Theorical Injectors Fuel Rate and Offsets

[The_Emard]

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.

[The_Emard]

4) Offset tables

For the offset tables the principle remains the same except that this time around you will use 4 different tables:

The first part of the process will be the same whether you are using a return or a returnless system.

a) Finding the offsets at target PD

Using the offsets and the voltages from the FNPW_OFFSET table, you will find an equation that will allow you to find values beyond the given range. I feel like the values before 6 and after 15 volts are not really necessary since supposedly the injectors are not suppose to work outside that range. But we will do it anyway for the purpose of having a full table and a clean conscience.

You can use Excel to plot the table and find the equation.

You will notice that it is not linear like the multipliers. You will have to find the best type of equation that gives you the most accurate offsets. In this case I tried Exponential and Power because they both looked almost accurate.

There are much better ways around to interpolate those values and get them to be almost exact. But for the purpose of this post I used the easiest and fastest way I could think of. So I compared the values from both the equations and averaged them. While comparing the average values with the ones from the FNPW_OFFSET table, the error percentage was so small that it should not have much influence.

These would be the offsets for a base pressure/Pressure differential of 39.15 PSI (the target pressure for 60# at which the multiplier is 1)

b) Finding PD multipliers for Returnless fuel systems

This is very simple. You do exactly the same as you did while looking for IFR.

Then you find the PD for every MAP and enter its value in the equation. You should get something like this if you use excel.

You end this stage by multiplying every target offset by the newly found PD multipliers to get your entire offset table.

!!! Converting the FORD offsets to GM offsets !!!

Since the format of the data sheet is meant for Ford ECUs, the offsets at this stage can't be directly copied to HPTuners if you mean to use them on a GM ECU.

You can simply see offsets as the distance from the Y axis at which the slope will begin. From a Ford standpoint, the offsets are the distance at which the LOW SLOPE starts. But since we are using the HIGH SLOPE our offsets will put the slope further from the Y axis than it should actually be and slightly or dramatically change the fuel injected (depending on the break point's position).

Now the goal is to find a way to bring the slope back to it's rightful place. The thing is that the breaking point must be used to make it possible but it also depends on MAP.

Using simple and linear y=mx+b I managed to find an equation to isolate the right offset using 2 variables and known constants.

Offset GM = [Breaking Point + (Low Slope x Ford Offset)]/Low Slope - (Breaking Point/High Slope)

I will not go into full details but using the methods used previously you should be able to:

1) Make a Breaking Point table for Volts vs MAP.

2) Convert your Ford offsets found earlier using the conversion equation.

c) Offsets for pressure referenced fuel system (return style)

This one is VERY VERY easy. You only have to enter your base pressure/PD in the multiplier equation. When it is done you multiply the answer by the target offsets column. Once gain you must convert you offsets to GM before making every column from the Offset table in HPTuners the same as the one you've calculated.


Thank you very much and sorry for the extremely long and boring post. Keep in mind that everything in it is theoretical. You should also consider that everything in this post could be optimized which would result in WAY WAY WAY more accurate offsets and IFRs.

P.S.: Thanks to DSteck for the Ford/GM conversion Trick.

[DSteck]

Just FYI, your voltage offset calculation is incorrect. The way Ford defines offset is very different from the way GM defines it.

[The_Emard]

I found something on Ford Offsets vs GM offsets. I'll be back.

[DSteck]

The values I see in the tables are not for FORD engines they are mechanical and electrical properties of the injectors so if it's what you mean then I don't get it.

Although my calculations may be wrong, I'd really like you to tell me exactly which part and why... cause I will correct my post and my tune.

Nope. Those values are part of dynamic flow characterization and do not represent a physical property of the injector. It's actually a pretty complex algebraic derivation for the equation to convert it.

Here's a question for you. Where did you read how the values relate to injector function?



If you search my threads, I actually posted an Excel file that does the conversion. It is protected though.

[The_Emard]

I think I get the difference between them. I'll look into it and correct my orignal post once it is done. Thanks

[jrsmith]

Anyone have a copy of post 1 & 2 with the pictures still present? Kind of hard to decipher with everything gone.