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Converting a mechanically injected engine to run electronic engine management.

by Julian Edgar

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This article was first published in October 1998.

Nothing gives you a better compromise of power, economy and emissions than electronic engine management. That's the system where both the ignition timing and the fuel injection are controlled by a single Electronic Control Unit (ECU). But that's all very well if you already have an engine fitted with all of this stuff, but what if your engine was born in the time of carbies and points? Easy - you just convert it to running engine management.

In this feature we'll use as an example a conversion that was carried out on a Mercedes 450SEL V8. Normally fitted with the Bosch CIS mechanical injection, Awesome Automotive in Adelaide (South Australia) successfully fitted the car with GM-Delco engine management.

The Air Intake System

EFI intakes consist of a large plenum chamber and individual runners that carry air to each intake valve. A single throttle body (butterfly) is usually positioned at the entrance to the plenum chamber. Most intake systems are made of cast alloy and feature very careful design. By far the best way of sourcing an EFI intake is to find an EFI version of your engine. Some engines were converted to EFI by their manufacturer sometime in their life, and so the intake manifold and plenum chamber can be simply bolted on. Other manufacturers have engines in the same family that are mechanically very close and are equipped with EFI.

Make sure that it any new intake is actually a bolt-on before you shell out money, though! The Australian Holden 5 litre V8, for example, had new-design heads fitted at the same time as engine management. This means that the EFI intake manifold cannot be used with the earlier engines. And if you need to change the
head(s) as well, you may as well buy the complete newer engine!

If nothing is available ex-factory, you can fabricate your own intake system. Depending on the type of engine, this can be a nightmare - or relatively simple. Points to keep in mind include making the intake runners as long as possible (this improves low-down torque) and remembering that the injectors have to fit in somewhere close to the intake valves! Mild steel or aluminium is usually used in fabrications of this sort.

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With the Mercedes 450SEL, an earlier model of the 4.5 litre V8 used EFI (not engine management, but that doesn't matter for the intake system), and this manifold and plenum chamber are a bolt-on fit to the 4.5 litre V8.

Throttle Body

The throttle body contains the butterfly valve that regulates how much air gets into the engine as you plant your right foot. On that basis, any decent sized throttle body can be used. However, the throttle body usually also contains a throttle position sensor, and an air bypass for idle speed control. This means that you should use a throttle body that has compatibility of these components with the new engine management computer that you are going to use. Most aftermarket systems have "preferred" throttle position sensors and idle air motors and so you should take note of this before selecting the throttle body.

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In the case of the Mercedes, a Holden 5 litre V8 throttle body was used. The throttle position sensor and idle speed control were then a direct match for the GM-Delco electronic control unit (ECU) being fitted. Some mechanical revisions were needed of the throttle linkage and the plenum mounting plate to take the new throttle body.

Temperature Sensors

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The new engine management system will require inputs from coolant and intake air temp sensors. Neither of these generally cause much problem in a conversion. Even if the new intake manifold already has an intake air temp sensor, it is unlikely that it will be able to be used with the new system. Instead it usually needs to be removed and replaced with a sensor suitable for the new ECU. This generally involves welding up the hole, drilling and re-tapping. The same applies for the coolant temp sensor, which is normally located on the thermostat housing.

On the Merc, the GM-Delco coolant temp sensor went neatly into the front of the alloy inlet system that contains a coolant gallery. The air temp sensor is on the back of the plenum chamber, hidden from view.

Load Sensor

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By far the majority of programmable management systems use a Manifold Absolute Pressure (MAP) sensor to detect manifold vacuum (or vacuum and boost). Together with engine rpm and intake air temp, the ECU can use this signal to work out engine load. The MAP sensor is normally attached to the firewall, with a small-bore rubber hose connecting it to the plenum chamber anywhere after the throttle body. Connecting this sensor was a 5 minute job on the Merc.

Crankshaft Position Sensor

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A variety of crankshaft position sensors are used in factory engine managed cars. Types include optical, inductive and Hall Effect. The latter is the most common in cars using programmable management.

