Injector offset or Volts vs adder

R100RT said:

Hi Tuners, It's been a while but I'm back fooling with my TT Vintage Mustang.
I want to check my understanding of this function. First, its easy to find charts showing volts vs offset for many injectors. The Tweecer tuning guide suggests values that get smaller as the injecto gets larger.

If I have a value of 1.0 entered for 13 VDC and I am logging a PW with of 1.8ms, at idle, then the actual calculated PW is 1.8 - 1.0 or .8ms. This function effects idle the most as it becomes increasing less of the total calculated PW.

My Mustang has been runing for years. I have a A9S and Delphi 50 lb/hr injectors. Current offset is like I sad above, 1.0 at 13 VDC with the curve shaped like stock. When I tried the Tweecer suggested values, the car would not idle. The suggest value is about .5 at 13 VDC. So, based on the idea that the calculated PW is .8ms plus the .5 we get a output pw of 1.3ms or 72% of the original PW that worked.

My MAF is factory ProM 3", 42# calibration. Slopes are 50 and 55.

I have seen some tuners suggest using the same high and low slope value and starting with an offset of .5 at 14VDC. The same tuner suggests using 1.0 if you are using different slopes.

What say you?

Eric

Click to expand...

I say they don't have a clue what they're talking about. Go to the Ford Racing Parts website, look up your exact injectors, and then find the factory injector specifications sheet that Ford is nice enough to list for you. Use the Ford data for the injectors, get an uncalibrated MAF, and tune the car properly.

The calibrated MAF lies to the ECU about how much air you're engine is breathing. It lowers the airflow numbers in an attempt to lower the fuel flow numbers to compensate for your larger injectors. It's a trick at best, designed to prevent the need for custom tuning.

Ford injector specs are broken down into four major settings, and one major table. The four settings are injector high slope, low slope, breakpoint, and minimum pulsewidth. High slope is how much flow (in #/second) that the injector flows when you first open it. Low slope is how much it flows (in #/sec) once the flow stabilizes. Breakpoint is the point where the injector goes from hi to lo slope. Minimum pulsewidth is the minimum amount of time that the injector can be opened.

Imagine a garden hose, pressure turned on, your thumb over the end of the hose. When you release your thumb, the hose squirts water 10 feet (hi slope). After about 2 seconds of that (breakpoint), the pressure stabilizes and the water now only squirts 8 feet (lo slope), and continues to squirt 8 feet for the rest of it's life, or until the water is turned off. Now, same hose and pressure, but you add a squirt handle to your hose and turn the water on. No matter how quickly you try to squeeze and release that handle, a minimum of X amount of water always comes out (minimum pulsewidth).

The injector is an electromagnet. Give it voltage, it opens. End voltage, it closes. The more voltage you give it, the faster it opens. The less voltage you give it, the slower it opens. Somewhere in the middle of this voltage spread, this injector is rated for flow. Lets say thats 14 volts on our sample injector. So if the car is only at 11 volts, the injector flows less fuel because it is open for slightly less time because of the reduced voltage opening the injector slower. At 19 volts, the opposite happens. The injector opens too fast, flowing more fuel in than what the normal voltage would have done. To fix this, EFI has a table (injector comp battery voltage) that compensates for this by multiplying the slope (flow rate) of the injector. A sample compensation table might look something like this:

12v = 1.21
13v = 1.14
14v = 1.00
15v = 0.96
16v = 0.92

Every injector has different characteristics about how much more the hi slope flows than the low slope, where the breakpoint is, what the minimum is, and how voltage affects the opening of the injector. This is not the same for any 2 injectors really. At higher RPM, some of these specs become less of a problem. But at idle, they have a much more pronounced effect. These specs are set in the tune, and no calibrated meter can change them. They must be changed in the tune.

To further prove the importance of this, take a look at Fords specs for their 30# injectors. The old Bosch style 30# injectors had very different specs than the new pencil style 30# injectors do, despite them both having the same static flow rate. Switching from 30# inj A to 30# inj B would therefore require retuning.

By the way, to find the flow rate in pounds per hour, multiply the hi slope or the low slope by 3600. (That's seconds to minutes, and minutes to hours). You'll find that the hi slope is usually about 15 - 25% more flow than the low slope.

Your battery voltage compensation table is a multiplier, not an adder! At least in terms of factory ECU thinking.