Hi ! I have been going around multiple boards and threads gathering informations on how to tune Injectors for LSJs. Fortunately there is a very good and helpful guides made by SJSchafer and spreadsheets made by Omega but the problem is that everywhere the answer for IFR and Offsets is that heaven made them available to a selected few. This is why I've been trying to decipher a data sheet directly from Siemens/Motoron as I think this might be how people found the offsets that most people are using right now. Keep in mind that it is all theoretical and that it might not at all be accurate. If it turns out to be true then I hope this will help more people understand injection and make their own offsets tables for whatever injector they are using.
1) Pressure differential
Every values from the data sheet is presented as PSID, which is the pressure differential (PD) between Fuel pressure (FP) and Manifold Air Pressure (MAP). It is very simple to calculate:
FP - MAP = PD
2) Fuel system
As you all know there are 2 types of fuel systems that are frequently used in LSJs
a) Returnless Fuel System
The way I understand how this one works on an Ecotec is that when the pressure changes in the intake manifold the pressure difference between Fuel and Manifold changes. This is all supposing that the fuel pressure is constantly at 58 PSI. This is why the offset table accounts for different pressure conditions. So PD is always different in this case.
b) Return Fuel System
This system makes it so that whenever the MAP goes down the fuel pressure also goes down. This is why PD is alway the same in this case.
3) Injector Flow Rates
Most people like me may not have the opportunity to bench flow and match their injectors. So like me you might be a bit insecure about what flow rate you should be using. Supposing that the following data sheet is accurate, here's how you can find the flow rate of your injectors:
There are 2 important tables here:
and
From the first table you find 2 values which represent the flow rate. Those values are ALOSL and AHISL. From looking around I found that the slopes are used in the ECU of Ford engines. In the case of a GM ECU, only the High slope will be used. This is can be verified by the next calculation:
From the second table you get the multiplier to use for finding the right flow rate depending on the pressure differential.
We already know from SJSchafer's guide that by his say he bench flowed is injectors and knows that for pressure differential of 43.5 PSI (assuming a vaccum referenced system) he gets 63.4 lb/h from his injectors. So using the Low and High Slope multiplier tables and interpolating for a PD of 43.5 I get:
LOW SLOPE
65.8 lb/h
HIGH SLOPE
63.4 lb/h
Seeing as I get the exact same value from those simple equations that SJSchafer got from NewEra's test numbers I would suggest that the High Slope is the one that is used by the LSJs. Now there are 2 possibilities to find your IFR, depending on what type of fuel system you are using.
a) Return Fuel system (vacuum/boost referenced)
Since this system has a constant PD it is very simple. Whatever pressure you chose as your base pressure will be exactly the PD when the engine is running. So per example if you take 39.15 PSI as your base fuel pressure you only have to multiply AHISL by 3600 seconds per hour to get the lb/h value for your flow. If you take 50.03 as your base pressure then you would multiply AHISL by the multiplier and then by 3600. You would then copy that number all across the IFR table.
b) Returnless fuel system
This is where things get messy. This time around you will have to find the PD for every MAP and then interpolate the right multiplier to find the IFR. This is also where I am absolutely guessing and can't be sure if what I'm doing is right.
So you have that table:
For an unknown reason (maybe someone can explain it to me, the formula to calculate the IFR with a returnless style system at a constant pressure seems to be FP+MAP=PD.
Now, since the multiplier form the FNPW_HSCOMP table are only for PD ranging from 20 to 60 PSI we will have to find an equation that will give us multipliers for higher PDs.
Using Microsoft Office Excel we will make a graph and find the equation:
The equation will not be exactly accurate but since it all looks fairly linear we can assume that the effect will not be so dramatic. Then using this equation we will find the corresponding IFR for each case of the table.
For that purpose you will try to find the PD for each one with FP+MAP=PD.
For example: When MAP is -120 kPa (17.4045 PSI), you will have 58 + (-17.4045) = 40.6 PSID. You will then use the same method as before which is to multiply AHISL by 3600 by Multiplier @ 40.6 PSID.
This should give you a table that looks like this: (remember that I am using the data sheet for Siemens 60# Injectors)
You would then proceed to use the classic method of adding a multiplier to prevent maxing out the values in HPTuners.