D-Jet Fuel Injection Pulse Phasing Options

[Not_A_Six]

Hi All-

I recently installed a 123Ignition dizzy in my '73 2.0. It apparently can be installed in any of 4 orientations, in 90 degree increments and operate properly when wired to the plugs appropriately.

The d-jet version of the 123 dizzy also has two wires that are used to time the two groups of fuel injection pulses (cyl 1+4 and 3+2). These wires can be connected to the ECU in either polarity, and the "correct" polarity to time/phase the injection pulses seems to depend upon both the distributor orientation, and the wire polarity. Instructions from 123 Ignition show a connector sketch that "suggests" a particular polarity, but I think the correct one also depends on the chosen dizzy orientation, as well as the firing order. (The dizzy can be used in non-914 applications. E.g., with a straight 4.)

I'm an EE. I've been through Paul Anders' ( @pbanders ) very helpful writeups on the ECU and d-jet operation. I've got d-jet schematics. And, I've got a generic (Volvo?) chart purporting to show the injection timing WRT the cylinders (which seems to have a lot of discrepancies with a 914 setup with different firing order.) I've also looked at the injection pulse/spark timing with an oscilloscope (with a high-v probe).

It looks like the orientation and polarity can be adjusted to inject fuel at any of the following times:

1) Inject cylinders 1 and 3 during intake stroke and cylinders 4 and 2 during exhaust stroke.

2) Inject cylinders 1 and 3 during power stroke and cylinders 4 and 2 during compression stroke.

3) Inject cylinders 1 and 3 during exhaust stroke and cylinders 4 and 2 during power stroke.

4) Inject cylinders 1 and 3 during compression stroke and cylinders 4 and 2 during intake stroke.

I think the choice of the 4 options above would affect the dwell time of the fuel charge in the intake runners and might also cause some raw fuel to be sent out the exhaust if a pulse occurs during valve overlap.

Which of the 4 configurations is correct? Is my analysis correct, or did I miss something? Any other advice you can give me?

Thanks for your help.

Cheers. (IMG:style_emoticons/default/beerchug.gif)

[pbanders]

IMO, if there's a choice on the 123 between vacuum advance or retard, you've got to use the retard. If you don't, when you set the mechanical advance to the stock setting, there's too much advance to bring the idle down to 1000 rpm and the throttle plate closed, even with the bleed screw turned all the way in. I know this is a modified motor with a different cam profile, but I'm pretty sure it would still have the same issue.

Vacuum advance (according to the Bosch Blue Book) is required for low load conditions, where the A/F mixture burns more slowly. According to other references I've seen, this additional advance was incorporated not for better acceleration or transition, but to reduce emissions. Apparently, the improvement in emissions wasn't significant in actual practice, because in later 914's, the vacuum advance port wasn't present on the throttle body and the cell was capped off.

If the vacuum advance is really providing significant driveability effects, it should be a simple thing to test on either stock or modified motors where both the vacuum advance and retard ports are present on the throttle body, and the stock distributor is installed and both cells are functioning properly. Simply clamp the advance vacuum line with a fuel line clamp and see if it makes any difference when driving. I'll try this out with my car when I get a chance.