For tuners, a Holy Grail of Injector voltage Offset

[XfireZ51]

Gentlemen,

The Holy Grail of Injector Voltage Offset:

http://injector-rehab.com/shop/lag.html

If you look through this you can see how different injectors act under varying voltages. You can compare the Bosch v Accel and RCs for example. With longer opening times you get less fuel for the same calc'd pulse width. That's why it's important to get this offset right since it makes tuning fuel more accurate. If you have ever felt that your car has idled roughly after a long hot run, this is one of the reasons why. Once you modify this parameter, you'll need to re-check your fuel tables by datalogging.

[mike100]

I like the listing to use for comparison purposes, but they neglected to show the unit of time.

[scottfab]

Originally Posted by mike100

I like the listing to use for comparison purposes, but they neglected to show the unit of time.

Must be milliseconds.
Interesting the variance in resistance from unit to unit even
within one vendor per part number.

[XfireZ51]

Originally Posted by scottfab

Must be milliseconds.
Interesting the variance in resistance from unit to unit even
within one vendor per part number.

Scott,

Yes, its msec. And the variance is the point of posting this. An injector isn't just an injector. Swapping different brands, although same fuel flow rating, has consequences in the tune. This is especially true at idle and short pulsewidth situations where the opening time is a proportionately larger part of the total calc'd pulsewidth.
Now some on this board might say, "yeah but.." when you tune to 14.7:1 or to a 128 BLM, doesn't the ECM overcome this. Without the correct injector offset, you'll undershoot or overshoot the actual calc'd pulsewidth the ECM is looking for. In some cases, you may "run out of injector" or have too high a duty cycle. By using the correct offset, you may find that you can lower the overall VE% and have a smoother VE map which means the motor operates more smoothly and predictably in varying conditions.

[scottfab]

Ok, let me open by saying I got up at 1:30am here while thinking about the injector duty cycle at 6000rpm. (when I say thinking I mean dreaming. Yes this is sad but I've always been kinda this way)

Anyway I came up with 20ms. Then I got to remembering there was a thread that had a table in it with various injector lag times and wanted to find it. Well I did. This is it. OK so now read on......

Originally Posted by XfireZ51

Scott,

Yes, its msec. And the variance is the point of posting this.

Yes, variance is the point of the post. I agree. Now, how much variance is too much and therefore a tuned chip is needed vs kinda wanted? (you know to be dead nuts on).

Originally Posted by XfireZ51

An injector isn't just an injector. Swapping different brands, although same fuel flow rating, has consequences in the tune. This is especially true at idle and short pulsewidth situations where the opening time is a proportionately larger part of the total calc'd pulsewidth.

I agree lower rpm and idle would feel the differences (make more difference). Now lets run some actual numbers.

Originally Posted by XfireZ51

Now some on this board might say, "yeah but.." when you tune to 14.7:1 or to a 128 BLM, doesn't the ECM overcome this. Without the correct injector offset, you'll undershoot or overshoot the actual calc'd pulsewidth the ECM is looking for. In some cases, you may "run out of injector" or have too high a duty cycle.

Now this is the area in which the dream centered. It's an accurate statement but..... what are some numbers? Given the worst case OT (also CT) in the table on the link given in your OP what can the ECM do to correct it? (how far down the BLM table must it go?) Does it in fact run out of room? Also how would the component to component variance (inj to inj) of a singe vendor fare? That is, given a batch of inj that were not "matched", when does trouble begin? (largest variance to be tolerated in OT as effected by internal resistance?)
For sake of computation we could even use an OT larger than any listed e.g. 2ms.

To do all this we'd need some info. What is the max swing (in ms) the ECM can effect on pulse with from min BLM to max BLM? I have no idea. Maybe you do?

Originally Posted by XfireZ51

By using the correct offset, you may find that you can lower the overall VE% and have a smoother VE map which means the motor operates more smoothly and predictably in varying conditions.

Yes, we may indeed find.

FYI did you notice in the table they talk about OT and CT being important but the table makes no effort to show CT. Seem it only shows OT. Surely they can not be suggesting they are the same.

So ok, I've written up all that was buzzing around in my head. Time for more ZZZs. I wonder if I'll return to this topic in my dream?

[XfireZ51]

This may help in understanding why tuning benefits from having the correct Injector Offsets. From a Thirdgen thread

http://www.thirdgen.org/techboard/po...545-post3.html

"There is no set formula for flow versus Injector offset.
Wish there was, but it is a little more difficult than that.
The offset (or "deadtime") is the result of several factors, which include:
o mass of the pintle and its lift off the seat
o fuel pressure
o coil inductance
o magnetic circuit characteristics - function of materials and design (like eddy currents)
o injector driver circuit design

So depending on these, there is a correction curve based on battery voltage that takes in account the above, which defines the "deadtime" as the opening delay time minus closing delay time, in milliseconds.

This is developed for each injector type and is then coded into the calibration. Any change in the above parameters can affect this offset correction curve in the calibration.

If you have a test bench (with an oscilloscope and an accelerometer) you can set the injector up and then feed it increasing PW using a function generator and using the accelerometer can determine how long at various voltages it takes to open and close the injector. This then becomes the battery correction curve, and then the BPW after that is the actual PW that provides the fuel."

Another good explanation of injector offset:

http://www.thirdgen.org/techboard/po...341-post8.html

"The spike you are seeing is the "peak" portion of a "peak and hold" injector pulse. This is a hardware function and has nothing to do with injector bias. The reason for the bias is because the injector requires a set amount of time to open to the point where fuel flows. This time can be as short as a few hundred microseconds for a TBI injector to over one millisecond for a saturated port injector. Say for examplethe ECM calculated that on a port injected setup, it requires a 1.5mS pulse width. Since it takes 1mS for the injector to open, fuel will only flow for .5mS or only 1/3 of what is needed. If a bias of 1mS is added to the final pulse width, a 2.5mS pulse occurs minus the 1mS for the injector to open and fuel flows for 1.5mS. Just what the ECM commanded. Make sense? "

Scott,

Perhaps you could use your oscilliscope to help in developing these correction curves for the various injectors available for the LT5.

Example of VE table w Injector Offset too low. Creates a "bathtub" area of VE.

[mike100]

Originally Posted by scottfab

Must be milliseconds.
Interesting the variance in resistance from unit to unit even
within one vendor per part number.

yeah, I fired up my TunerPro file and checked the usec values against the table here assumed as msec. Looks like the RC Engineering 23 lb injector falls into the stock table very closely.

I have the Lucas ones- I can't recall if they are 19 or 21 lb rated though.

[XfireZ51]

Originally Posted by mike100

yeah, I fired up my TunerPro file and checked the usec values against the table here assumed as msec. Looks like the RC Engineering 23 lb injector falls into the stock table very closely.

I have the Lucas ones- I can't recall if they are 19 or 21 lb rated though.

Not sure but when I look at the stock table v the RC values, they're off but 300msec or so. At 14v, its double the stock time of 300 v 620 for
RC injectors.

[mike100]

This was the example I was comparing to:

__________cc. lbs/hr. ohm -> 10v 11v 12v 13v 14v 15v

RC Engineering: 240 23 16--> 1.31 1.10 0.97 0.77 0.62 0.46

[XfireZ51]

Originally Posted by mike100

This was the example I was comparing to:

__________cc. lbs/hr. ohm -> 10v 11v 12v 13v 14v 15v

RC Engineering: 240 23 16--> 1.31 1.10 0.97 0.77 0.62 0.46

Yes that's right. And at 14v the stock Inj. Offset is .3msec compared to .62. You would note a difference if you made that change in the cal.