403/700R4
we dont veiw voltages but the resistance withen the netres chip that changes the voltage on the pins.TBI to pfi v8 actually looks like it needs a couple of mods with resisters to be factory correct. anyone care to measure the resistance of the pins above on a TBI memcal to see if it has the 7.5K between the 2 pins of the v8 memcal setup
The only Memcal information I have is for the '7727, '7730, & '7749 ECM I downloaded from Thridgen.org years ago. See attached pic. I DO NOT KNOW if the '7427 Memcal is exactly same? I'm not sure it's a good idea to post the pic of a different Memcal? I strongly suspect that GM would not re-invent the wheel when it came time to design the '7427 Memcal. It's likely GM would leverage / re-purpose a Memcal it already has / had in production for the '7427 Memcal with maybe a different component (limp home mode) on the Memcal for TBI?
dave w
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