This testcase was about seeing how much different fuel injector brands require a different time delay set within the PCM injector offset voltage time table and air/fuel multiplier table.

Installed in our 1999 C5 372 stroker engine was the GM C5R 42 lb injector. The fuel trims cells were right on at about 0% readings for the long and short term fuel trims

Thanks to teammate Mark D for donating his used SV030 36 lb injectors that we swapped in with the C5Rs.

First we did a tune making the injector flow with the difference of using stock 28 lb GM injectors to the 36s.
You see from the results in doing so the fuel trims were way too lean due to the makeup of this stroker engine.

We then made fuel flow changes in another tune to make up for that but fuel trims were still a bit too lean and testruns showed engine was sluggish.

We analyzed all the OBD data and then made changes to the injector offset voltage and air/fuel multiplier tables and tested that to find the internal coil of the SVO30 injectors required more delay time for the coil to react.

We the proper values put in those tables as you see those changes alone caused the fuel trims to get richer, the engine response times were better and the fuel mileage bettered by 2 MPG.

You also notice as the offset times were set to more suit the injector that the left/right fuel imbalance decreased

This clearly shows not all injector coil reaction times are the same and must be factored in when changing injector brands.

The PCM determines how long the fuel injectors will be pulsed on to meet the fuel needs as what the feedback sensors such as the 02 report.

The PCM though needs to command the injectors on at the right time else the fuel could be flowing too early or late to when the sparkplug is being fired on and what position the cylinder cycle is.

The PCM has no way of knowing how slow or fast the internal coil of injector is. Each brand or size of injector's coil reacts differently.

Inside the PCM calibration is a injector offset voltage table related to engine vaccum/MAP and battery voltage.
This is the measured voltage at the PCM and not at the battery since there is some voltage loss.

GM in testing the stock injector knew what the delay time was and added that delay time in mSecs to the table.

Now when the PCM commands an injector on the delay time is added to the pulse width time

The problem is when the voltage changes the PCM has to adjust to assure the proper fuel flow is commanded.

If the car, battery or alt has a problem and the voltage is low, the PCM then has to add more of a delay time to bring the proper fuel volume in control so it is important for you to monitor voltage and know if voltage is lower then of course the PCM is in fact commanding the pulse width on longer which effects fuel trims and performance

Suppose you add a 0.5 millisecond (ms) offset to that table at your normal battery operating voltage.
If the PCM calculates an idle pulsewidth of 2.0 milliseconds before applying the offset, the final pulsewidth after adding the offset will be 2.5 milliseconds.

The absolute change is 0.5 milliseconds, and the relative change caused by the .5 ms offset is 0.5 / 2.0 = 0.25, or 25%. This is a big change!

Now suppose the PCM calculates a WOT pulsewidth of 15.0 milliseconds before applying the same 0.5 ms offset.

After applying the offset the pulsewidth becomes 15.5 milliseconds. In this case, while the absolute change is still 0.5, the relative change caused by the offset is only 0.5/15.0 = 0.03, or 3%. The same offset has almost no affect on large pulsewidths.

Since not all injectors are made equal it is important then when changing injectors is a MUST to correctly change the offset times to assure proper fuel flow and injector being commanded ON when the sparkplug is also fired off.
Demand from the seller they provide the specs for the injector including what the injector offset values are and then have PCM tuned to include the values for the injector offset voltage table

If this is not done the injector can be flowing at an incorrect firing cycle of the cylinder and cause incorrect fuel trim values.

As you notice the chart shows as the voltage is lower the offset delay is larger.

It has been recommended to fire injector earlier rather then later.

Firing the injectors too late can cause the fuel to wash down the cylinder wall causing excessive wear and oil contamination.

While firing the injector early will puddle fuel on the top of the valve, this does allow the fuel to absorb heat from the intake track and valve. This heat will help keep the fuel close to vaporization.

