I saw this thread in the single turbo forum and I figured I would post it here because it discusses hesitations and other driveability issues with low impedence injectors, although some of them are FD specific.

Here's the original thread: https://www.rx7club.com/single-turbo-rx-7s-23/goodby-resistors-hello-peak-hold-666741/

The ubiquitous resistor in the typical Power FC modded FD secondary fuel injector circuit is a band-aid that needs to go.

Since replacing my resistors with an FJO (AEM also offers) Peak and Hold module I pull the same AFR w 17% less injector ontime. Further, the mountainous peak in my base fuel map at transition is a now a linear slope.

Since there are differing levels of understanding re this important issue let’s start at square one…

There are 2 types of fuel injectors.

High Resistance (10-17 ohms)
Most OEM, including the FD are High Resistance injectors, and are run on a linear voltage basis. At a given moment they are hit w 14.4 volts. Figuring 12 ohms resistance that results in a current of 1.2 Amps. The flow is constant until close.



Low Resistance (1 to 4 ohms)
Generally higher flow and non-OEM. They are directed in an entirely different manner….

National Semiconductor builds most of the driver chips that run fuel injectors… here’s what they have to say on the subject:

“The current required to open a (fuel injector) solenoid is several times greater than the current necessary to merely hold it open therefore the LM1949 by directly sensing the actual solenoid current initially saturates the driver until the ‘‘peak’’ injector current is four times that of the idle or ‘‘holding’’ current. This guarantees opening of the injector. The current is then automatically reduced to the sufficient holding level for the duration of the input pulse. In this way the total power consumed by the system is dramatically reduced. Also a higher degree of correlation of fuel to the input voltage pulse (or duty cycle) is achieved since opening and closing delays of the solenoid will be reduced.”

So instead of the single voltage flow operation of the High Resistance injector, an initial flow four times that of the flow necessary to hold it open initiates fuel flow within the Low Resistance Injector.

Further, according to National Semi, who makes the drivers in the Power FC…

“Note that for large currents above one amp IR drops within the component leads or printed circuit board may create substantial errors unless appropriate care is taken…”

Bad things can happen to the components inside the Power FC if you run the fuel injectors substantially above one amp.

That’s why most of us have 10 ohm resistors in our secondary (Low Resistance) fuel injector circuits. 14.4 V / 12 ohms = 1.2 Amps. 1.2 Amps is O K as to protecting the Power FC circuits.

Fine. So we aren’t burning up our PFC, but what have we done?

We are attempting to run an injector designed to be Peak and Hold directed in a Saturated linear manner.

We solved the ECU protection at a cost of speaking Chinese to an Anglophile.

How’s that work?

Not as good as you might think.

There’s the theory and the real world observations.

Since installing my 850/1600s I have always had to build a mountain of injector ontime Ms in my base fuel map to maintain constant AFRs. I have consulted loads of people and have been all over the map messing with lag time, transition timing etc.

the mountain remained. Oh well, my AFRs were constant. But what a way to run a railroad.

Then I bumped into the Peak and Hold Convertor Module products. AEM and FJO make such a product. I found a helpful person at FJO and bought the unit.

Running an unchanged base fuel map I went PIG RICH at transition. After dialing down the Ms I now have a linear base fuel map.

And NO RESISTORS.

That’s when things got really interesting.

I had way too much fuel dialed into my map. By my calculations somewhere around 15%!!!! That’s right, I dialed down my fuel and maintained the same AFR as before.

That tells me that resistor-runners can accomplish a number of positive things w a switch to a Peak and Hold Module.

a rationalized fuel map at transition
increased fuel delivery

to quote RC Engineering:

there is more area under the curve w a Peak and Hold injector than a Saturated injector. Take both types, put them in the same fuel log, run them for the same Ms and you will get 8% more fuel w the Peak and Hold.

Perhaps more than 8%…. If you try and run a P&H injector Saturated you will probably get a lot less than the 8% drop quoted by RC. More like my 15% number.

Backing this up:

National Semi:
“a higher degree of correlation of fuel to the input voltage pulse (or duty cycle) is achieved since opening and closing delays of the solenoid will be reduced”

from Stealth 316:
Because the amps are higher in the Peak and Hold circuit, and because the P&H injectors are designed to act faster, the ECM decreases the duration of the pulse (that is, the amount of time the current is applied or the circuit is grounded) to compensate for the faster solenoid response. The ECM for Saturated injectors knows to use a slightly longer pulse duration.

EFI 101

“Your results are a direct consequence of the fact that putting a 10 ohm resistor in series with your injector limits the instantaneous injector current (no such thing as a perfect inductor), and the series resistance coupled with the parasitic coil resistance and the coil inductance, slows the magnetic field from forming in the injector.

This gives more injector "dead time", which means that for a considerable period of time, the injectors are on, but not flowing.”

EFI 101

“By putting a resistor inline with a low-impedance injector, you're limiting the current through the injector, and slowing down the opening time.”


And finally from FJO:

"I'm not surprised that you reduced your injector time to maintain the same
AFR. The difference depends on the peak current that the injector can draw
and the fuel pressure, while you were using a resistor.


Here's a non-tech explanation behind this:

A peak-n-hold injector (low impedance) is designed to draw more current
during the open cycle but requires less during the holding cycle. If the
current is not controlled and the injector is allowed to run saturated, it
will overheat.
The common way of adding a resistor in series with the injector will prevent
it from overheating, however it prevents the injector from reaching it's
peak current during an open cycle - which ultimately slows down its opening
time and makes it more sensitive to changes in fuel pressure and voltage.

The FJO driver allows the injector to hit its peak current, causing it to
open faster and more consistently. It then cuts back the current to the
minimum required to keep the injector open. This is lower than the current
regulated by a resistor. Because of this, when it is time to close, the
magnetic field in the injector will collapse faster and the injector will
close sooner and more consistently. This whole process results in the
injector opening and closing faster and more consistently.

Hope that helps explain it a bit. What you have seen is a common result in
cases where the "resistor box" was replaced. Another improvement is in the
idle stability. The faster closing time allows for shorter open times, which
is important if you are running large injectors."

here's my FJO module before installation:



FJO's site:
http://www.fjoracing.com/products.php
2 models. the 4 injector model is approx $195. built to MilSpec.

a few dealers:

florida www.cyberauto.com
oregon www.horsepowerfreaks.com/buy/fjo
texas www.trickracingproducts.com i am working w Rick at Trick on other FJO rx7 products...

AEM
runs up to 10 injectors $349 retail.
http://www.aempower.com/ViewCategory.aspx?CategoryID=83


finally, those who are at the edge of their injectors deliverability might benefit greatly from an increased delivery from their current injectors.

any and all tech comments on this welcomed. i will be extensively dynoing july 15 and will no doubt have further data.

howard coleman[/QUOTE]