Old School Fuel Adder Injector Drivers TBI version for BOOST

On the my intro posts I was talking about a fuel adder that I have used to add fuel on boost.
Fuel Adder circuit explained.


Fuel Pressure


Since the adder is using a common fuel rail with the existing injectors we use the same fuel pressure to determine all injector flow rates.


If you can read block learns you can add an adjustable manifold pressure referenced fuel pressure regulator.
Adjust the fuel pressure up until your 3000 rpm block learns are about middle. IE if scale is 0-255 128 if 0-128 64 if 0-64 32.


If your fuel pressure is at least (2 x barometric pressure) + manifold pressure then you do not need an additional fuel regulator.


If neither block learn or high fuel pressure are present then you should add a boost referenced fuel pressure regulator A boost regulator only restricts fuel under boost and is used on the return line from your throttle body.


Measurements


You have to measure the pulse width at lowest demand for fuel.
Typically this is idle at hi vacuum.
This pulse width can be measured with an oscilloscope at the injector.
You want to find the time that the injector is on for.
On GM TBI there is a Running power feed wire from the injector fuses and a return to ground wire that is switched by the ECM.
Since the GM TBI injectors are low impedance the ECM uses a FET to ground the wire.
When measuring you can connect one lead of a 10K Ohm resistor to the battery + and the positive of the scope to the other lead of the resistor.
Connect a test jumper between the scope positive lead and the ground side of the injector.
So one side of the resistor is connected to battery the other side to scope and injector.
Connect the negative lead of the scope to the ground side of the battery.


Start and run the engine until it reaches operating temperature allow to idle and read the scope trace. The time that the scope trace is 0 or near zero is your base injector pulse width.
Now have an assistant power brake the engine to simulate full load.
Read the scope trace agan this time we want the time that the pulse is positive between 0 readings.
This is called injector off time. It is rough estimate some websites may even list these values for various controllers.
Read the total pulse width at WOT and the RPM of WOT.
You will need the vacuum reading at WOT as well.


The existing engine control system will typically handle MAP readings up to the 1 ATM or zero vacuum we are adding fuel for boost above zero vacuum.


We have to calculate what the minimum pulse width for the new injector can be.
First the existing injector minimum period is the base injector pulse width + the minimum injector off time. The maximum injector frequency is 1/injector minimum period.


A little of air fuel shop tech.
Boost engines hate lean and to show it they make you spend lots of money when you do let them lean out.
On Gasoline AFR of max power is 13.1:1 under boost you may go as low as 12:1 to prevent knock and overheating. Anything above 15:1 under pressure and you will be using a parts washer to remove metal from your pan.


To figure this out we have to know how many pounds of air your engine will suck in under full boost.


First we figure the natural asperation in CFM,
(Max RPM x Engine Cubic Inch Displacement)/(1728 x 2) = CFM of Four stroke
(Max RPM x Engine Cubic Inch Displacement)/1728 = CFM of two stroke


Then we calculate the Ideal Gas Weight
We need the barometric pressure to start with.
On old fuelies we had a mechanical barometer bult in, on new cars built since the stone age there are different schemes to get barro reading.
On GM single MAP sensor the first reading key on engine off TPS minimum the reading is stored as Barometric pressure. Some cars use a separate sensor to get continuous readings. Hint if climbing steep mountains in GM car or truck and you start losing power stop and restart engine to obtain higher altitude reading.


Anyhow I live at sea level so mostly it is 14.7psi unless there is a hurricane coming then it will drop lower, at altitude you will see a lower pressure.


So no boost wide open throttle max rpm manifold vacuum + 14.7 (or your average pressure) is the ABS value we need. we will call this NAPSIA


We also need the max boost you intend to generate.
Max Boost PSI + 14.7 (or your average pressure) = BPSIA


Now we need the temperature of the intake manifold air naturally aspirated and under full boost.


NAR = intake air temperature + 460
BR = boosted intake air temperature + 460


Ideal N/A Pounds of air per minute = (NAPSIA x CFM x 29) / (10.73 x NAR)


Ideal Boosted Pounds of air per minute = (BPSIA x CFM x 29) / (10.73 x BR)


Volumetric Efficiency
No this is not the same as the EPA star rating it is the difference between the theoretical IDEAL and the actual expressed as a percent.
For most naturally asperated engines this is the area we make the largest power gains.
So for calculating purposes without extensive dyno testing here are some basic values.
Gen 1 SBC non-corvette L05 @76%
Gen 1 SBC corvette 2.02/1.6 @80%
Gen 1 SBC nascar 2.02/1.6 @100%
Gen 1 SBC tunnel ram @100%
Gen II SBC non-corvette LT-1 L43 @80%
Gen II SBC corvette LT-4 @83%
Gen III SBC LS-1 LS-2 @90%
Gen IV SBC LT1 @100%
NorthStar 32V @85%
Chevy/ Lotus ZR1 @100%
V-tech 2.0l @100%
V-tech >2.0L @90%


Actual Air Flow Pounds per minute = Ideal Pounds of air per minute x Volumetric Efficiency


Air flow pounds per hour = Actual Air Flow Pounds per minute x 60


Fuel Flow Full Rich (12:1) pph = Air flow pounds per hour / 12


Now you have your calculated amount of fuel you need you can calculate how much you currently have and subtract that from what you need.


