Accel Injector Info Needed

MichBC3

Member
I am changing to a set of low impedance injectors from Accel, part number 74616. Does anyone know the injector delay time/rise time for these?

One of the Greg Banish books says that typical rise time for low impedance injectors is 0.2 ms to 0.4 ms. But I'd like to know the real number for this particular part rather than guess.

Are Accel injectors made by someone else perhaps? If I knew that I might be able to find the information elsewhere.

Thanks, Bob
 

jjkv

Member
I am changing to a set of low impedance injectors from Accel, part number 74616. Does anyone know the injector delay time/rise time for these?

One of the Greg Banish books says that typical rise time for low impedance injectors is 0.2 ms to 0.4 ms. But I'd like to know the real number for this particular part rather than guess.

Are Accel injectors made by someone else perhaps? If I knew that I might be able to find the information elsewhere.

Thanks, Bob

A book would only give you a ball park estimation...If you want to get the real number...You could put an oscilloscope across the injector to get a better picture ...best to log it since it happens fast....john
 

MichBC3

Member
Injector Rise/Delay Time

This number has to be input into the GEN7 ECM on the Configuration screen - also known as injector offset. I currently am running a high impedance injector, so 0.8 ms is shown.

Basic part characteristics like this should really appear in the product information of every injector that is sold. I called the Accel tech line and they didn't know what to tell me. Pathetic.

If anybody has a GEN7 and is using low impedance injectors (hopefully Accel brand), I'd appreciate knowing the value that they have entered for this.
 

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    Config.jpg
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MichBC3

Member
Thanks for the good wishes.

It seems to me that knowing the correct injector offset/rise time/delay would be important to anyone that runs an aftermarket ECM. If the ECM has the wrong value then the whole fueling of the engine could be off - creating a crappy idle at the least.

My car is street/strip so street idle quality, fuel transitions and fuel economy are all important to me.

Injector offset should be listed in the product information of every injector - especially a larger low impedance one like this that is more than likely to be used with an aftermarket ECM/computer.

That's my soapbox for today..!
 

Craig Smith

That was easy!
I really don't think that .2 to .4 mS is a valid offset time to use. The time it takes for those things to actually start spraying fuel from the instant the signal is applied is going to be closer to 1.0 to 1.2 milliseconds for a low-impedance injector.

If you are looking for a way to get those 72s installed ahead of your trip to Michigan and you just aren't sure what the "real" number will be, you can use some trial-and-error data to get the car close to how it is now. Data log the car running at idle and observe the a/f while idling. Change the injector flow rate, swap the injectors, and fire it back up. You can tweak the injector offset value to get the idle a/f to be close to what it was before.

I acknowledge that this is not a good way to determine what the value really should be, but it should be an acceptable workaround method to get the car running well enough to get it up here so we can work on it.
 

MichBC3

Member
Injector Offset

Craig -

I will take your advice on the trial and error method. Sounds reasonable - and it's my only option as of now. As also suggested, I could put a scope across an injector and look at the voltage. The shape of that curve might tell me something since these are peak and hold type injectors.

The main reason for switching to a low impedance (LI) injector in my case is to reduce the injector rise time and therefore improve idle quality. The Accel 72 LI injector product literature claims faster rise time because of it's improved disc design. But it doesn't cite the actual value. As a user, it would be nice to see more part characteristics to go along with the marketing statements. If they have room on a product sheet for marketing, they also have room for a number.

The secondary reason was to prepare the car for higher future power levels (because the injector that I chose is larger).

My car runs already. I can drive it to Michigan as-is, but until then, I've got time on my hands to experiment and try to improve it.

Looking forward to seeing you in May.

Bob
 

MichBC3

Member
Maybe some clarification here

According to a description of the Accel 74616 injector on the Summit Racing web site, this is a 4/1 injector.

On pages 18 and 19 of the book "Tuning Accel/DFI 6.0 Programmable Fuel Injection" by Ray Bohacz, a 4/1 injector takes 4 amps to open it and 1 amp to keep it open. Also, tests have shown that a low impedance injector will have a rise time of just below 1.5 ms and a high impedance can approach 2 ms.

This information at first seems to conflict with the Banish books (typical low impedance rise time is cited as 0.2 ms to 0.4 ms and typical high impedance is 0.8 ms).

But what is going on here is that Banish is addressing pintle travel. Bohacz is talking about the electrical pulse length. These can be very different numbers.

So Craig, your memory of the typical range for low impedance being 1.0 ms to 1.2 ms of injector offset is most likely correct. I have to assume that the GEN7+ configuration screen entry is supposed to be the electrical pulse length, not pintle travel. That would most likely mean that my current entry of 0.8 ms is incorrect for the Siemens high impedance injectors that are currently in my car. Before changing out the Siemens injectors, I will increase the offset value and see if that helps stabilize things.
 

Craig Smith

That was easy!
The offset is a global value intended to encompass all these things. As I mentioned before, it is intended to represent the time elapsed from the instant the driver signal goes low to the time that fuel actually starts spraying out of the injector. Coil rise times, pintle movement, and fuel flow through the pintle are the major components that are represented by the offset value entered into the ECM. It is difficult to precisely measure what this offset really is, but as you suggested, monitoring the signal with an oscilloscope can get you really close.

It should also be noted that there's no such thing as a 4/1 injector. There are 4/1 injector drivers, though. That's what is in the Gen 7 and many other engine controllers. The output switches to ground as quickly as possible and monitors the current flow. As soon as it reaches 4 amps, the voltage on the injector driver output will actually increase until the measured current through the injector is reduced to 1 amp, and it holds steady for the duration of the injector pulse. This all happens very quickly, but when you zoom in and view the voltage over time with a scope, this is why the "voltage curve" that is mentioned exists.

