O.E. 5.7 Vortec CPI intake with 16197427 PCM ?

[1project2many]

Stock L31 intake enhances low rpm power like L98, but without the large choke at 4500 rpm. It's going to feel far stronger than the TBI system.

[dave w]

I've been thinking the injector voltage offset of the black box PCM is for sequential firing injectors, so batch fire would have different impedance (resistors in parallel)? I'm wondering what the impedance of four SCPI to MFI conversion injectors would measure in parallel? Four injectors in parallel would have a very low impedance, maybe very close to the impedance of one TBI injector?

dave w

[dave w]

I measured the impedance of one stock L31 vortec injector = 13.2 ohms. Four stock L31 vortec L31 injectors in parallel = 3.3 ohms. One factory TBI injector will typically measure 4.4 ohms. I would think a 1.1 ohm 10 watt resistor in series with four stock L31 injectors in parallel would allow the '7427 PCM to operate in TBI mode. I would think using the four stock L31 vortec injectors in parallel only would require the '7427 sense resistor modification if wanting to use the '7427 in MPFI mode.

dave w

[1project2many]

I've never tried driving saturated injectors in P&H mode. Typically they'd be connected four to a bank in parallel in saturated mode and the sense resistor would not be used.

With either driver circuit configuration I would select MPFI mode for injector trigger timing.

I believe the injector voltage corrections are ultimately a system based value, not exclusively an injector based value. After all, wire routing, wire length, manifold design, connector types, and typical underhood temps are all going to play a part in what happens to fuel delivery when system voltage changes. Additionally you might find small changes in fuel supply from pump come into play as well. For most people I believe that starting with OE based values (when available) is the best final answer. But for anyone with the desire and knowledge I recommend determining the values empirically by disabling the charging system and adjusting the corrections as system voltage drops. In this case AFAIK this will be a first for this combination of injector and pcm, so it will be interesting to see how it works out.

[pmkls1]

Well, I definitely got a lot more responses and info than I expected. I'm excited to get working on the project and even more excited to get my car back on the road again. The combination of several factors have kept me from doing anything except just looking at the car sitting in the driveway for a few months now. So it's safe to say that I'm chomping at the bit to be able to drive my car again. With that said, I have some questions about wiring and the driver circuits and some tuning questions. Forgive me, but I'm in the middle of a lot of situations that require a lot of This is where I reach the limits of my knowledge and experience. I have training in electrical theory, but I still have to sketch things out on paper once you get past typical wiring schematics and such. Anyway, I am going to list my questions so as to avoid confusing myself. lol

1. Was the L31 injector that was measured an original poppet-type or the MFI upgrade type ?
2. I plan on selecting MPFI mode in the PCM, so I would obviously wire the 4 injectors in paralell. But I would only perform the sense resistor mod and not need a resistor wired in paralell with the injectors, correct ?
3. I will take resistance measurements of the unit that I end up purchasing, but ultimately the driver circuit should be configured the same regardless of whether the injector assembly is the original poppet-style or the MFI upgrade, correct ?
4. I would like to better understand the process of how you determine the injector offsets with the charging system disabled.
I will be out roaming the junkyards tomorrow in search of a manifold assembly and a few other items. Once I actually acquire a manifold setup I will be able to get going on the project. Once I get into the wiring and tuning I will share my results as I go and will inevitably have more questions. If the manifold works as good ad I think it is then I am going to go that route with my S-10 V8 conversion because it is so cheap, seemingly simple, and shouldn't have any hood clearance issues.

[1project2many]

1. Was the L31 injector that was measured an original poppet-type or the MFI upgrade type ?

The replacement injectors are saturated type as are the original. Electrical characteristics are close enough that they can be interchanged without a recalibration.

2. I plan on selecting MPFI mode in the PCM, so I would obviously wire the 4 injectors in paralell. But I would only perform the sense resistor mod and not need a resistor wired in paralell with the injectors, correct ?

Apples and oranges. MPFI mode refers to when the injectors are triggered in relation to the Distributor Reference Pulse. Peak and Hold or Saturated mode refers to the current control mechanism used when the injectors are energized. TBI injectors are typically peak and hold injectors but so are low impedance pfi injectors. 12 Ohms resistance typically puts the injectors in the "saturated mode" category.

3. I will take resistance measurements of the unit that I end up purchasing, but ultimately the driver circuit should be configured the same regardless of whether the injector assembly is the original poppet-style or the MFI upgrade, correct ?

