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Digital Ignition with MegaSquirt

Introduction

These notes concern the use of a digital engine management system on the Murena to control ignition. At the time the Murena was built, ignition control was mechanical. Murena 2.2 is equipped with an electronic transistorised ignition system controlled by a reluctor pickup in the distributor, and this offers greater reliability and better dwell angle control than the traditional contact breaker system. Upgrading to a digital ignition system will improve control of ignition advance (particularly load based advance) and dwell, and has tuning options far better than the mechanical system.

While the next step of converting to full digital fuel injection seems attractive, experience shows that a well tuned carburettor can give results that are almost as good. The advantage of taking the next step is not that big. Quoting Dave Walkers book (see reference below): "Ignition control via an ECU is probably more beneficial to the engine than fuelling" and then he continues explaining why (pp 66f).

Furter more, while ignition is electrical only, it is easier and safer to implement: It seems a good start even if you have already decided to go the full way to digital fuel injection too.

A few digital ignition systems are available commercially, including the popular ignition controller from Weber Alpha. But the price for a setup with a Weber Alpha 3D controller is in the area of 700 EUR. An alternative, the Emerald M3D is somewhat cheaper at around 600 EUR.

With several other projects on my car and therefore always on the lookout for ways to save money without compromising performance and quality, I am therefore studying the option of using the DIY and open engine management system MegaSquirt, considering the use of it as a dedicated ignition controller. The objective with these notes is to evaluate and conclude on this.

MegaSquirt background

MegaSquirt was originally a DIY fuel injection only system, designed to convert carburettor cars to fuel injection or to convert cars that were already fuel injected to an open system that allows tuning and working with the parameters. Latest version of the system offers fairly advanced ignition management, however.

MegaSquirt is based on a Motorola 8 bit embedded CPU and the software is written in assembler. It is realised on a relatively small board with leaded components. It is designed to be simple to build yourself.

Assembled MegaSquirt v. 3 board
Assembled MegaSquirt v. 3 board

While the original project did not allow for ignition managament (but was based on the assumption that the original mechanical ignition system could be kept unchanged), the current version 3 of the board does suppoert ignition management in hardware. To facilitate this, it has among other components, a high current FET driver to drive an ignition coil.

Similarly, the basic software version of MegaSquirt does not support ignition, but a supported and semi-official version called MSnS-E does. This is basically an enhanced version of the original software, still implemented in assembler and on the original CPU, but with several added features, including 12x12 maps and ignition control.

By running MSnS-E code on a MegaSquirt V3 board, it will be possible to configure MegaSquirt to do only ignition management, leaving fuel injection control unused or spared for later projects.

System design

The system design is based on a standard MegaSquirt setup, except modified with fuel control left out. Software is the standard MSnS-E code.


The diagram is based on this one

The ignition controller drives the ignition coil directly, which in turn is connected to the distributor and through that to the spark plugs. The distributor is modified with the spark advance mechanisms removed, thus fixing the trigger points on the VR pickup and leaving advance and dwell controlled by the igition controller.

The trigger input to MegaSquirt is taken from the VR pickup. A better solution would be to fit a toothed crank wheel which will give more stable control of advance and dwell, but this is not strictly necessary for the software to run, and mechanical system implementation is greatly simplified by relying on the basic VR pickup which fires only twice per crank revolution.

A throttle potentiometer is fitted on the carburettor(s) giving information about engine load. The MAP sensor in the MegaSquirt controller is fitted too and can be configured to be used to measure load with, but is expected to not give reliable results on high overlap cams (e.g. Holbay cams) and/or short S-manifolds.

Holbay Fast Road cam for the Murena 2.2 engine
Holbay Fast Road cam for the Murena 2.2 engine

A water temperature sensor is fitted to allow the ignition controller to compensate for slow combustion with a cold engine by increasing ignition advance.

Finally, an air temperature sensor is fitted to allow for timing retard when the engine is breathing warmer air.

A pinking sensor is not fitted as this is not part of the MegaSquirt basic hardware. Designs do exist for this to be incorporated and the MSnS-E code does contain support for pinking sensor retard, so this can be implemented at a later stage.

