Tuning '94 chev Suburban Big Block

[Nasty-Z]

if you have a multiport intake and injectors I have no idea why you would want to run them in a tbi fueling mode. ya it'll run but leaves a lot on the table no?

The calibration from Edelbrock runs the 8 MPFI injectors in a batch fire mode , this setup is not a "true" MPFI setup but rather a small upgrade to get the fuel flow from the factory TBI out of the air stream and directly into the ports. The 16197427 will not support true MPFI.

As I said in my earlier post , I tried to use the factory Edelbrock calibration on both my original 454 and the replacement 502 (509) in the '7427 , It ran OK , but I believe it left a lot on the table with the PCM in the original TBI mode. I switched to PFI mode in the PCM (Jumper mod only , no sense resistor mods) and it runs and idles much better.

Just my experience , the OP states his idles just fine the way it is so he should be good to go.

TOM

[rsicard]

The calibration from Edelbrock runs the 8 MPFI injectors in a batch fire mode , this setup is not a "true" MPFI setup but rather a small upgrade to get the fuel flow from the factory TBI out of the air stream and directly into the ports. The 16197427 will not support true MPFI.

As I said in my earlier post , I tried to use the factory Edelbrock calibration on both my original 454 and the replacement 502 (509) in the '7427 , It ran OK , but I believe it left a lot on the table with the PCM in the original TBI mode. I switched to PFI mode in the PCM (Jumper mod only , no sense resistor mods) and it runs and idles much better.

Just my experience , the OP states his idles just fine the way it is so he should be good to go.

TOM

By true MPFI, does that mean sequential MPFI versus batch MPFI? Still do not understand from an electronics standpoint why the resistors are removed and a jumpers installed.

[Nasty-Z]

By true MPFI, does that mean sequential MPFI versus batch MPFI? Still do not understand from an electronics standpoint why the resistors are removed and a jumpers installed.

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

[rsicard]

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

[rsicard]

Just came to me what DRP is. Must be Distributor Reference Pulse. Still unable to find something about Netres mod.

[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]

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

Nasty-Z: Trying to understand 2nd DRP for TBI and 4th DRP for PFI. Seems to be backwards as far as idle quality is concerned. Need to know a lot more about how DRP is being processed by the ECU.

Rolly

[Nasty-Z]

Nasty-Z: Trying to understand 2nd DRP for TBI and 4th DRP for PFI. Seems to be backwards as far as idle quality is concerned. Need to know a lot more about how DRP is being processed by the ECU.

Rolly

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

[delcowizzid]

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

[rsicard]

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

Thanks much for the diagram. Don't understand the purpose for the resistors in the Memcal. Will try to measure the resistance between the two pins on the Memcal. Heard something about the Memcal providing the limp home function of the ECU.

[rsicard]

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

Don't understand HOW netres chip voltage dividers are converted to discriminate 2nd or 4th Distributor Reference Pulses. Which two pins does the netres resistance need to be measured. Need a memcal pin layout reference to start with to be certain where to measure.

[1project2many]

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

I have made a series of measurements including TBI memcals and posted the results. TBI memcals show 7.5k between the two pins.
http://www.gearhead-efi.com/Fuel-Inj...ull=1#post3374

The images above are for specific chips in the memcal. Those chips are connected to numbered pins on the memcal. Understanding how the ecm uses voltage on pin 56 to determine fueling mode involves studying the ecm schematics and / or having access to GM documentation. It may be a distraction if the goal is simply to make your engine run well.

Here is some work done by Cliff Harris which shows the memcal pinouts which correspond to the specific chip in the pictures above. Cliff's work has been very beneficial to this hobby. Notice how pins 53, 56, and 56 work together to provide the desired voltage for fueling mode select.

http://www.misterpeachy.com/VettePics/MEMCAL-ECM.pdf