Is it possible that the 7747 ECU cannot drive/operate 8 MPFI injectors? I understand the 8 (x) MPFI injector LOAD to the drivers is lower than the 2 TBI injector load, but will the MPFI injectors operate, ie, open and close???

I have fuel pressure and spark. No fuel is entering, though. I spray raw fuel down the TB and it fires but then stalls. either the injectors cannot be driven, the PROM is not right or I blew the drivers. I am sure there are other possiblilities but I am in doubt as to whether this project is feasible. I am getting bummed out and need some reassurance that the 2 drivers can and will operate any high-rssistance 16 ohm injector.

Any hints?

My next thoughts:

1) Switch to TB prom with MPFI injectors = much bigger BPW to see if I can get fuel that way?

2) switch BACK to the TBI injectors and PROM to make sure ECU is ok???

3) ?????????????????

 

a couple things, maybe you've already done them:
check for 12v on the batt side of the injectors.
unplug the 4 injectors from one of the drivers, and hook up a standard tbi injector(if you do it with an adapter plug, you don't have to cut any wires), and see if you can hear the injectors clicking.
the driver should be able to drive the 4 ohm load(4-16's in parallel), but maybe not. you can try lightening the load by only trying 2 or 3 injectors as a test.
lastly, use an LED as a visual indicator, either a noid light or an LED with a 1.2Kohm resistor should be good(around 10mA of current)
if these don't work, especially the replacing with a standard tbi injector, odds are that the ecm itself has some problems.
hope that helps.

 

Got 12V to the injectors, checked that first.

I am no EE, but this is what I understand so far?:


I was under the assumption that the drivers could "drive" any injector combo BUT the current they "see" would be GREATER through them and blow them if the seen resistance was less than the TBI injectors. I am safe here.

no adapter plug, unfortunately. I had high hopes of multiple weatherpak connections, but could not find any in time.

But, Your point makes sense to me: If the drivers only "put out" x amount of current, how can they possibly run the 4 injectors AND, how come Edelbrock has a system that works??? Are the Edelbrock injectors some special ones withdiff internals???

Can I crank 12volts to the MPFI injectors to see if they open???

 

Are you wiring them in parallel?

 

How about removing a injector, disconnect fuel pump relay and start the car, see if the injectors is making some noise. Wouldn't you be able to hear it clicking away if it was working?
If you aren't getting an SES light then the eprom has a slim chance of being bad. I didn't change any of the variables you sent to me and if something got wacky before or after the burn it would have not verified. I hope it didn't get damaged by the x-ray machines and the postal office.
Just stick a multimeter in and test for voltage is another simple and easy way to make sure it isn't the drivers at fault.

 

What is the resistance of the MPI injectors? I know the stock TBI's are 14 ohms. If they are the same, then you can run them in a configuration of both series and parallel and still see 14 ohms at the ECM.

Since Voltage = Ohms x Amps then as long as the ohm level is the same, then the amps and voltage will stay the same.

Of course there are many more formulas that can be used to come up with this conclusion, but the end result will be the same.

Basically it will all come down to the resistance of each MPI injector. The only way this simple configuration will work is if they are the same resistance as the TBI injectors.

But that still leaves me puzzled as to how GM made it work with that 9th injector in the older TPI system.

AJ

 

I think I heard somewhere that the Edelbrock injectors put less of a load on the drivers.
Maybe check the website.

 

The 9th injector was on a completely independant circuit controlled by the cold-start switch on the front of the intake manifold. The circuit was only energized during CRANKing, and the switch would only come on when the coolant was 35°C (95°F) or below and only for 8 seconds (an internal resistor would heat it up and open the circuit after 8 seconds). No part of the cold-start circuit ever came near the ECM, so: no injector driver problems.

 

Got it running, but had to use TBI PROM. VERY rich. Eyes burning for the first time in more than 10 years. Not good for cylinder wall lube.

TBI injectors are 2 ohms only, mine are 16 apiece, with 4ohms seen by each driver, which is more resistance = less load to the drivers??? Also, the winding length and wire diameter seems to make a difference in the injector behavior.

So, I need to make a PROM with the BPW somewhere in between the TBI PROM i used to get it running and the one I had JP burn for me.


I'll update you some more, but I need to beg for another chip, as well, until I get a burner...

I'll pay 20bucks PLUS will send you a burned PROM back. (that includes shipping) I hope JP can call me at work at (860)557-4529.

 

If each injector is 16 ohms, and if you wired two in parallel, then you'd have 8 ohms effective impedance. If you had four wired in parallel, then you'd have 4 ohms effective.

The drivers (the FET that sends the signal to open the injector) will have a limit on either (a) the voltage ceiling that they'll operate at, or (b) the current ceiling that they can provide. If you have injectors that are two low in impedance, then they will demand a current flow that the drivers can't provide... and the injectors won't open or will open too slowly to allow operation.

