Tuning '94 chev Suburban Big Block

[tayto]

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?

[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.

[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]

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

Is it that the pulse width remains the same, yet the net effect by going from 2nd DRP to 4th DRP is that there is less fuel per unit time thus yielding a leaner (cleaner) idle mixture to the engine? Don't have a clue about the electronics on the Memcal. Will look for something about it by 1Project2Many on this site. Don't understand the Memcal voltage dividers and if they are being sampled by what?

[rsicard]

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

Switching from the 2nd DRP to the 4th DRP should lean out both the on-idle and off-idle operation of the engine. Is this correct? Please advise. Thanks.