An Inexpensive and Practical way to Retrofit EFI


Legal Disclaimer: The following is my opinion for which I hold no liability. Use this information at your own risk. It involves using pressurized flammable material which can cause injury to property, person, or death. You read this at your own risk and agree to not sue me for any information provided here or its influence on you.


Outline

Foreword
Personal Background
Introduction
General Problem and Challenges of Retrofitting
Fuel Injection System
Hydraulics
Electronics
The Simple Approach and Rationale
Mounting the Injectors
Adding Sensors and Computer
Adding Fuel Pump, Regulators, etc.
Practical Details
Mounting Fuel injectors
Type of Holders
Ring
Wire
Bowl
Flat
Modifying the Injectors
Locating and mounting Sensors
Locating and Mounting Computer
Making and installing Wiring Harness
Characterizing Performance


Foreword
I have converted a few cars to fuel injection by making my own injector holders to place in the carburetor bodies, and using a suite of sensors and a Megasquirt ecu to do the electronic engine control. I have tuned the cars using Megatune software, which is free. There are quite a few details to keep in mind, but the overall process seems straightforward to me.
So far, I have concentrated in finding an economic and efficient path to retrofitting fuel injection to cars fitted with 1.5 inch and 1.75 inch carbs (SU's and Zenith Strombergs), and also one a few 2 inch SU's. To date I have made injector holders to fit for all of these, and I have tested the various 1.5 and 1,75 setups on my cars. In general, I have been surprised by how straightforward the process is, and by how relatively inexpensive it is. The cars run very well-better than with a carb or carbs.







Personal Background

This project began when I added fuel injection (EFI) to my son’s GT6 spitfire. I found a simple and inexpensive way to retrofit EFI for his car and then generalized the approach so I could convert my 5 MGB’s and my 6 XKE’s to EFI. The carb adapters available were pricey and the recommended computer by Patton was also very expensive. I decided to learn about the area and do it myself.

Introduction

Most of the common European sports cars from the 50’s through the 70’s suffer from unreliable and sub-optimum performance because of problems inherent in their fuel/air intake systems. Until very recently, the only reasonable route to address these problems has been to buy aftermarket carburetors and intake manifolds. The cost of these has been about $500-$1500, and they have been fairly popular over the years. The recent advent of reasonable priced EFI components has now made possible the retrofitting of EFI to these cars. EFI holds the promise of significant improvements in reliability, power, gas mileage and emissions reduction at a reasonable cost. I chose to retrofit efi to cars in my family (Triumphs, MG's and Jags).

General Problem and Challenges Retrofitting EFI

Retrofitting any new system (like EFI) to an established design requires capturing the essence and advantages of the new device while minimizing cost and the collateral changes to the whole unit (car). Important ways to do this include using parts which are already in common use in the cars (sometimes for other applications), and making reasonable changes, and as few changes as possible to the original unit. In other words, change as little as possible, but still capture the advantages and functionality of EFI for these cars. The more that’s changed, the more is the expense and complications to the conversion.

Fuel Injection System -hydraulics and electricals

Hydraulics
A fuel injection system uses a fuel pump, pressure regulator and fuel injectors to spray fuel into the air that goes into the engine.(Figure 1). The fuel pump is located in the tank or in the fuel line, and the pressure regulator is usually located downstream from the injectors. The excess fuel from the pressure regulator is returned to the inlet of the pump or to the tank.

Electronics
The EFI system is essentially a pressurized gasoline spray system. It uses a computer and sensors to spray the amount of fuel needed based on the operating conditions of the car. The pump and regulator keep they system at a constant pressure (usually ~40 ps1). The injectors are electrically operated spray nozzles that are controlled by the computer (ECU) and provide pulses of fuel as appropriate.
Before each combustion cycle (i.e. each time the spark plug fires), the computer calculates amount of fuel needed and then opens the injector(s) long enough for that amount of fuel to be sprayed into the intake air. This mixture is drawn into the motor and burned. Sensors measure the conditions (manifold vacuum, water temperature, amount of residual oxygen in exhaust, etc.) and the computer uses this information to make corrections in its calculations for each combustion cycle as operating conditions change.

Figure 2. Electrical Schematic of Typical EFI System
(See Megasquirt.info for more details)




Practical Considerations and choices


There are a number of considerations:

Item My choice + rationale

Injector choice Port style injectors
Much better than TBI style
Tried TBI's. Not good
spray pattern. Poor seals.

Port style injectors have
finer spray, more versatile,
modern and less expensive.

