Tuning '94 chev Suburban Big Block

[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.

[Nasty-Z]

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

[rsicard]

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

[Nasty-Z]

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