The longer it sits on top of the valve the more heat it can absorb. But this could also be very hazardous under boost, were you want the coolest charge possible.
It has been stated that fuel on the valve could cause carbon buildup.
I don’t believe this to be the case. Carbon buildup is most likely caused at the point where the exhaust valve is closing, the intake is opening and there is residual pressure in the chamber is higher then intake pressure (nearing TDC, just as the intake stroke starts).

Much like any other spring-mass system being driven by an electric current, opening of the injector is not an exact step change.

It takes a small amount of time to build up enough energy in the coil to begin to move the pintle of the injector off the seat and allow fuel to flow into the manifold.
The initial delay is known as “dead-time,” but is also followed by a period of exponential movement of the pintle until it hits the fully open position.
Likewise, closing the valve takes time as well. Once the current to the coil is removed, the pintle is pushed back to its seat by internal spring pressure as well as fuel pressure behind it. The difference between the opening delay and closing delays is called “injector offset.”

Remembering that fuel injectors are actuated by electromagnets, it is important to further understand how their performance can change. The strength of the electromagnet in the injector varies relative to voltage.

Having more voltage across the field of the coil increases the strength and allows the injector to open quicker. This in turn means that fuel begins to flow into the manifold slightly sooner if voltage is higher.

Knowing that cars almost never have constant voltage, the PCM needs to be able to adjust. A failed alternator, dead battery, or even normal cranking can send voltage to 11.5 or lower. Normal charging usually keeps voltage around 14 volts, and a failed voltage regulator can send output above 17 volts.

The bottom line here is that the same injector under these varying conditions can change its actual output by 40% or more. All modern PCMs have tables built into their software code to model this change, even if they aren’t overtly visible to the calibrator.
Various injectors exhibit different voltage compensation curves. While all injectors change relative to voltage in a similar manner, the exact offsets at a given voltage are slightly different as internal construction of the injector changes.
To best model the actual fuel delivery to the engine, it is ideal to accurately input the voltage compensation for the injector used. A quick Internet search can often yield the exact voltage offsets for most injectors.

To add more complexity, the actual flow rate changes based on pulsewidth. As the injector first opens, more fuel flows for the split-second that pressure differences are the highest.

Additionally, when the PCM commands an injector-opening event for a short duration, there is a tendency for the injector’s spring-mass system to overshoot the desired duration.
The net result is that at small pulsewidths, the injector tends to deliver fuel at a rate slightly higher than the static flow rate of the injector.

This often leads to modeling the injector with two different flow rates, one for the normal pulsewidths of cruising and power delivery and another for the shorter pulsewidths of idle and starting.

This in turn leads to the need to determine where this change, known as the “break point,” occurs. This break point is usually relatively small, on the order of 1 to 3 ms, so the effect is often only seen at idle and very low loads.
Again, entering this break point into the PCM routine allows for more accurate modeling of exactly how much fuel can be expected to enter the engine for a given commanded pulsewidth.

PCM fuel injector flow process

After allowing the PCM to relearn over a few days driving we see in the only changes made was to the injector offset table that before was high lean conditions causing more fuel use and now fuel trims about perfect with a 2 MPG increase in saving fuel costs.

Final test results on this project after another week of driving and PCM settling in on it's adaptive strategy

You notice that the fuel trims have remained around a perfect zero for both long and short terms.

Also the injector maximum duty cycle was only 73% and in this testrun flywheel horsepower was 461 yet the math shows for a 36 lb injector that would have been at the same HP thus data shows that the maximum injector needed in this run was 32 lb/hr.

This gives a pointer to how much the water/methanol injection and our smart 3D controller reduces the load on the injector and to increase fuel mileage.

I just compared the injector offset voltage values of a 97-98 Corvette with those of a 98 LS1 Firebird (both stock programs!)

The Corvette has slightly higher values (example: 13.5 V and 60% load) Corvette = 0.4407, Firebird = 0.4103.