When you wire injectors in parallel, as compared to a stock configuration, then you will need to have them driven with more current than stock. This *may* be your problem. So you might try wiring the injectors in a parallel -then- series.
that is, put 2 in parallel on the passenger side (say cylinder 2 and 4), and put the other pair in parallel (6 and 8), but then wire the feeds to these in series. This will give you the resistance equal to one injector by itself (16 ohms).

The above assumes you are opening the injectors in a batch mode (4 at a time). I also don't know if it is preferable to batch all the cylinders on one side of the engine, or to spread them across both banks. I suppose it should make no performance difference (in batch mode).

I also know that injectors come in two flavors, a peak-and-hold which IMS requires a lot of current to open it but then very little to keep it open (it probably has a nonlinear mechanical spring, like a Belleville) within it, while the other kind of injector requires constant current to open it and keep it open (probably a linear mechanical spring within the injector body). Which type of injector you have will alter how you tune the ECM. I know little else about the above, so I won't offer any further advice. I suspect that there is ample info on these items (injector types, and the implications of using either type) in the FAQ, or if you search the archives, either here on the various DIY ECM websites.

You also wrote:
More resistance means more ohmic load, so there will be less current flow for a fixed drive voltage.

V = I R, and you just described increasing R. So if V is 5 volts, for example, and you go from R=5 ohms to R=15 ohms, then the current (I) will drop from 1 amp to 1/3 amp.

I'm intentionally leaving out a discussion of an injector impedance (that's it's more than just an ohmic resistance), because if you aren't an EE and don't understand electro-mechanical transduction, then you won't follow it. And it has no bearing on helping to solve the problem. HTH. - Ken

- Ken

 

Thanks for that info.

I have an Edelbrock parallel wiring harness in there. The OEM TBI injectors have 2ohm resistance EACH (NOT 16ohm EACH AS SUGGESTED ABOVE) and my parallel banks each have about 5ohms resistance, shown on the ohmeter. So each drivers is seeing a higher resistance than before, now 5ohm as compared to 2. So, less current should flow through the circuit, right? How can LESS current fow still allow proper injector operation?


I was told to run these injectors in parallel so that the ECu would see 4ohm resistance and provide optimum current. Does it still makes sense?

I was also told that the Peak and hold function of our ECU drivers would make the MPFI injectors operate better in parallel as it would overcome the initial load of the 4 injectors and the hold function would do the rest.

All I know is that started and ran so I believe that PROM burning will cure my probs. Gonna try the JP PROM again tonight with a little go-juice in the TB. I'll get it running well enough to get some BLM and another PROM should be MUCH closer.

I think this might be none of those Rumpity Rump cams that Grumpy wrote about in days past... We shall see.

 

Right. You are now seeing 2/5 ths (or 40%) of the current you should have.

> How can LESS current fow still allow
> proper injector operation?

It probably can't operate properly.

BTW if your parallel banks are at 5 ohms, then each injector (by itself) is at 20 ohms, assuming you have one in each port.

Let me see if I have this right:

- you are using a 747 ECM to drive 8 injectors in batch mode, correct?

- the 747 usually drives two 2-ohm injectors in a TBI. So that means the drivers that are controlled by the ECM would like to each be connected to something in the 2 ohm range; correct?

- right now you have two banks that each look like 5 ohms, rather than 2 ohms. You can't do anything more to those injectors on each bank to lower the combined load.

A. So you need to change the drivers (to one that is more suited to a 5 ohm load); or

B. Add a transformer between the driver and the wires to the injector bank, to step down the voltage and step up the current. This will make the 5 ohm load on each bank feel like 2.5 ohm load to the driver, which is more like what you want. Actually you could wind-your-own transformer to get the load exactly at 2 ohms; or

C. Change the injectors from ones that are 20 ohms each (singly) to ones that are around 8 ohms each.


Item C would be the easiest option, but costly and I don't know if they are easily available. Item B would be the next best option, but you'd have to learn how to wire a small transformer, and you'd have to get a core on which to wind them. This isn't that hard to do btw... but it is probably something new for you. Item A is probably also not hard to do but you'd probably want a hardware-oriented EE holding your hand, so to speak, while choosing the part and doing the soldering.

BTW that doesn't guarantee this will solve the problem, but I suspect it will.



Yes, but the 4 ohm value you were given doesn't jive with the 2 ohm value the ECU/driver was used to seeing. I don't know enough about the specs of the factory driver to be able to say if they are out of range, currentwise, or not... but I probably do know more about this than the tech help people at Edelbrock.



I don't know enough to reliably comment on that.



