Tuning '94 chev Suburban Big Block

[rsicard]

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.

[dave w]

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

[Nasty-Z]

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.

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

[JeepsAndGuns]

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.

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.

[rsicard]

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.

[rsicard]

Gentlemen: Another question, does modifying the ECU cause the VE on and off idle maps to be all screwed up? Please advise. Thanks.

Rolly

Tucson, Arizona

[JeepsAndGuns]

From what I understand, installing the jumpers changes the way the ecm fuels the engine, so I would say that yes, it will effect the fuel tables and require them to be re tuned.
Installing the jumpers was only part of the conversion. There was also changes to some of the parameters in the bin also.

[rsicard]

Sure would be nice to know what parameters of the binary as affected by the modification. Sure would be nice to know by what percentage the fuel tables have to change. I am going to hold off on the modification until I get more experience under my belt with tuning the VE maps. Can't understand by reliability is an issue unless firing 4 injectors versus 1 is causing excessive injector driver current in the ECU.

[JeepsAndGuns]

The resistor mods change the way it fires the injectors.
Copied and pasted from the tbi to mpfi zip file:
"Next you will need to change two resisters in the PCM. This modification is necessary to disable the Peak and Hold function of the injector drivers. The resistors are called sense resistors and sense the current to the injectors. The resistors have to be changed in order to run 8 MPFI injectors, 6 is OK but 8 puts too much current load on the resistors.
Port injectors come in two types: Peak and Hold (low impedance) and Saturated (high impedance). This modification only works for Saturated (high impedance) injectors."

So you might want to check the resistance on your injectors.