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Thread: Tuning '94 chev Suburban Big Block

  1. #16
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    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

  2. #17
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    Just came to me what DRP is. Must be Distributor Reference Pulse. Still unable to find something about Netres mod.

  3. #18
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    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.

  4. #19
    Super Moderator dave w's Avatar
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    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

  5. #20
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    Quote Originally Posted by rsicard View Post
    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
    Last edited by Nasty-Z; 01-22-2015 at 01:48 PM.
    1994 3500 Dually , 502 (509) , 264HR , Edelbrock MPFI , PFI '7427
    2006 Forest river-Sierra 37' 5th wheel
    1999 Astro Van, L31 Vortec , 4x4 conversion
    1994 Pontiac Sunbird "Work ride"
    1993 K-3500 "Ex-diesel" 454/4L80E - '7427
    1992 454SS "Eh Stock" , Blower & '7427 swap .
    1992 S-10 434 SBC/Tremec - '7427
    1984 S-10 , SAS, 496/700R4/205 , D44/14BFF -'7427
    1980 Z-28 454/700R4
    1977 Olds 98 Regency 403/700R4

  6. #21
    Fuel Injected! JeepsAndGuns's Avatar
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    Quote Originally Posted by rsicard View Post
    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.
    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.

  7. #22
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    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.

  8. #23
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    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

  9. #24
    Fuel Injected! JeepsAndGuns's Avatar
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    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.
    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.

  10. #25
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    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.

  11. #26
    Fuel Injected! JeepsAndGuns's Avatar
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    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.
    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.

  12. #27
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    The sense resistors (and the modification of them by bypassing them) in and of themselves do not change the firing strategy of the injectors , this is accomplished by the Memcal modification. The Memcal mod changes the firing mode of the Injector drivers. The sense resistor mod basically assures 100% reliability in the PFI mod by not allowing the injector drivers to enter peak and hold mode. This is the way it was explained to me by Ken from HP tuners and Dimented from TGO years ago in posts I had questioning it.

    Once again , the sense resistor mod is NOT a requirement for proper operation as mine has been fine for years with only the Memcal mod. I have questioned the reasoning of doing only the Memcal mod and both the Memcal mod AND the sense resistor mod and was never able to find an acceptable answer as to which is the best way to do it . That is one of the reasons I never did the sense resistor mod on mine.


    The Edelbrock injectors are manufactured by Magnetti Marelli and are of the saturated (High impeadance) type , therefore 4 injectors on each driver work fine in both TBI and PFI mode with the '7427 .

    Hope this helps.


    TOM
    Last edited by Nasty-Z; 01-23-2015 at 05:21 PM.
    1994 3500 Dually , 502 (509) , 264HR , Edelbrock MPFI , PFI '7427
    2006 Forest river-Sierra 37' 5th wheel
    1999 Astro Van, L31 Vortec , 4x4 conversion
    1994 Pontiac Sunbird "Work ride"
    1993 K-3500 "Ex-diesel" 454/4L80E - '7427
    1992 454SS "Eh Stock" , Blower & '7427 swap .
    1992 S-10 434 SBC/Tremec - '7427
    1984 S-10 , SAS, 496/700R4/205 , D44/14BFF -'7427
    1980 Z-28 454/700R4
    1977 Olds 98 Regency 403/700R4

  13. #28
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    Nasty-Z:

    Thanks very much for the reply as it has cleared up the sense resistor and Memcal mods. Apparently, Ken at HP tuners has the knowledge to impart upon others and this does not affect his present business. I need to go back and review the Memcal mod and how it affects the firing stategy of the injectors. I will jumper the sense resistors to keep the ECU from going into the peak and hold mode. Again, thanks very much for clearing up the understanding of these aspects.

    Regards,

    Rolly

  14. #29
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    Quote Originally Posted by rsicard View Post
    Apparently, Ken at HP tuners has the knowledge to impart upon others and this does not affect his present business.
    There are many with a very good working knowledge of the '7427 , Ken was/is one of them . He helped me tremendously. I believe he and I were one of the first to try the PFI mode in the '7427 to work with the shortcomings of the Edelbrock MPFI conversion. Doug Stalter (ECTune on many forums) who worked for Edelbrock basically had his hands tied when the original calibrations were designed in house at Edelbrock for the conversions. This is why workarounds were discovered and it was a good thing they were , the Edelbrock calibrations were bad on a good day .

    TOM
    1994 3500 Dually , 502 (509) , 264HR , Edelbrock MPFI , PFI '7427
    2006 Forest river-Sierra 37' 5th wheel
    1999 Astro Van, L31 Vortec , 4x4 conversion
    1994 Pontiac Sunbird "Work ride"
    1993 K-3500 "Ex-diesel" 454/4L80E - '7427
    1992 454SS "Eh Stock" , Blower & '7427 swap .
    1992 S-10 434 SBC/Tremec - '7427
    1984 S-10 , SAS, 496/700R4/205 , D44/14BFF -'7427
    1980 Z-28 454/700R4
    1977 Olds 98 Regency 403/700R4

  15. #30
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    Quote Originally Posted by Nasty-Z View Post
    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

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