Fuel pump control is optional, but is useful when running an electrical fuel pump.

Not shown in the diagram is the pneumatic hose connection directly into the MegaSquirt box. This should be connected to the vacuum retard tube on the carburettor.

Software configuration and tuning

The MegaSquirt software to be used for this project is the MSnS-E software which features:
  • 12x12 spark tables
  • Alpha-N and Hybrid Alpha-N load compensation
  • Coolant temperature timing advance
  • Inlet temperature timing retard
  • Soft and hard rev limit
  • Distributor and wasted spark ignition coil systems
The Murena S-distributor ignition chart is shown below:

Murena 2.2 ignition advance chart
(Source: Zuendung 17.11.2001)

This can be loaded directly into the spark tables.

Murena 2.2S vacuum advance
(Source: Zuendung 17.11.2001)

The distributor must be bolted down with advance mechanisms removed, and the distributor set for 10 degrees fixed advance. This value is programmed into MegaSquirt during configuration as the syncronisation point for the ignition timing.

Installation considerations and economy

An assembled MegaSquirt unit with the MSnS-E software preloaded and appropriate wiring loom can be supplied by www.extraefi.co.uk for GBP 260.00. The unit would ideally be fitted in the passenger compartment, but this is impractical on the Murena where wires would have had to be run under the car to the front. Running the loom through the firewall would not be acceptible. A suitable place is on the right side of the engine room, above the vacuum reservoir, where the unit could be bolted up under the "ceiling" of the engine room. This would keep it out of the way of most temperature variations, but unfortunately not out of moisture. A sealed enclosure is therefore necessary instead of the standard box most suppliers seem to offer.

Coolant temperature sensor can be fitted in the thermostat housing below the temperature sensor for the dash board. There is a 10 mm threaded drain plug fitted there, where an Infineon KTY 19 6 M sensor can be fitted. See part no 186465 on www.conrad.biz.

Air temperature sensor should be fitted in the inlet before the carburettor.

The distributor will need to have the advance mechanisms removed. It will also have to be set for fixed timing advance of 10 degrees when run directly on the ignition module. The original ignition system can be kept in place and can work as a backup system in case the MegaSquirt controller should fail.

It will not be necessary to fit a new VR or hall effect pickup.

Conclusion

While digital engine management of all kinds has many advantages, it also introduces a level of complexity that one must understand and master.

Mechanically, the implementation proposed here is as simple as it gets. The changes needed on the engine and carburettor only amounts to fitting a throttle potentiometer, fitting air and water temperature sensors, and locking the advance mechanism of the distributor. The temperature sensors could even be left out for a first installation. This compares favorably to some other engine management systems which additionally needs a toothed wheel and new crank sensor to be fitted. Fitting that would only be possible by removing the engine from the car, whereas the MegaSquirt and distributor based ignition system can actually be installed with the engine still in the car.

To learn about ignition systems, I purchased Des Hammil's book about distributor-type ignition systems (see below). This book does not concern digital systems, only the mechanically controlled ignition advance, but does on the other hand discuss various tuning aspects of ignition management, which apply equally well to the digital system as the mechanical one. I can therefore recommend the book to anyone considering ignition tuning, be it mechanical or digitally controlled.

But the book gave me an insight in the tuning possibilities with the mechanical system, and also practial instructions on how to actually carry out this kind of tuning. The digital system will offer simpler tuning once installed, but as both will practially require a rolling road and an experienced expert to finalise the tuning, the advantage is voided. And the mechanical system certainly has some advantages over the digital one: It's simple, reliable and completely standard. Whereas the digital one is home made, has unknown reliability, and is non-standard. The mechanical system is also cheapea,r and tuning and replacement parts (e.g. advance springs) for the Bosch distributors are available from almost every car breaker at virtually zero cost.

So to cut the long story short, my conclusion so far is that this is an interesting project, but not interesting enough compared to tuning of the original distributor based ignition system. I have therefore shelved the idea.

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Page updated: 2007-07-12 22:50:46