The injectors are the same (GM part # 12533952) and I think the PCM's are basically the same too.

Do you know why the offset values are different?


Thanks

Stefano

That is about a 7% difference

I do not know as fact but I'd assume the difference is due to the CAM grinds are different.

Best is to experiment with this table and review scanner recordings to determine if you need to add or subtract from the stock values in that table.

OK I can try that.
Are there some kind of danger involved?
Like backfire or overheating some component?

No not if you do small changes and then via scanner seeing if fuel trims go leaner or richer.
Try adding 10% to stock values and then another test taking out 10% from the stock values.
See where your car normally is as to voltage so if it runs mostly around 13.5 volts then change the rows in that area.

Once you have the values right for the makeup of your car then you can adjust the whole table to fit your engine's need.

Here the results:

0% offset change (stock) => Fuel Trims = +1.73% (FTC 6-14) and +1.70% (FTC 1-19)
+10% offset change => Fuel Trims = +0.14% (FTC 6-14) and -0.91% (FTC 1-19)
-10% offset change => Fuel Trims = +3.12% (FTC 6-14) and 3.39% (FTC 1-19)

The fuel trims values are a sum of LTFT and STFT (this to give a more precide result).
I averaged the counts and not the cells, so those numbers are a weighted average and give the real pictuer of the thing.

The change in the fuel trims is pretty logical.

With +10% offset the car feels stronger while accelerating ant the engine reacts quickly.

With -10% offset something is strange... it's not too bad but it's like an old car with a compression problem. A little asleep

Any comment? What else to try? The result with +10% is really interesting!

I'm sorry but I can't post the graphs. JR: is there a way to upload pictures directly from my PC?

In your case

After offset is changed and PCM has had relearn time then scan and adjust fuel flow to complete the changes related to offset values

I would need to see scanner data to say add or remove fuel as to the offset changes but assume -10 would make it richer and require less injector flow

To upload images simply use the options FILE MANAGER function listed below when entering a new post or full reply mode content window and it allows you to select images off your PC and embeds into the post.

Image cannot be larger then about 400K bytes

Here the results as a graph

I would do tuning tests with both -10 and +10 as to injector IFR table and see which one you like best.

OK let me try.
would you try with even bigger changes? like +/- 20%?

Do you see a reason why increasing the delay time (= advancing the fuel injection) the engine reacts better?
I can't express it in numbers, but it's one of those improvements that put a smile on your face when you test drive the car.

Better fuel vaporisation?

I think with stock injectors you should not do but a tweak of offset as you did and not as big a change as +/- 20 % as you could force the injectors to command on before or after the sparkplug fired

The delay, 2 parts, 1 is was the existing values not correct for the engine makeup and 2 did offset values for engine makeup require values different then what you had ( assumed stock values) not valid for your engine's needs.


Keep in mind if voltage level changes it then has effect on which offset row and its values are used.

Thus if for some reason voltage is low then so it the voltage to injectors thus different offset value row is being used and those values for delay might not suit the engine's needs.

I see.
The voltage is between 13.5 and 14 depending in the electrical load (lights, defogger, fan...).
I'll probably tweak the region between 12.5 and 15 V

My version of LS1Edit can read only 97-98 programs.
Could you take a look at the differences beween the years? Let's say 98 - 02?
If the cam grind plays a rule on how GM programmed those values each year should be different. If they are the same it would be interesting to understand why F-bodies LS1 have different values than Corvettes.

Stefano

The lower the voltage is at the battery means even lower voltage drop to the injectors and less the injector is spraying so it is important to assure good voltage level is maintained

If not then lower voltage also means the PCM is using the offset voltage values for that voltage row in the table.

Looking at a 1999 and 2002 Fbody you see there is offset value differences

Here the 1998 F-Body delay table, which is pretty different than the 99 and the 02

And here the cam specs of different F and Y bodies.
Notice how the 98 and the 99 F-body have the same cam specs but completely different injectors offset.