Injector holders "Top Down" work great
"Bottom Up" look prettier,
work as well, but more work

Fuel system pump(s) Recycle without return to
tank. It works and is much
simpler.Variable regulator
adds flexibility.

Computer Megasquirt. Works well and is
well supported. Easy to tune

Fuel Rail Made connections from
plumbing fittings. Cheap



Retrofitting EFI- The Simple Approach

Rationale: The carburetor body normally holds a spray system (needle and jet) that sends a spray of fuel into the air intake of the motor. The carburetor meters the amount of air going into the motor based on the position of the throttle plate . Changing the carburetor over to fuel injection just involves fitting the carburetor with a different, much better spray system. This is done by:

(1 )Mounting the EFI injectors into the carburetor body and
(2) Adding a computer plus sensors to control the injectors
(3) Adding a suitable fuel pump, regulator and tubing

Mounting the Injectors into the Carburetor Body

The most direct and simple way to mount the injectors into the carburetor body is to add an injector mount atop the main body of the carburetor so that the injectors spray the gasoline into the air stream going into the intake manifold. I made several types of mounts from flat sheet metal using simple tools (drill, shears, hammer, screwdriver, etc.). My favorite right now is the flat 2-piece style. I have used it in 5 different carburetors (1.5, 1.75 and 2 inch SU’s, and also 1.5 and 1.75 inch Zenith Strombergs). I secured the injectors and the fuel connectors to the holders using brake safety wire. Figure 8 and 9 can be used as patterns by simply printing the figure to scale, adhering the print to the piece of metal, then drilling 2 of the 4 holes and then cutting out the profile. Next I bent the piece to fit into place in the carb body, and then used the "dome as a drill jig to place the last 2 holes into the piece.


I have also made flat style holders that holds the injectors underneath the carburetor body, so it looks like a stock carburetor setup from above. These are shown elswhere on this site.

Choice of injectors:

I chose to use port style injectors because they spray very well and are easy to mount. These injectors are a distinct improvement over earlier “TBI” injector designs. (‘TBI’ (throttle body) injectors were made popular by GM as GM’s first way to convert their carburetors to fuel injection. They are relatively large, have poor spray patterns, take a lot of current to operate, and need to be cooled. They usually have weak seals and are therefore operated at lower pressures.)

'Fuel Rail' I made my own connections to the injectors from plumbing fittings. I used 3/8 NPT fittings, drilled them out to 1/2 inch, and then out to 17/32 inch. I found that, if I slightly countersunk the hole, I could get the injector o-rings started into the hole, and I could then twist and press them into the fitting. Using this approach I got sealing to over 50 psi, and never have had an injector and fitting try to separate.

I dicovered that it is easy to remove the o-ring and retainer at the 'nozzle end" of the injector. This makes it possible to better fit injectors in smaller spaces


Adding Sensors and Computer
Manifold vacuum, water temperature, air temperature, oxygen and throttle position sensors are easily added to the car, but some attention to detail is required for best results.

(1) A dedicated port in the intake manifold is needed to sense the vacuum properly. (I found that using the vacuum ports on the carburetor body does not work well.). This is usually done by installing a barbed hose fitting into an existing hole in the manifold or adding one a hole (drill and tap) and fitting.
(2) Water Temperature is measured using a GM sensor. They are mounted in a T in the hot water circulation hose.
(3) The air temperature sensor is a standard GM air temp sensor mounted next to the air filter or in the air filter housing.
(4) The Throttle position sensor (TPS) is mounted onto the end of the carburetor shaft. A flattened bolt provides the correct shape for the TPS to fit properly.
(5) The computer (Megasquirt) and relays are mounted inside the passenger compartment. I make my own wiring harness (silly me), but a very good one is available for about $50. I recommend buying an assembled, ready-to-use Megasquirt ECU, rather than a kit, and the wiring harness (see DIYautotune.com).

Mounting the Injectors

The injectors can be mounted into many different carburetors in 4 main ways: Ring style holders, Bowl style mounts, Flat style mounts, and safety wire holders. The first three are made from copper; the fourth is made from Stainless safety wire. The ring holders, made from some small pieces of 1/2" copper copper tubing and flat stock, capture both ends of the injector. The bowl style holders are made from two pieces of copper which are fashioned to fit inside the top ‘bowl’ region of the carb and hold the injectors. The flat style holders consist of two pieces of flat copper which are formed to fit into the top if the carburetor and hold the injectors firmly in place. This last design is now preferred because it is easiest to make and easiest to use with many carburetors. The flat style holders can also be mounted on the underside of the carburetor so the system still appears ‘stock’ when viewed from above. This last one requires more work to implement(1-2 hrs per carb), but it looks neat. This is shown in figure 6. Notice the injectors are mounted below the main carb body air path.