Maybe. That would be ideal. HTH. - Ken

 

The real question is:

can I use changes in the PROM to get the engine run well from 800rpm to 6000rpm and everywhere in between with the 7747 as a controller?

I sure hope so... Before I go buy/make things to make a 7747 work, I'd just as soon switch to GM TPI ECU/sensor hardware...

I'll get you guys more accurate ohm measurememtns tomorrow and also update on if I can even get it to start with the JP PROM. I hope I get it running so taht I can get some BLM's and a new BPW for the next PROM. That should help TONS.

One thing I did notice is that when cranking, moving the pedal makes the accel enrichment circuit activate which aids in the start procedure. Sounds like a AFR WHEN CRANKING table change as well.

 

I found some bigger 35+pph injectors and tried the with JP's "lean" Prom. Justed and worked pretty well in the garege but didn't want to idle.

I realized that I sent JP a BIN I had made months ago and it has terrible VE Fuel and Spark distribution in the idle areas, which has given me worse problems on lesser cams. ie, when she goes into closed loop at 750 rpm, the hunt begins.

Next step, drive with bigger injectors, get feel for VE table, make VE changes and BLM change and burn PROM to use with 28pph injectors. Or maybe I'll just leave the 36's in as they work...

Comments?

 

36 pph * 8 *0.80 (duty cycle)= 253 lb/hr max

Conservatively:

253 lb/hr * 0.85 (% VE at max power)/0.50 = 430 fwhp

where the 0.50 is an assumed BSFC for the rpm near peak power. Is 430 fwhp enough? If you use 28s, then you'll be at 430*28/36 = 335 fwhp. The thing you don't know is how well the 28s and 36s would run at part throttle cruise, and this is in part due to how much FP you plan to run and how well you have, or haven't, aimed the injectors at the underside of each intake.

Said differently, both will work and make decent power, and one (the 36s) can make a lot more power, but you won't know about the driveability of each at all engine speeds/loads until you try both, and get both tuned well (via FP trim and ECM tweak). So there's no way to really answer this question.

 

Your mathematical calcs are correct. The 28pph injectors are off one of my retired Edelbrock ProFlo systems (now RPM ProFol, P/N3500) and have made me 400hp more than a few times now. The 36ers are off an old DFI unit I had YEARS ago. Quite dirty, but they still worked.

I am looking for 450hp min, but not 450hp continuous, so I believe that either injector set will work. I am going to try the 36pph'ers and I just realized, too, that I had the vac ref line to the FPR off, so that will further reduce FP and allow an increase in pulse width which will help the low pulse-width (idle) areas.

I guess I reallly need to get the WINALDL running to see where I am at. The rain last night prevented open garage door running for more than a minute or so. Don't want the tools/cars getting all rusty and all.

Thanks. Update tomorrow.

 

Can a 747 run TPI injectors?.
Yes.
How well is going to depend on your prom burning, and selection of parts.

I see your relying on someone else for the prom, so that's going to be a crap shoot, even trying to get that right over the phone.

The TBI is a wet manifold, the TPI is dry, so your going to have to do some serious AE work. It's going to take a bunch of tweaking to get right, make no mistake about that.

Your injector sizing may give you fits.
Peak + Holds are intended to run at 1-5 msec operating times. The saturated ones at 2-10. Being in a TBI firing strategy you might have PW problems at idle. You need to look at the 42 code real close, I think there may be a switch in there for TPI/TBI modes. Do a file compare with your edlebrock bin and see what is different from the oem one.

But, when you all done, you just be up to where you were, if you'd used a TPI type ecm to begin with, except you'll chugging along with a real slow ALDL rate.

Feel free to search the archives at DIY-EFI, there might be some old posts that might be more specific in nature about what folks have done to get the 747 TPIs to run.

 

I haven't done much since my last post.

My plan now is to set the PE enrichment TPS% to 100% for all RPM values, which will esentially force closed loop operation all the time and thus allow WINALDL to capture data in all the VE table cells.

Then, I'll burn the next chip (or have it burned for me, that is) with PE% set back to normal.

I drove it a bit and it actually runs ok. Little on the lean side in OL, but not too bad. My TPS enrichments need more juice and I need FP and VE table changes, but it is surely drivable to work and back. Lost more low-end than I expected, more vacuum, too.

Thanks for the help, Grumpy. I hope that the adjusting the vac ref'd AFPR and VE table changes (with WINALDL help, of course) can get me where I want to be.

I am using the small OEM TB unit for air control right now. If I do not get the GM 454 2" TB unit soon, I guess I'll be trying the ProJection TB unit, as I planned before. NO IAC, though, in that one. I tried the one I have on with the IAC unplugged and it gave me no trouble at all. Oh, probably need to idle the thing at 800 or so...

Have fun. I'll have to work on it this week at night in the garage...

Thundering right now. wassup w/that?