EDIS-4 in an MGB

By Matt Kimmins

The system I designed takes the EDIS-4 Module used by Ford in their Mondeo and combines it with some electronics that essentially replace the EEC Box in the original vehicle.

The Box!

The module and Atmel processor electronics, are housed within an extruded aluminium box. This helps screen the system from RF noise produced by the coil, alternator etc. The inputs to the actual EDIS-4 module are very sensitive and for this reason the resulting connections to the sensor, coil pack and TPS are all done through locking metal connectors and high quality OFC screened cable. The box is mounted under the glovebox so as to be handy for connection to a laptop .

 The image below show the partially completed module in the box plus the locking connectors.

The edis module is partially dismantled and sits in the base of the box benefiting from the heatsinking effect of the aluminium case. I found the MG engine bay a very noisy environment and as such the circuitry is heavily screened.

Why Bother?

 The MGB distributor set-up can be described as a 2D ignition system if the vacuum advance is no longer used, especially as is the case on many replacement units. It derives its timing from the engine speed and not load on the engine. At full throttle this is acceptable, however on part throttle, economy and driveability are affected. In another vein with some performance engines the required advance may not alter in a linear manner, there may be places in the engines speed range where required advance can fall even though RPM is rising. With the MGB relying on weights and springs this system although workable can be seriously improved upon.

The standard distributor goes part of the way towards varying the ignition timing for load by fitting a vacuum advance device which dependant on year, advances/ retards the ignition based on vacuum. The trouble is it isn't very precise and does a rough job. Performance distributors even remove it.

A mapped system can give precisely the right ignition advance whatever the engine speed or load. This improves the tractability of the engine dramatically as well as giving far better economy.

To appreciate the difference between a 2D and a 3D mapped ignition system you have to understand a little about combustion within the engine. When a fuel and air mixture ignites within the combustion chamber, the burning of the charge starts at the sparking plug and spreads throughout the mixture from that point. It takes a given amount of time for the whole charge in the chamber to burn, expand, and hence force the piston down the bore. This is why we have to start the ignition process before the piston reaches top dead centre. This lead-time is called "ignition advance".

It follows that as engine revs rise and the engine turns faster there is less time for the charge in the chamber to burn hence the need to increase the ignition advance with increasing engine speed.

 Within the standard MGB as engine speed increases the centrifugal force acting on the weights increases and causes them to move outwards, against the resistance of a couple of springs,  in doing so advancing the ignition. The springs pull the weights back as the engine slows again reducing the advance. A series of stops and different tension springs allows the ignition advance progress to be controlled. On an MG engine in two stages.

But there is another factor effecting advance that needs to be taken into account - cylinder filling. The speed at which the mixture in the combustion chamber burns varies with the amount of compression that the charge is under. This in turn depends on how full the cylinder is before compression takes place. For example: on a small throttle opening at higher rpm, the cylinder will only partially fill, compared to wide-open throttle at the same engine speed. It follows that you need different ignition timings even though the engine speed remains the same. The timings being dependent on throttle position or engine load.

An EDIS system can control the ignition with very few moving parts; all it needs is a trigger and a load sensor. We use a throttle position sensor instead of a vacuum sensor just because the siamesed port design of the B tends to pulse and doesn’t provide a very steady vacuum signal. The EDIS knows the load on the engine from the throttle position sensor, as well as the engine RPM. Since the ignition timing is mapped for each engine speed and load, the timing is optimised for essentially all load conditions including part throttle. This gives the best possible performance and economy whatever the throttle position. In addition, unlike points which wear down over time the triggering system has no physical components. Thus the timing remains correctly set more or less indefinitely and is maintenance free. There are other spin-offs included within the system such as rev-limiting, shift light, accurate tacho etc as well as the certainty that the timing is never likely to ‘go off’.

The benefits from a mapped system have to be experienced to be appreciated, throttle response is razor sharp, economy is improved and tractability (especially with more radical cams) is amazing. In my own experience an engine converted from a centrifugal advance type of system to a mapped system undergoes a transformation.

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Copyright Matthew Kimmins 2006-2009