Ultimately, a driver circuit configuration that works with one injector will work with the other.

4. I would like to better understand the process of how you determine the injector offsets with the charging system disabled.

I don't actually think I'd try it. Adjusting small PW offsets can be tricky enough without the added complexity of falling system voltage. I'd stick with adjusting the voltage scalars and leave the offsets alone.

I have a '99 L31 calibration for comparison. There are three tables of interest. Voltage correction to injector flow rate vs voltage, injector latency vs voltage, and small PW correction. For this cal I'd start with the flow rate correction vs voltage table.

Volt  Correction
4.5   0.800
5.5   0.850
6.5   0.917
7.5   0.958
8.5   0.970
9.5   0.981
10.5  0.992
11.5  1.000
12.5  1.000
13.5  1.000
14.5  1.006
15.5  1.016
16.5  1.026
17.5  1.035

Since these values represent change in flow, and the $0D table represents change in time, you'll need to invert them (time = 1 / flow). As a sanity check, when the numbers are correct, the highest voltage values will have the smallest corrections. Here's the table with inverted values:

V  Multiplier
4.5   1.25
5.5   1.176470588
6.5   1.090512541
7.5   1.043841336
8.5   1.030927835
9.5   1.019367992
10.5  1.008064516
11.5  1
12.5  1
13.5  1
14.5  0.994035785
15.5  0.984251969
16.5  0.974658869
17.5  0.966183575

Use Excel or another program to fit this curve to the voltage steps in $0D.

Small PW offsets look a little different than the typical TBI offset table:

ms  offset
0.0  -0.34
0.2  -0.34
0.4  -0.34
0.6  -0.15
0.8  -0.07
1.0  -0.04
1.2  -0.04
1.4  -0.04
1.6  -0.04
1.8  -0.01
2.0  -0.01
2.2  -0.01
2.4  -0.01
2.6  -0.01
2.8  0.00
3.0  0.00

There are two other values to consider as well:
Base injector latency: .94ms
Low injector PW Correction Threshold: 2.49 ms.

The low injector PW correction threshold indicates that no corrections are needed below 2.49ms and it's easy to populate the $0D table with zeros from 2.44ms to 3.9ms. The negative values in the correction table may be the result of table biasing using the base latency of .94ms. I'd start by inputting the base latency value into Async Short BPW Offset Bias (as 940us) then populating the Async Short BPW Offset vs. BPW table with the values from the L31 (again, multiply them by 1000 to get microseconds).

Finally, there's the latency vs voltage change table.

4.5V 2.424
5.5V 2.234
6.5V 1.457
7.5V 1.167
8.5V 1.024
9.5V 0.926
10.5V 0.835
11.5V 0.761
12.5V 0.701
13.5V 0.653
14.5V 0.611
15.5V 0.578
16.5V 0.547
17.5V 0.549

This table may benefit from adjusting on the vehicle. Use Excel or another program to fit this curve to the $0D voltage correction table as a starting point. For this to work you must have the tune so it's good enough for the car to idle in closed loop with BLM and INT showing minimal corrections. The process to get actual changes is to idle the car in closed loop and disable the alternator. Monitor system voltage and BLM / INT. As system voltage approaches the next lower voltage in the correction table, adjust the table entry so there's no BLM / INT correction needed.

I've uploaded a new $0D definition file version 251 which properly biases the Async Short BPW Offset vs. BPW table table. You might want to head over to the $0D thread to get a copy. http://www.gearhead-efi.com/Fuel-Inj...ull=1#post2202

^^^^^^
Correct file should now be in place.

HTH.

[pmkls1]

I tried to find the definition and I couldn't, but now I see why. Anyway, I guess what I was asking about the injector driver setup was wouldn't it be best to remove the sense resistors in order to drive the injectors in saturated mode ? This procedure is outlined in the mpfi conversion file in the $OD thread.

I went to the junkyard and got a complete intake setup with the TB, sensors, pigtails, fuel lines, etc.... It still has the original poppet style spider in it, but while I am cleaning everything up I am going to bench test the spider for leaks and sticky poppets. If it isn't that bad then I will just perform the poppet cleaning procedure and reuse the spider for now. I got a good deal, though, as I only paid $40 for the entire assembly. I expect it to take a few days for me to get everything cleaned up and tested before I begin installing the intake so it will still be at least a week before I get to the tuning part. Looking forward to hearing the car fire up for the first time in months though...........