Austin Healey Electronic Fuel Injection - Page 6

Speed Sensor


Speed Sensor (included in mine but optional):  The speed sensor translates the mechanical output of the transmission rotation into an electrical signal that is input to the ECU.  The speed sensor is placed between the transmission output and the speedometer cable.  You will need an adapter to connect these pieces.  The ECU uses this "speed" information in relationship to other information (such as TPS, MAP, etc.) to asses the correct action to take on the engine management functions, i.e. injection firing duration, timing.  The additional benefit of adding the speed sensor to the ECU is that if you plan on fitting a cruise control, the cruise control can use the VSS (variable speed sensor) signal output from the ECU for the cruise control input.  This eleminates the need for having to add a driveshaft rotation sensor for your cruise control and makes your cruise control installation just that much neater.  I have cruise control on my car and a write-up on it can be found in the Project section of this website. 

The other major component of the EFI system is fuel delivery.  The fuel delivery portion of the EFI system is made up of a 1) high pressure fuel pump, 2) fuel filter, 3) fuel rail, 4) fuel pressure regulator, and 5) fuel injectors.  The fuel delivery system must be capable of providing a constant high quality fuel source.  The ECU expects that its fuel demands will be met quickly.  The injectors require a clean source of fuel to respond to those demands and to stay operable.

The fuel pump must be capable of delivering a constant 40-50 psi at a minimum of 35 GPH.  To accomplish this you need a good pump made expressly for the demands of fuel injection.  Even high performance pumps (not designed for EFI) will not be capable of meeting the demands of an EFI system.  There are now external tank pumps that can deliver that supply if you don't want to go to the trouble of getting an in-tank pump system fabricated.  The advantage of the in-tank pump is that the fuel helps to cool the pump which will allow for a longer life for the pump and quieter operation.  I have chosen to go with a GM in-tank fuel injection pump which has the added benefit of being generally available at parts stores across the country.  Another factor to consider if you have an original Austin Healey Lucas generator: fuel injection pumps will have an amperage draw of approximately 5 amps per hour. Each injector draws approximately 1 amp.  I'm using 6 injectors (some systems use only 4 injectors), so my electrical system needs to be capable of a minimum of 11 amps without discharging the battery.  So your old Lucas generator may be marginal....but then it was probably marginal before the EFI if you have driving lights or modern halogen headlights.

The fuel filter needs to be a high quality element capable of filtering 10 to 20 microns to keep the injectors clean and to allow sufficient fuel delivery.  Most fuel filters designed for EFI will provide that level of quality.  A regular fuel filter is NOT good enough to ensure a clean fuel supply for the injectors. 

The fuel rail serves a number of purposes.  It's a pressurized reservoir for delivering fuel to the injectors, it stabilizes the fuel pressure at the injectors, and the fuel passing the injectors cools the injectors.  Since the pressure can rise and fall rapidly inside the fuel rail as the injectors open and close, the supply must be stable enough to match the engine fuel requirement.  If the volume inside the fuel rail is insufficient, this fluctuating pressure can affect the amount of fuel that is injected.  Therefore, it is best to over engineer that fuel rail capacity, since if it is marginal it would be difficult to diagnose and is not easily corrected.

The fuel pressure regulator ensures that the ECU is always dealing with a consistent fuel supply.  The ECU's ability to achieve a goal air/fuel ratio is dependent on injector size and fuel delivered at a defined fuel pressure, the ECU can then alter the duration of the injector firing for predicable results.  The fuel pump does not always pump at the same pressure, and the fuel supply is also affected by temperature, so the pressure regulator is designed to maintain the fuel supply at a predetermined pressure.  In the case of the Healey's system, we set the pressure regulator at 42 psi.  We have installed the pressure regulator on the return side of the fuel system, down stream from the injectors.  Because the fuel pump delivers more fuel than the injectors can use, the excess fuel is returned to the fuel tank via a return, you need to plan on plumbing in a return line from the fuel rail (or after the pressure regulator, depending how you elect to plumb your fuel system).  The return line should enter the tank at the top and extends down into the fuel supply.  An additional consideration on the fuel delivery plumbing:  A 5/16" (or - AN4) fuel supply line can handle up to 250 HP (the stock A.H. is a 5/16" line)....which should be sufficient for most of you.  The return line needs to be at least the size of the supply line, this is dependent on fuel pump size, not a function of the engine HP, a 5/16" line will handle up to a 45 GPH fuel injection delivery system at 40-45 psi which is what I suggest for this AH system.  To maximize the system, it is best to avoid restrictive fittings such as sharp right angles and "T" fittings.  When right turns are needed it is best to use tube style fittings which smooth out the fuel flow.  I'm using AN-6 (3/8") Kevlar lines for both supply and return.  All fittings in the fuel system are AN-6 to provide the best connections and highest quality fuel flow.  I did this, not because my engine HP requires that much fuel, but because, as stated earlier, I wanted to over engineer the fuel deliver system so there was no question of fuel availability for the ECU.  I might add, it's no more difficult to do AN-6 during the build so it's an easy safeguard. 

The injectors are Bosch disc injectors.  These are electromechanical devices that spray fuel into the injector block (mounted between the intake manifold and the SU throttle bodies) under the control of the ECU.  The ECU controls the timing and duration of the injector spraying.  The spray pattern is a function of the design of the injector.  The delivery capabilities of injectors are expressed in lb/hr.  The injectors we used are based on delivery requirements and physical size restrictions for the mounting of the injectors.  I ended up using 22 lb/hr injectors that I had from my C5 Corvette, these more than met the requirements (at about a 68% duty cycle)..... 19 lb/hr injectors would have sufficed at a safe 80% duty cycle.  The requirements of this system were calculated based on horsepower expectations, duty cycle requirements, and number of cylinders.  (See the formula for determining the required injector size at the end of the Addendum.)

So you may be wondering what function the SU carburetors are performing......well not much.  They are serving as the throttle bodies upstream from the fuel injectors to control the amount of air that is available to the injectors.  The remaining inner workings of the SU's have been removed.  It is important that the throttle shafts be "as new" since leakage around the shafts would cause confusion to the ECU and the IAC.  To that end, I would strongly encourage replacing old throttle shafts when converting to EFI.  Leaving the SU's in place, and having them act as the throttle bodies, adds to the stealth factor.

The injectors are mounted on custom aluminum injector blocks that are fitted between the intake manifold and the SU's.  Since the injector blocks are 1" thick, it is necessary to install longer carburetor mounting bolts (3" worked for mine, this is somewhat dependent on the size insulator blocks you install, I used two 1/4" insulator blocks, one on each side of the injector block).  The injector blocks are also tapped for the lines to the IAC and one of the blocks is tapped for the mounting bracket for the TPS.  The injectors are mounted on the bottom side of the injector blocks....once again to add to the stealth factor.

There are also three custom aluminum fuel rail mounting brackets that attach the injector blocks to the fuel rail and stabilize it (these can be seen in the above image).  This is an important safety feature.  Some of you may have seen the pictures of Jim Hockert's beautiful green BJ8 with EFI that had an engine fire... the fire was caused by an injector connection that failed.  The attachment method you see pictured here can not fail in the way that Jim's did.  In fact, I can't imagine how this can fail as the brackets are attached with lock washers and loctite creating an extremely ridgid structure.  When Jim rebuilt his engine he discarded the injectors clips, the component that failed, and went with a more positive fuel rail connection similar to this.