Injector 101

[Bruce]

We have Low Impedace (Peak and Hold), and Hi Impedance (Saturated).

The P+H's have a coil resistance of about 2 ohms (in the port injector series), and the Sat.'s about 12.

The reason for the term Peak and Hold, is that thru clever current sensing, a P+H can initially use 4 Amps, and then be throttled back to 1 Amp., once it's opened. Allowing it to draw all that extra current allows for the magnetic field to build and open the injector must quicker. And with the larger injectors, the opening speed of the injector is a critical issue. You want it to be fast, and repeatible. Erratic injector openings can be a major PITB to work with tune wise, over the years the chip guys have figured this out. And the quality of injectors themselves have improved over the years. Now, with the large injectors, also means heavier valving in the injector to turn the fuel on. So they all seem to be kind of lazy on the low end of the Pulse Widths, and this shows itself in them not being as linear in flow as the smaller injectors, at short Pulse Widths.

Then the P+Hs are designed to be operated at 1-5 msec PWs, and the Saturated at 2-10.

Now, there are several ways to use an injector at Wide Open Throttle.
One is to just run it static, ie just turn it on all the time. The other is to follow good design procedures, and run it at a max Duty Cycle of about 85%, and some will go 90%, but at anything just over 90%, you can get to the stage of at higher RPM there isn't enough time for the injector to fully turn off, so the internals of the injector just start to chatter. Problem, is that when it chatters, it stays OFF TOO LONG. Meaning Lean. Since this is a physical characteristic of the injector, you won't see any tell tale signs on a Scanner other then a possible trace amount (or worse) amount of detonation. The physical symtoms of this are blown headgaskets.
If you choise to run a large enough injector so as not to run them static, then you have to actually work on getting the fuel curve right. It's actually easy, but kind of hard on the car just make passes, and work on the % Power Enrichment vs RPM table. For me, I just fire up the ecm bench, and tinker with the table until I get the DC where I like it, then it's a matter of adding boost until I get the AFR I want.

On the GNs, for a *Hot Stocker*, 50s will do. And you might need Water Injection to get up past 16-18 PSI.
Anything more then Hot, and you need 55s.
72s get you into having some really good control of the injector, and fuel curve.
One secret to safe high boost numbers is accuracy in you fuel curve, and timing. Boost is your friend, conservative timing, and enough fuel works.
A real engine saver is just biting the bullet, and installing a decent Wide Band.

[Bruce]

There are 2 basic ways of sizing injectors.
One is by using the BMFC numbers, and estimated HP.
This first one is mostly used by the aftermarket ecms.
A N/A engine will normally run a BMDC number of about .5, meaning for every .5 pound of fuel used, the engine will make 1 HP. So using an imaginary v6 with 6, 50 PPH injectors and running them static you should make 600 HP. But, in truth a GN is going to be at .55 or .6 BMFC, and you don't want to run over a 90% DC. So that 600 quickly drops to more like ~480.

Now, in the world of closed loop ecms, things change somewhat. If you want to double the HP level, then you want to double the injector size. 3x the HP then 3x the injector. That way the BLs/Ints numbers tend to line up, without having huge swings to correct for being off Stoich..

What's often ignored is weither the ecm is batch fire or SEFI. With the SEFI you in effect pick up an extra 5% of flow, since the *off* period occurs only once per two engine revoltions.

Then the actual application, has some being on injector sizing. Since a lighter, car with a high numberical rear end, will want alot of pump shot, you'll have to factor that in.

[Bruce]

The term DC has come into being as a matter of convention. It's a convient
way that's been accepted as the norm, to discribe a design/ operating consideration, when discussing injectors.

It's just something that's evolved.

In talking about injectors, each group, has a specific min on time, and
specific off time for it to function properly. The min on and off times are
effected by operating voltage. The injector turn on time decreases with the
increase in voltage.

If you can find any of the DIY-EFI Flow Bench pics still on line, there are
some pics showing how the flow begins as an injector opens. Intially it's
just a few globs. No at all like the spray pics people show of a fully open
injector. Depending on injector, and a host of variables, at idle, as much
as 50% of the PW time at idle is just about the injector discharging
globbules. And some injectors generate globbules as they close.

When you turn a coil of wire off, the voltage doesn't instantly disappear.
In the case of an ignition coil the point open signal causes the voltage in
the coil to ring, rather then shut off, this ringing back thru the secondary
coil is what boosts the voltage up high enough to bridge the gaps in the
secondary side. Anyway, this ringing of voltage takes place in the injector
as it's turned off. So it can't close until the voltage in the coil decays
and the magnetic field collapses. This takes time. The amount of voltage to
keep the injector open is going to vary by the injectors design. Two primary
elements are weight, and the way the injector deals with stopping the actual
flow of fuel.

There is no way, that I've seen to close an injector instantly, some get
close, but again not instantly. So as long as it can't close instantly there
will be a gray area. Call it what you want, but the injector will behave
erratically as the variables to a slight degree will vary. While slight in
some applications, just the pressure surges with in the fuel rail can cause
poor injector behaviour. You might even see the injector going richer then
prescribed, then the flow dropping, ie going lean, and then the flow
increasing again.

At least this is how it looks to me, from what I've read and looked at, and
again looking at some DC files will show how at times, at high DCs, the O2s
drop, and knock begins. If you have any doubts, PLEASE start looking at as many data logs as you possibly can. Especially from cars that aren't running right, they can be extremely telling in what's going on.

This is just a simple explaination, just trying to explain some complex events.

I've included two pictures that may shed some light on events. Typically all you see are patterns of an injector firing with a text book spray pattern.
In the pic titled opening, you'll note there is an intial discharge which forms a ring, and then as the flow continues, you notice there is a cone that forms inside that ring, the cone being the fuel after the injector first cracks open. So the injector releases the fuel right around the sealing devise, and then it takes an instant for the fuel to actually start to flow. Not a huge issue but it shows the dynamics of what the initial flow is, now, with that in mind, when your firing some really large injectors this pause as the flow actually builds takes somewhat longer. And in the second photo is why some cars like Batch fire more then SEFI. Please note how a large part of the fuel Spray is actually a steady ribbon of fuel. With that type of injector, you might do well with batch fire, since the injector is fired twice as often and that will help slightly break up that stream, and in the intervening time, the first shot of fuel gets some vaporization going.

Again, all just a lay persons interuptations, and explanations

[Bruce]

The one good thing about using DC as a suggested limit is that it accounts for RPM. ie if someone says not to exceed a 90% DC the only time you'll approach that figure is at high RPM when the time available for the Injector to fire is at it's min. Well, unless you really have some odd configuration.

Other then when in AE, cranking, or special modes.
Again, this is about TRs.