Great FIC article on Injector Offset

[XfireZ51]

Here's a very good article from FIClinic.com on the value of using the correct injector offset when replacing injectors. And FIC provides these values specifically for the injectors you purchase which is a great service. So its not just a question of slapping the injectors in.
For those with larger displacement motors who are "running out" of injector,
using mixed sets of injectors is also not recommended since the offset values would be different for each set.
All of this has little effect on WOT, but significant impact on idle and part throttle, which constitutes the majority of engine usage.

http://fuelinjectorclinic.com/data-match-technology

[tpepmeie]

you sure that's the same company? Thought it was fuel injector connection, and out of Georgia.

[XfireZ51]

Originally Posted by tpepmeie

you sure that's the same company? Thought it was Fuel Injector Connection, and out of Georgia.

Good catch Todd. Worthwhile article anyway. Does FI Connection provide similar info on their injectors?

[Paul Workman]

Thanks for posting this Dom. I'd say you get the "Veddy Intorestink" award!




(Remembering Artie Johnson's Rowe and Martin "Laugh-in" character)

[Dynomite]

This from RC.....I have three sets of RC 205s in two 90's and one 91' (The 91' is 500+ hp)

Hello Mr. Dynomite,

A fuel injector’s offset time is the minimum amount of time it takes for an injector valve to move from fully-closed to fully-opened. Generally, most injectors will open fully within 1.0 milliseconds, at 13.5 Volts and 43.5 psi (3 BAR) of fuel pressure. With that said, small flow rate injectors usually open faster than large flow rate injectors because small flowing injectors will have smaller valves, less mass, and less weight. Also, the higher the voltage, the quicker the opening time (smaller offset). Conversely, the higher the fuel pressure, the slower the opening time.

Similar-sized injectors, like the stock LT5 injectors and RC Engineering’s SL4-205 injectors, will have very similar offset times and similar electronic characteristic all together. If you change to a significantly larger injector (20% more or larger), than you’ll need to reprogram your ECU fuel map, including the injector offset tables.




On another subject....I just read that the original LT5 injectors which were rated at 22.5 lbs actually provided around 20.5 lbs.....and further the GM replacements are actually rated at 19 lbs......so there

[XfireZ51]

If RC has "very similar" offset times, they should publish them so that can be verified. Other injector providers like Bosch, Accel and FI Clinic now do that.
Having the correct offsets makes a big difference in arriving at a successful tune particularly idle and part throttle. The calibration we use today is now 25 years old. I imagine there's been improvements made to injector design since that time. Seems like offsets may have changed since 1990.
If current GM replacement injectors are flowing 19# that's ~17.4% differential.
Sounds significant to me.

[Dynomite]

Originally Posted by XfireZ51

If RC has "very similar" offset times, they should publish them so that can be verified. Other injector providers like Bosch, Accel and FI Clinic now do that.
Having the correct offsets makes a big difference in arriving at a successful tune particularly idle and part throttle. The calibration we use today is now 25 years old. I imagine there's been improvements made to injector design since that time. Seems like offsets may have changed since 1990.
If current GM replacement injectors are flowing 19# that's ~17.4% differential.
Sounds significant to me.

Are those offset tables in the form of a graph as dependent on voltage and fuel pressure?

Also.....if you program a fixed offset, is that offset an average offset of a range you select as operational (in terms of voltage and fuel pressure)?

I am assuming the offset does vary with voltage and fuel pressures but not by much variation ......say in the voltage range of 12.5 to say 14 volts.
Say in the fuel pressure range of 40 lbs to 50 lbs?

Also....I am assuming we are using relatively small injectors given we have 16 of them as compared to an engine with two cams.

Then we have Duty Cycle.......which I have been assuming 80% for calculations of potential Horsepower.

But then again.....You are the Tuner....I am the wrencher

I get em close.....you get em closer

[XfireZ51]

As you have pointed out, injector opening and closing times vary by voltage and pressure. The calibration has both tables and constants for injector parameters.
There are offset tables for both voltage and PW. The constant is for fuel flow which is 22.8#.
The offsets allow the ECM to maintain as constant a fuel flow as possible under various conditions. Voltage varies for several reasons, and the ECM needs to compensate for that. It particularly affects idle and part throttle operation since the opening and closing times for injectors constitute a significant fraction of the calculated PW.
The purpose of a VAFPR in port fuel injection systems is to vary fuel pressure in order to maintain a constant fuel flow. It compensates for the changing vacuum the injector "sees" during engine operation.
The accuracy of this basic information increases the accuracy of the ECM calculations which in turn makes a difference in engine operation, ie idle quality, part throttle response, fuel economy blah blah blah.

The use of 16 injectors I speculate had to do with maintaining a controllable PW for idle while being able to supply enough fuel for WOT operation. That's why even those of us that run secondary injectors continuously, still only idle on the primary injectors.
Using BSFC to calc supportable power we come up w

(22.8# * .8DC) * 16injectors/ .5#/hp = 583hp. So several of the big cube motors are "running out of injector" and going static at WOT requiring an upgrade in injectors. In turn, this will likely affect the injector offsets.

[mike100]

The only published data i ever saw for Lucas style injectors was for their 24lb/hr units (or was it the 26's?). Quickly comparing that to the stock offsets of the multecs showed that they were pretty similar, but for 19 or 22lb replacements...?

I am curious to maybe try to figure out what the opening time would be experimentally. the hard part for me would be getting a variable pressure fuel rail test bench. I have enough stuff here at work to drive a signal into an injector, monitor the current, and simply measure the time. My theory is it would be like a multec, but possibly a little different since the disc type has a lighter mass and is supposed to be pretty snappy when it comes to opening speed.

[XfireZ51]

Mike,

Currently I run Accel 150121 injectors which I believe are Lucas injectors. However, I am using the offset values for the 24# injectors. They're rated at 43.5# FP so I am hoping that the values would be accurate. I'm in process of verifying that.