Figure 4. Two similar Ring Style Holders and Zenith Stromberg Carburetors

A rather inelegant approach using safety wire to hold the injectors seems to work pretty well. The wires hold the injectors so they spray in the proper place. A second set of wires (not shown here) secures the upper part of the injectors. This is shown on a 1.75 inch Datsun Carb of the ‘70’s. The ‘top hat’ of the carburetor can be cut out in the back so the carbsuretor retain the ‘original carburetor’ look and hide the safety wires and injector bodies.

I presently prefer the flat style mounts over the safety wire approach because they close up the air hole in the carburetor top nicely, hold the injectors firmly in place well, and are simple to make for most any carburetor and port style injector. They consist of two pieces: (1)a lower piece that holds the injectors in place, aiming the spray at the center of the throttle plate, and (2)an upper piece which closes the air opening and also provides secondary support for the injectors. There are several holes in the lower piece through which securing wire can be run to secure the injectors in place. Some of these are shown in the figures below.

Figure 6 Flat style injector holders

Another example is the view from the top of a 1.75 SU carburetor, with the two pieces and injectors installed.

The flat style holders can be added to SU’s or Zenith Stromberg carburetors atop the main body of the carburetor where the ‘dome’ usually is located.


Figure 7. Shows two different kinds of carbs converted to EFI devices using the injectors mounted below the air pathway. This view is usually hidden by the air filter, which is not in place in this photo.



Comparison of holders.

The best ring style mounts require modifying the carburetor body. (I used this style mount on the GT6 two years ago.). It was an early design. The bowl style design does not require modifying the carburetor body and works about as well.

Slightly modifying the injectors makes the installation neater. Using modified plumbing fittings saves the expense of a fuel rail. This is shown in the picture below.

Figure X Fuel Injectors and Fittings


Locating and Mounting The Sensors

The GM water temperature sensor is mounted in the fill port atop the motor on the MGB. Inthis arrangement, the sensor is immediately and correctly responsive to engine temperature. Note that the MG temperature sensor is left in its original place to report temperature to the meter in the dash. On the GT6, a barbed plumbing ‘T’ fitting was added to the hot water line so a GM water temperature sensor could be mounted in one side of the ‘T’. This arrangement is not ideal, but it was convenient at the time. It works fairly well but reports lower temperatures and lags behind the true water temperature. This was compensated for with the tuning parameters associated with water temperature.


Figure x Water Temperature Sensor



Oxygen sensors were installed in the exhaust manifolds of the cars. This was accomplished by brazing oxygen sensor bungs into the manifold so the sensors could be screwed into them.

The TPS is mounted onto the carburetor body with two 10-32 screws and a couple of small flat plates. A ‘through drilled’ 3/8 inch bolt with its sides filed flat serves to couple the shaft to the TPS itself.

Figure X TPS mounting

Locating and Mounting the Computer

The computer and relays were mounted inside the passenger compartment. In all cases, care was taken to make them accessible to a laptop connection and yet out of harm’s way during routine car operation. In the MGB, the computer and relays were mounted in the glove box; in the Spitfire the computer was mounted above and alongside the passenger foot-well region. The recommendations from the Megasquirt.info site is to mount the computer in the passenger compartment to minimize harsh temperatures and changes in temperature. Care should also be taken to keep the unit dry. It is therefore mounted up off the floor and away from any source of water. The units should be mounted so that there is a simple route for the wiring to go to the engine compartment, to a good ground place, and with access to the wires associated with the ignition switch.



Wiring
The simplest way to prepare a wiring harness is to first mount the computer and then route the wires out from the D37 connector on the ECU, leaving enough slack so that it is easily unplugged, etc. The harness from DIYautotune has every wire marked, so it is just a matter of finding a neat and tidy way to route the various wires. The wires go as a bundled unit out through the firewall in a grommeted hole, and then separate into smaller groups as they go to the various places: On the drivers’ side they go to the sensors and injectors. On the passenger’s side they go to the ignition sparking system. The wires are bundled together using an aftermarket wire covering material similar to that used by GM.
I use a Crane XR700 and send the Crane sensor signal to as the “tach” signal to the Megasquirt.
Below is a copy of the schematic that was used on the MGB and the Triumph 6. (Not shown is the inertia switch in the power line to the fuel pump).

