Lower my fuel pressure to help w/fuel injectors.., ...being too large??? Options

[Mueller]

Running 2.0 injectors on my stock 1.8, new fuel injection fires all 4 at one time (batch), I have a feeling new injectors are too much....anyone lower their fuel pressure to compensate for large injectors??

I'm wondering how low can I go yet still retain a nice reliable pattern?????

[lapuwali]

You have to be careful on open/close times. The numbers I've seen quoted for "modern" injectors is 1.25-1.75ms just to open the injector fully. That's not a low v. high impendance issue, just a time for the pintle to move from fully closed to fully open. The close times are similar.

One thing I've not figured out yet is if the pulsewidth includes the open and close times, or only the open time, or neither. Electrically, I'd expect it to include the open time (pulse goes high, injector starts to open, pulse goes low, injector STARTS to close). If that guess is correct, then 1.75ms is a reasonable minimum time, and the fuel flow only includes whatever comes out while the injector is opening and closing, with no "fully open" time (other than whatever is there from the inertia of the pintle reversing direction). I'd bet there would be little to no measurable difference between a 1.5ms pulsewidth and a 2.0ms pulsewidth on most injectors. Operating in this area is also going to give high inconsistent fuelling from cycle to cycle, since the open and close times aren't fixed, but vary by 0.1-0.2ms each time.

Even if I'm wrong, and the pulsewidth only includes the fully open time, the fuzziness introduced by the variable open/close times will still be a very significant portion of the total fuel injected. If the times both vary by 0.2ms each, then at 1.75ms, fuel flow is going to vary by as much as 10% from cycle to cycle. (0.4ms/1.75ms is 23%, but there's not 100% of fuel flow while opening or closing, so take a swag at a bit less than half of full flow over the variation in times).

Perhaps none of this makes any practical difference (who knows how bad carbs are in flow variation?), but there's certainly going to be some lower limit. I'd expect 1.75ms to be very close if not over that limit. Dave Hunt noticed that, when running 2.0 injectors on a 2.0 with MS, that only 0.1ms in pulsewidth made a noticable difference to idle quality.

Here's a quote from John De Armond, who's forgotten more about EFI than any of us ever knew:

under normal conditions, the injector spends so much time opening
and closing. A typical injector can open in a millisecond. It
takes a similar amount of time to close. During the opening
process, the flow goes from zero to full flow in a very non-linear
fashion. High speed photographic studies I've done on the old style
Pintle injector (where the valve pintle is visible) shows that the
flow doesn't start until about 750 microseconds into the open
event. The closing process is also nonlinear but of a different
shape. During idle and cruise, the injector is running with an open
period of from about 0.5 ms to perhaps 1.5 ms. Under these
conditions, the injector is continuously either opening or closing,
never reaching the full open state (or just barely doing so.)

I'll note that the D-Jet injectors are the "old-style pintle" injectors he's referring to.