Hey, I was hoping someone could give me a hand figuring out how to adjust my ve tables to cut the amount of fuel in half.
I got a 383, with a tunnel ram and dual tbi(I know iam gonna have fun tunning) but here is the thing, i did the calculations and i roughly figured my btw should be at 83.
The car runs rich but it runs.
Next i tried cutting all the values in both VE tables in half, and the car ran WAY!!! to rich, to the point it was nothing but black smoke.
So i thought maybe i went the wrong way, so i doubled the numbers and still tons of black smoke.
Iam really getting frustrated/confused, please can someone give me a bit of insight to what needs to be change to cut injector pulse down.
I have the TUNER PRO RT and all the hook ups(still trying to make them work)
But i can burn chips atleast, plus i converted to Eprom
Any ideas would be great

 

First, I need to state that there may be a little more to it then just BPC. If the cam is a little (to a lot) ratty then the lower RPM VE values also need to be reduced.

For the BPC, I would do the calculation for the typical 2bbl TBI. Then cut it in half (twice the injectors, should work out).

Then go from there. See how it is at low and higher RPMs, different loads. And adjust accordingly. For an idea, an engine that idles at 850 RPM, 62 KPa, has a VE of 20%.

The BPC is 74.

This is with a 2bbl TBI unit.

RBob.

 

thanks i'll have to try that, too bad nobody else has posted a reply

 

No one else has posted a reply because no one has done what your doing. Your in uncharted territory in an inflatable chair, when you need a boat. Most of us have stayed away from using dual TBI's because of one the stock code would take so much work to make it run correctly it's not even funny, and two it's questionable as to whether or not the stock injector drivers can drive additional injectors without either burnign up or causing some funky things to happen with the injectors. What your trying to do is so much more complicated than running the injectors in parrallel and halving your VE curves.

Lucky for you Rbob has just released the EBL which has support for dual TBI's and he has the code know how to make it work correctly.

 

Yeah, I have a 650+ horse Big Block Chevy to that use 2 TBI's. hehe. I just have to get a manifold.....

Good luck with your efforts. Like BMmonteSS said, not many people have tried it yet, let alone got it to work "great" and keep it. Keep us posted.

 

Well i got a big WIEAND tunnel ram, and all this is in a '79 camaro.
I solved the injector problem all u gotta due is run them ins series rather than parallel.
I think i just found my biggest problem it would appear that my fuel pressure regulator is either stuck closed or is set too high, either way iam pulling the spring out and getting an adjustable one hopefully by tuesday or wensday, then i'll play with the VE tables again and it should run ok.
I used vice grips on the fuel line and even though it was lean it ran pretty good, it bogged a bit when i snapped the throttle but i still had the VE tables cut in half.
I'll have to post a pic of my setup and details as i go.
If this works, then iam gonna go really big and take a 427 with a 871 blower and put 3 tbi's from a 2.8L and see if i can get that workin

 

ohh jeez, ok I'm not an electrical engineer but I'll try to explain this. When you run your injectors in series like that the second injector is going to see less voltage than the first injector, so it's going to open slower or maybe not at all. The concensus is that using an external board or modifiying the stock ecm to accept two extra injector drivers is the only way to do this correctly. Some one else can explain this better.

I don't doubt for a minute that you have your motor running, but I would almost bet a bunch of money that you won't get it to run very well. There's a huge leap between getting something fired up to driving it down the road and making power. Keep in mind that the first gasoline engine used a gas soaked rag for a carb, it ran but not very well.

Your using a sledg hammer to drive finishing nails.....you just haven't realized it yet. Do yourself a favor and wait till Rbob releases his modifications to allow the stock ecm to run 4 injectors with the EBL. The EBL has the provisions to make the code side of things work correctly.

 

If you run the injectors in series they're both going to see half the voltage, not just the 2nd one.

figured out my 2nd question so i'm deleting it on edit

 

Here's a thought on the process, don't know if it will work.

If you are running twice as many injectors as before you've effectively doubled the output of the fuel system in terms of its delivery of pounds per hour of fuel. Another way of looking at it might be that the computer needs to know you've doubled the output of the injectors, rather than asking it to cut the fuel in half.


Have you tried doubling the injector constant instead? It's as if you put the original number of injectors back in, but doubled their flow rate.


Once again, just a thought on the logic of the process.

 

That's just it, it's not as simple as cutting your VE in half or doubling your injector constant (BPC). Things like your pump shot don't use the injector constant, so they would need attention as well as a dozen other constants and tables. It's doable, but that still doesn't fix the hardware issue of running your injectors at HALF the required voltage. Your not going to get a consistent amount of fuel injected every time you open the injector, that’s if they even open at higher RPM's. An extra injector driver isn't an accessory it's a requirement.

 

I wonder what would happen if you let the stock injector driver fire (close) a solid-state relay and then hooked the "contacts" of that solid-state relay to your injectors.

The software tuning would likely be extensive, but at least you wouldn't burn out your injector driver boards.