Figure X. Wiring Schematic for EFI retrofit

Work is now undertaken to compare a performance impact of a number of different inlet systems

Tuning the Car
Once the whole EFI system is installed in the car, there may be adjustments to make in the relative richness of the air fuel mixture under various operating conditions (e.g. starting cold, starting hot, accelerating, etc.). This can be done by uploading a file that has already been developed for that model car or by doing some simple tuning and driving runs.
I have done both using a laptop and its USB port connected to the serial port on the Megasquirt. I used the free tuning software called ‘Megatune.’ Adjustments can be made while running the car and then ‘burned into’ the computer so it remembers these parameters and runs the car accordingly. Alternatively, some automatic tuning software can be used. This is thoroughly discussed at: Megasquirt.info.com. Below is one of the displays on Megatune (see photograph X). Detailed, easy-to-read and easy–to-follow instructions are available free at Megasquirt.info.

Figure X Megatune Main Screen

Characterizing Performance- Horsepower and Torque

Before starting, it’s informative to characterize the performance of the car. Later, there is always a temptation to make tuning adjustments and see just how good it can get. Measuring torque and horsepower can be done by using with a dynomometer or by doing a few simple road tests.
The acceleration rate, unpowered deceleration rate, and mass of the car can be used to calculate the torque and horsepower of the car. It is especially easy to run the road test using the Megasquirt data logging function and a spreadsheet like Excel®.

Legal Disclaimer: The following is my opinion for which I hold no liability. Use this information at your own risk. It involves using pressurized flammable material which can cause injury to property, person, or death. You read this at your own risk and agree to not sue me for any information provided here or its influence on you.

I built a GT6 spitfire for my son and converted the twin carb 2 liter motor to fuel injection. It was my very first project in that area. The fuel injection conversion process had 4 main parts:

adapting the carbs to hold fuel injectors
adding a high pressure fuel system with regulated pressure
setting it up for computer control
Tuning it to run

Part 1 Adapting the carbs


The carbs can be adapted for EFI by adding holders for the injectors, and retaining the main carb structure to act as a TBI. The first holders add holders from the top of the carb, and are quick, easy and reversible. The second type of adaptation puts the holders underneath the carb body-and essentially HIDDEN. This second type of conversion does require modifying the carb bodies. It is shown in some detail on page 3 and 4 of this journal.


HOLDERS WITH INJECTORS ON TOP:

(On these first holders I cut out a small part of the back center part of the carb to give the best possible angle for the injectors. I later found that cutting out the back slot in the carb bodies was not necessary)

The first adapters to hold the fuel injectors in place were made out of copper sheet and copper tubing. I used household scissors, a propane torch, a bench vice, a hacksaw and an electric drill. In the first step, I made a pattern out of masking tape which conforms to the inside of the top part of the carb. I used it to cut out the copper liner piece as shown in the picture below. A later design which works as well is given on page 2 or 3 of the journals, and it doesnt even require soldering.

Figure 1

I next pressed it it into the top of the carb so it would conform to that shape.

Figure 2

Once this was done, I soldered the ends of the coper piece together so the piece was fairly rigid. I did this in while the piece was sitting on a vice. I used a small propane (bernzomatic) torch. I could have done it in place in the carb body, being careful not to over-heat melt the piece. (I did this later and had problems.)

Figure 3
I then soldered a couple of small pieces of 1/2" copper tubing in place to hold the injectors so they would squirt directly at the back of the throttle plate.
For the gt6 carbs I cut out a section of the top cap mounting surface, leaving the screws in place for later mounting. In retrospect, I found this was not necessary.
I drilled the copper insert so the holes which which are used to hold the 'dome' on could be used to hold the adapter plate in place.


I recently have made some adapters like this which do not require cutting the back of the carb and they seem to work just as well. I had originally cut out the back so the injectors would have perfect alignment with the center of the throttle plate. This level of perfection is not required, I have now found. The new pattern is very similar to the old one but there is just two cuts on the sides rather than a cutout to clear the carb ears.
The newest 'from the top' does not require modifyong the carb body at all and it uses a simple injector holder. See the next pages of the journal for a drawing and details.

ADAPTATION WITH THE INJECTORS HELD UNDERNEATH THE CARB

This approach requires removing the float setup frtom the carb body and then making a passageway for the injectors. It can be done fairly neatly using a drill and file. It can be done very neatly using a die grindeer or a router.