Low-impedance injectors will draw more current (initially) than high-impedance injectors, and the additional current creates a stronger magnetic field that causes the moving parts in the injector to move faster. This is why low-impedance injectors open more quickly than low-impedance injectors (assuming all other variables are identical). The reduction in opening time tends to provide better fuel control when running at very low duty cycles.
 

MichBC3

Member
Tech Info

Thanks again Craig. That's what forums like this need is good, clear technical information discussions so that everybody can benefit. This is a great thread.

Today the weather is going to be pretty good in VA. So I'll probably get out there to try a few new (longer than 0.8 ms) injector offset settings - while using the currently installed high impedance Siemens 55's. This may give me a little more insight and build experience before switching to the 72's. If I find out anything interesting, I'll post it here.
 

MichBC3

Member
Still a mystery..

Well, this hot topic has blown over like a lead balloon. I warmed up the car thoroughly with a trip to the store and back. Then I got out the laptop. I changed the injector offset from it's original setting at 0.8 ms to 1.2, then 1.5 and even tried 1.9 ms. It didn't change the idle quality or even the A/F ratio. :confused:

Yes, I used the "enter" key, then F10 to send the value to the ECM. I even closed CALMAP and re-opened it to verify that my changes went into the ECM.

So I just set it back at 0.8 ms. I guess I can install the larger low impedance injectors now and basically set the injector offset at a guess of 1.2 ms. Perhaps this value only makes a difference at high RPM.

So, sorry everybody - nothing can be learned from my experience today... :(
 

TurboDave

RIP DAVE
Staff member
TurboBuick.Com Supporter!
May or may not be exactly on topic but here's some good info on injector waveforms.
 

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HighPSI

Moderator
Staff member
Well, this hot topic has blown over like a lead balloon. I warmed up the car thoroughly with a trip to the store and back. Then I got out the laptop. I changed the injector offset from it's original setting at 0.8 ms to 1.2, then 1.5 and even tried 1.9 ms. It didn't change the idle quality or even the A/F ratio. :confused:

Yes, I used the "enter" key, then F10 to send the value to the ECM. I even closed CALMAP and re-opened it to verify that my changes went into the ECM.

So I just set it back at 0.8 ms. I guess I can install the larger low impedance injectors now and basically set the injector offset at a guess of 1.2 ms. Perhaps this value only makes a difference at high RPM.

So, sorry everybody - nothing can be learned from my experience today... :(

That is very odd. Adding 1.1 ms at idle should make a big diference even with small injectors. I am tuning a Ford GT today with a Gen7, I will change the injector offset and see how it reacts.
 

Craig Smith

That was easy!
That change doesn't take effect in the Gen 7 until you turn the car off and back on. Hang on the next time you start it, it's probably gonna get real stinky real fast.
 

Craig Smith

That was easy!
Depending on where you set the offset as compared to the original value of 0.8 it may not run at all. If you kicked it up to 1.9 my guess is it will not run for more than a few seconds if you start it up with that setting.
 

MichBC3

Member
Thank you for your insight Craig. I kind of suspected something like that (need to stop, then re-start engine for the change to take effect). I'll try again - probably tomorrow. I believe it's supposed to be a good weather day. Today's rain will be "out" of here.

The other day I didn't go to 1.9 ms right away. I snuck up on it gradually by trying 1.2 first, then 1.5, then finally 1.9 when the lower values were not making any apparent difference.

If the idle improves a little bit from doing this, I'm going to be extremely pleased.
 

norbs

Classic fast, XFI, SPortsman & MS3 programming
Just as an example the chip in my car has a min injector pw of .58 ms, and I can run perfectly at 14.7 a/f with 95 lb low impedance injectors at idle. What is your actual idle pw? If your running your injectors above the min pw rating, having a low number in that box is not going to effect anything, because your above that operating point already. I would run the lowest number possible in that setting that it runs the best at, any more its going to be too rich.
 

MichBC3

Member
Hi Norb - Interesting info. Something to ponder. Thanks.

With an idle speed of 850 rpm, my injector pulse width is reading out 2.0 ms and the duty cycle varies between 1.2% and 1.6%. As per Accel instructions, my throttle body is set to make the IAC run at 7% to 8%.

Recall that my present injectors are high impedance Siemens 55 lb/hour. I am hopeful that changing to the low impedance 72 lb/hour Accel injectors that I bought will help improve fuel metering precision at idle because of being low impedance with quicker action.

But before swapping injectors, I want to first see if I can improve things by changing the injector offset setting (currently 0.8 ms) - which appears to be incorrect (too short) according to most.
 

MichBC3

Member
Test Run

I just returned from a test. I adjusted the injector offset from 0.8 ms to 1.0 ms and this time I shut the key off and back on to effect the change in the ECM. This small change made a HUGE difference as far as A/F goes. Closed loop fueling went from an average value of about 5% rich to almost off scale at 20% to 25% rich. So now I know that I can affect a change. I adjusted the 9 VE cells around my idle down quite a bit to compensate for the richness but I don't think that the idle quality was any better or worse.

I guess that it's as good as it's going to be. So I set everything back to the way that it was.

What I learned from this little experiment is that this injector offset setting is very touchy. So since Accel doesn't know the injector offset specification for their 74616 low impedance injectors, I don't think that I'm going to switch to them prior to having Craig calibrate my car in May. I have no idea where to set the offset for the 74616.
 

jjkv

Member
A book would only give you a ball park estimation...If you want to get the real number...You could put an oscilloscope across the injector to get a better picture ...best to log it since it happens fast....john

Actually I adjust the injector opening time to match the Actual A/F with the Target A/F Map @ Idle, then drive and log the Fuel Correction % . Just make the correction in the VE Table if needed..Also I keep the highest Map Number @80 or lower....john
 
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