 

I will try to explain this better. I agree that to fire the extra 2 injectors properly requires added injector drivers and I have just the device to do it. I recently completed a project where I converetd a truck TBI PCM to run a TPI setup with the proper injectir timing. At one time I considered running P/H port injectors so I designed a box with 4 P/H drivers to go between the PCM and injectors. I was going to run two 2.4 Ohm port injectors on each driver but this would also work for one TBI injector per driver. If anyone is interested in this, get with me offline. Now, having said this, running the TBI injectors in series should work fine and here's why. P/H drivers are current controlled devices, not voltage controlled. When the injector is commanded to fire, voltage is applied by the driver and current begins to flow until the peak current is reached, 4 Amps in the case of a TBI injector, at this point the applied current is dropped to the hold level of 1 Amp. Ohm's law states that E=IR where E=voltage, I=curreent, and R=resistance. By putting two 1.2 Ohm injectors in series and using the same P/H current values of 4A/1A and solving for voltage, 4A * 2.4Ohms = 9.6V. This means that only 9.6 of the available 12V is needed to fire the injectors in series with the full peak current. The only drawback to this setup is that full voltage is not available at the instant the injectors are commanded to fire which will slow the open time ever so slightly but the time is negligible. A microsecond or two. HTH

 

Makes sense, Rbob has figured out a way to do this correctly making some form of modifications to the stock ECM. I'm not well versed in what it would take to make it work, so I'm relegated to saying that if Rbob decides the stock ECM needs modification to fire 4 injectors.....then it needs it.

 

The equation I = Imax (1-e^(-(R/L)t)) governs the rise of the current in the injector. R = resistance and L = inductance. Since 2 injectors in parallel doubles both R and L they cancel and the time constant stays the same. The time required to reach 4 amps in each case can be figured in the number of time constants. Imax = V / R. For 1 injector at 12 volts and 1.25 ohms Imax = 9.6 amps and it takes .54 time constants to reach 4 amps. For two injectors at 12 volts and 2.5 ohms Imax = 4.8 amps and it takes 1.79 time constants to reach 4 amps. This is 3.3 times as long to reach peak amperage. At high RPM this becomes significant as it eats away at flow time.

John

 

This is exactly the drawback I was refering to but it is actually more prevalent at idle where the pulse widths are low. Fortunately this can be tuned out by adjusting the injector bias.

 

Build an ecm bench, and fire the injectors, normally, and then with them in series.
Then report back with how different they sound and how at ~3,000 RPM the series ones, start sounding like they're chattering....
Be sure to have enough power supply to run them, correctly.

 

Eloquently said by some one smarter than I.....way to bring the tech

 

well i haven't bench tested it but i have watched it on the car and the issue has never been the injectors firing they work just fine, my problem is its running way too rich, so if the injectors weren't working good at high rpm then it would be lean which this isn't the case.
I just got an adjustable fuel pressure regulator, gonna hook that up when it stops rainning.
iam pretty sure that will fix the rich issues then i can start messin with my VE tables and then start road testing and start making some HP.
hopefully by thursday i'll have another post to update how things are goin

 

Your rich issue has everything to do with having double the injector capacity. No one said they wouldn't fire, they said your not going to have much control over them at high rpm's. So your AFR is going to be all over the place and not very consistant. Your basically going to have an uncontrolled fuel leak above 3000 rpms.

 

This is a question on the subject that I can't answer, but hopefully someone can.

Are injectors for a stock 305TBI of lower lb/hr rating tha those on a 350? If so, then maybe four 305 injectors would be better than four 350 injectors? If they exist, then it would be less flow per same the time interval opened.

Once again only a thought. I don't even know if lower pound per hour units were used or if it was all programming!

 

It takes energy into the coil to open the injector. Since you have them in series then you have decreased the voltage across them by 1/2. Therefore, you decreased the energy and will hear them start to "buzz" at some point because there isn't enough energy to open them properly.

By putting them in series you have doubled the inductance the ECM sees. That is twice as hard on the FET driver Zener because of the large injector off voltage kick. It will also take longer for the injectors to turn off.

Check the heat on the FET driver Zener at high RPMs. You might have to install a larger one power wise and voltage wise. The injector BPW vs FET on time is going to be different due to running the injectors at half power. It takes longer to store energy to open the coil. If you don't understand electronics..........this makes a buzzing sound. It isn't an out of control fuel leak.

That is one way to look at it. Check the ECM electronics for heat at high RPM. It wasn't designed for the application you are using it in. You would have a more reliable system if you used low-Z driver modules for the dual TBIs.

 

I finally have the pic's available. The young'n had the camara & cable elsewhere, and the cable has returned (well behind the return of the camara ) . The pic's in the post(s) are of a C3 ECM with a stock PnH driver, driving a single injector and then two injectors in series. These are scope traces that show both the base drive (volts) and the injector current.

The current traces are the important ones (upper trace). The injectors used are from a 2.8l GM TBI setup. There is no pressure on them. As in no fuel pressure. Because of that the results are a little different then if they had to open against fuel pressure. With fuel pressure they will start to open later, with a faster transition to fully open.

An area to understand when evaluating the scope traces is inductance. A straight forward inductor will produce a current slope that is a straight line. Injectors don't do this. As the magnetic field builds and the pintle starts to move, the inductance changes.

This movement can be seen as a change in the current slope. I've placed the 'boxed X' cursor where it looks like the slope becomes straight. Before that the slope has a curve to it as the pintle moves within the core. This inflection point is where the injector is fully open. Let's post some pic's.

RBob.

 

This pic is of a single injector on a C3 ECM as GM designed it. Note that it takes 740usec to fully open the injector. This is shown in the upper trace with the X cursors (boxed at 740 usec). Once a tad over the 4 A peak current is met then driver goes into the 1 A hold state.

The lower trace is chan 1 showing the base drive voltage.

On the upper trace notice how the line from the X cursor to the boxed X cursor changes. This is the injector pintle moving.

RBob.

{edited for spelling}

 

Here is the killer trace. This is the result of placing 2 injectors in series on 1 driver. Note how the trace is ill-defined. With it taking over twice as long to reach the inflection point (which is also ill-defined), and after over 4 msec peaks off. From a look of the current trace I don't believe the injectors ever fully opened.

Place the traces side-by-side. They both use the same scales to make a comparison easy. If a series setup was viable it would be common practice. With fuel pressure behind the pintle these affects are multiplied.

RBob.