The 97-98 injectors flow is 28.5 Lbs-Hr @ 4 bar
The 99-00 injectors flow is 26.3 Lbs-Hr @ 4 bar

Is it possible that the injector design changed so much from 98 to 99 to request such a big change in the delay table?

Or did GM recognise an advantage firing the injectors a little sooner?

That is the whole purpose of this thread, injectors cannot just be changed and NOT also redo the offset table to suit the design and flow of the injectors.

Pretty bad that not only are those installing injectors but ask the vendors selling them what the values are for the injector being sold and they have no clue.

Also as to F-body, look at how much the LSA changes were which effect vaccum and timing

So you think that in case of the 98-99 F-body the injectors were so different (28.5 vs. 26.3 Lbs-Hr) that the delay had to be increased by 30%?

Notice that the camshaft was exctly the same for both years.
Heads are the same too.

At the beginnig of this thread you describe the situation before and after programming the correct delay time.
How did you find out this time? From the manufacturer? Try and error? If by try and error how did you find out the opitmal values? Dyno? Scanner?

Stefano

As I stated all injectors will have their own reaction delays and/or with CAM changes require the offset table tuned for the makeup of each powertrain.

I do not believe in dyno tuning as known the PCM requires good amount of relearn time not done on most static dyno's

All testing and tuning should be with realworld street driving and OBD scanner rscording.

Here is long term test after 1 1/2 months since the SVO30s were installed and injector offset table modified.

Weather was 57 degrees, elevation from 3500 to 4,000 feet ( loss of 52 HP due to elevation).

PCM non WOT recorded OBD-II cycles was 42,800 with only 624 WOT cycles over 33 miles in 38 minutes drive time. OBD-II scanner capture rate was 40 F/ps of 25 concurrent PIDs.

We see both long and short term fuel trims right around a perfect zero. Max air flow at 43 lbs or 610 CFM.

WOT timing good at 28 degrees.

SVO30s around max duty cycle of 78% with average NON WOT fuel injector pulse width of 4 mSeconds with a WOT peak of 16.3 mSecs.

Engine produced in short WOT 405 ft/lbs torque at 468 flywheel HP at 122 MPH with just under the 6,800 rev limiter.

WOT AFR was between 12.7 to 13 as water/methanol is being used spraying at around 600 mL a minute.

Again this 1999 LS1 is 372 CI stroked with 11.3:1 AFR with 91 octane gas and during this run had zero knock or misfire.

Non WOT fuel mileage was 20 for city and averaged between 28 and 32 MPG depending on how stable speeds were maintained.

Here is the data when going from slow speed to WOT to 122 MPH and then to see how our model/simulator reported to see how it matched up to real world

Simulator results at top and scanner data in bottom graph. Numbers closely matched.

Here is another case if where not properly tuning or understanding how PCM functions costs how engine functions which effects performance

In this real world case a C5 Corvette using 18 PSI of boost was having engine knock problems even though the owner installed and ported water/methanol so that there was a nozzle for each cylinder that flows as much as 3 gallons an hour per cylinder.
They are using 70 plus lb/hr injectors and still had real lean conditions by only 4,000 RPMs.

Owner had PCM tuned by others and problem still existed.

Owner sent us a scanner recording and our results showing fuel trim values ( top graph) was as lean as 13% even with those large injectors.

Owner sent us the PCM flash file we then analyzed to find the injector offset table values incorrect for the injectors used.

We corrected that and the fuel trims went from real lean to at least 10% rich which again shows that values for offset that are wrong will cost more fuel being used as PCM was trying to drive the lean trims values back down and by properly making the adjustments for those injectors that we then not only gor rid of the lean conditions but was still able to reduce injector flow by 10%.

Bottom graph is after base tune, after owner does a drive relearn we will do a final tweak to lean up even more at the low engine loads.