Regardless of what the specifications read we can determine actual flow rate of the base engine injectors at this point.


From the WOT measurement above
(WOT RPM x Engine Cubic Inch Displacement)/(1728 x 2) = CFM of Four stroke
(WOT RPM x Engine Cubic Inch Displacement)/1728 = CFM of two stroke


Actual Total Flow Rate of existing injectors = (Volumetric Efficiency x ((WOT Vacuum + Barometric Pressure) x CFM x 29)) x 3.53 )


Amount of fuel per squirt
GM TBI squirts twice per firing of each cylinder
pounds per squirt = Actual Total FLow Rate of existing injectors /(WOT RPM x 120)


Now the fuel adder needs


Fuel Flow Full Rich (12:1) pph - Actual Total Flow Rate of existing injectors = Balance of fuel per hour needed pph


Now we can size the adder injectors


total pulse width at WOT x number of injectors x Max RPM x 120 = maximum pulses per hour


Balance of fuel per hour needed pph / maximum pulses per hour = PPH flow rate of adder injectors.


Select an injector slightly higher than you need we will have provision to adjust down adjusting up with fuel pressure would require a retune of the ECM.


The power adder injectors should start adding fuel when the engine reaches the WOT vacuum reading from above.


Ok so back to the electronics


We have the base injector pulse width (BIPW) of the existing injector from above.
We also have the injector off time (IOT) of the existing injector from above.


The BIPW is the window for our trigger. What this means is that a signal this size or larger is recognised as an input pulse.
The timeout window is slightly longer than the IOT this allows the system to turn off if there is no injector pulses.


So we know that the injector is connected to ground when turned on to measure the time period we will need a circuit that inverts the ground pulse to a positive one.
Then we charge a capacitor through a resistor until it reaches a preset voltage.
The voltage we are working at in the circuit is 5V provided by a 7805 1A 5V regulator U1.
We use a simple circuit called a TTL inverter with the output tied to a capacitor.

Attachment 12433
The capacitor value C1 is calculated as the RC time constant equal to the BIPW so the resistor R3 is 10K the capacitor value is BIPW / R3 value.
http://referencedesigner.com/rfcal/cal_05.php


The voltage of the capacitor is compared on 1/2 of U2 a LM 358 Dual Op Amp with a preset voltage provided by VR1.
Attachment 12434


The 3BAR MAP sensor we need to buffer the signal to help reject noise.
The signal is fed into a unity gain amplifier using the other half of U2.
From the amplifier it goes to pin 5 of U3 a 555 timer the voltage acts as a pulse width VCO with single pulse per original injector pulse at pin 2.
The MAX pulse width is tunable with VR2 output is at pin 3.
Attachment 12435


Now we duplicate the circuit again.
From the amplifier it goes to pin 5 of U4 a 555 timer the voltage acts as a pulse width VCO with single pulse per original injector pulse at pin 2.
Threshold like VR1 setting is at VR3
The MAX pulse width is tunable with VR4 output is at pin 3.

Attachment 12437

Now we want to add to the circuit the turn on MAP threshold we measured as MAP reading of WOT no boost. The moment you cross the threshold the adder injectors will pulse.
Attachment 12438
The highlighted trace is providing the MAP signal to another comparator the out put we will use some TTL logic to drive the switching MOSFETS.
Attachment 12439
Q5-13 form a TTL inverted AND gate. The stock injector pulses are used to trigger the First stage VR1 and VR3 are set to the smallest length of an injection pulse. This does two things it will limit the amount of noise in the pickup circuit by ignoring anything shorter than the injector pulse.
The second thing is that when the stock injectors are not firing there will be mo trigger signal to the secondary timing circuit so no squirt. If ignition fails or rev limiter kicks in the secondary injectors will not squirt.
VR2 and VR4 are used to set the maximum secondary injector pulse length.
VR5 adjusts the system active point based on MAP voltage.

Attached is the Fritzing file with board design.
Feel free to make changes and use at will.



Happy New Year
Dave