Generally it is required that the dizzy be gutted of its ignition timing components. These are replaced with a single Hall Effect or optical sensor, which is triggered by the passing of a metal vane. In the Mercedes, there was plenty of room to fit a Hall Effect sensor inside the body. This was triggered using a GM-Delco Australian Holden 5 litre V8 chopper disc.

The complete assembly can be seen below.

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The simplest technique to adopt with ignition is to retain the original dizzy as the means of distributing the high voltage spark. You can if you like use direct fire ignition (for example, one coil per plug), but most older engines don't warrant this level of sophistication.

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This means that the original coil (or a higher voltage replacement) is triggered by an ignition module that is in turn controlled by the ECU. The ignition module must be compatible with the new ECU, and again the manufacturers of programmable systems list which ignition modules are the right ones to use.

In the case of the Merc, the Holden 5 litre V8 Bosch dedicated ignition module was used.

Oxygen Sensor

If the new engine management uses a closed loop feedback signal (like all modern cars do), you will need to fit an oxygen sensor in the exhaust. Many aftermarket systems don't use such a sensor, but this means that emissions are often illegal and fuel economy not as good as it could be.

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The oxy sensor should be mounted as close to the engine as possible, but at the same time having the gas from as many cylinders as possible passing it. That means that a location at the end of the exhaust manifold (or where header pipes come together) is usually the spot. If the oxy sensor is then going to end up more than 300mm or so from the exhaust valves, a heated type should be used.

Fuel System

An EFI car's fuel delivery system differs from a carby fuel system in basically two ways - both flow and pressure are higher. This means that a high pressure EFI fuel pump needs to be fitted.

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The feed to the pump must be organised so that it will never starve, even during cornering or acceleration. This requires that either the tank is well baffled (so that fuel can't slosh away from the pump pick-up), or an external or internal swirl pot needs to be added.

An external swirl pot is a small container continuously filled by a low pressure pump drawing from the main tank. The high pressure EFI pump then draws from the swirl pot. An internal swirl pot requires that the tank be cut open (done only by experts after thorough cleaning!) and a small open cylinder welded around the tank pick-up.

A return fuel line back to the tank may also need to be fitted, but in many cases carby cars already have this facility.

In the case of the Mercedes, the standard CIS Bosch mechanical fuel injection already runs a suitable high pressure electric pump drawing from a well-baffled tank. This meant that no changes were needed to the fuel delivery system.

Mounting the ECU

The ECU needs to be mounted inside the cabin to keep it away from dust, heat and water. A number of locations can be used, depending on the size of the ECU and the room available in the car. Under the dash, inside the passenger side kick-panel or under the seat are common locations.

Harder is to get the wiring loom through the firewall to connect the engine bay with the cabin. Drilling a hole large enough to fit the engine connectors through and then feeding the loom through from the firewall side is the best technique. However, when there are sandwich firewalls, sound deadening, all-curved firewalls and heaps of small fittings in the way - it's often easier said than done.

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Make sure that the hole is adequately sealed after the loom has been passed through, otherwise noise and other nasties will be transmitted into the cabin. Using a rubber grommet with a long "tail" that extends along the loom is one approach that works well.

With the Mercedes, a 2-inch hole saw was used to make the opening, with silicone rubber sealing it up. The ECU was mounted behind the passenger side kick panel.


You could be forgiven for thinking that when all of the mechanical and electronic bits are installed - that's it! However, unless you have a pre-programmed computer already set up for your engine, the car will need to spend a good few hours on the dyno.

During this time the software that controls the fuel, ignition, idle speed and other factors must be programmed. This means that the car needs to be run at all the different loads and speeds that it will encounter on the road. At each point the correct injector opening time and ignition advance will be programmed into the ECU, creating "maps" of the information.

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Make sure that you budget for the programming, and remember that - irrespective of how good the hardware is - if the programming is poor, the car will drive like a dog.

The Mercedes took a few days of (not continuous) work to program the ECU. At the end of this process a car drove out of the workshop that had a power gain of 17 per cent! This is partly the result of CIS mechanical injection airflow vane no longer obstructing the intake air path, and partly because the management system could be fine-tuned so well. Economy also improved!

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