Megajolt Ignition – Why you should fit one

By Dave Robson

There are a lot of car racing today still using carburetors and distributors. The more modern formulae have fuel injection and Electronic Control Units (ECUs) to control the fuel and ignition needs of their engines. The choice is sometimes made for you by the regulations that govern your particular racing formula and sometimes it is a personal preference. Some of us like carburetors and distributors because they have ‘grown up’ with them and can understand the mechanics of their operation. Others like to tinker with electronics and software and relish the precision that digital control offers.

I race in 750 Formula which stipulates the use of a single choke carburetor and ignition with only one trigger mechanism. In an effort to get the best out of the ignition system, many of us have used electronic ignition systems such as the Lumination system for a while. In fact the standard FIAT Punto system that comes with the engine is triggered magnetically and amplified electronically. At least these systems have a strong spark and eliminate contact breakers with their unreliable condensers, wear problems and variable timing.  So why consider anything different?

What’s wrong with a distributor?

The down side of the ‘standard’ electronic ignition system on a distributor is their dependence on mechanical devices to manage the ignition timing. In the Reliant days the distributor was driven through a skew gear on the cam. Current overhead cam set ups tend to have the distributor driven off the end of the cam which at least eliminates the slop in the skew gear. Unfortunately the ignition advance is inevitably controlled by a mechanism driven by weights that push against springs to create ignition advance as engine speed increases. Some systems also have vacuum advance mechanisms driven by inlet manifold vacuum. These mechanisms not only have the problems associated with setting up the timing accurately, maintaining the timing with wear but also do not allow adjustment of timing between tick-over and maximum revs without a great deal of very detailed effort. Even then, it’s virtually impossible to generate the advance curve that you want because you are dependent upon the availability of springs of the correct rate and length. In addition, the spark passes from the coil across the air gap at the rotor arm before it reaches the spark plug. There must be a better way!

Why should I bother?

All aspects of tuning an engine are based on achieving the optimum in each system for each operating condition of the engine. The ignition is one of the most important systems in this respect. We want to get a strong consistent spark at the right time in the combustion cycle at all engine revs and load conditions. Distributors get us part the way there, but can it be done better and is it worth the effort?

Electronics to the rescue!

What we need is some way of triggering a spark in an accurate and reliable way such that the ignition timing will change as required in response to changes in engine parameters such as revs and load. Now ECUs do this very well and they take notice of air temperature, atmospheric pressure and engine temperature as well. The trouble is that the ones on road cars are not easily adjustable and the race bred ones are often very expensive and are too complicated. They are often configured to control injection and turbos too. They take a lot of experience, equipment and knowledge to set up properly. If you don’t set them up properly then you are wasting your time and money.

Electronic systems rely on sensors to feed information into them. They undertake calculations using these inputs and the software in their brain to decide when to instruct the ignition system to generate a spark. As a minimum they need to know the engine revs and the position of the piston relative to top dead centre. Other sensed data provides refinement to this information. The common way to generate this information is to use a toothed wheel mounted on the crankshaft. The teeth are counted by a proximity sensor that allows the engine revs to be determined. The position of top dead centre is identified by a missing tooth in the wheel. The clever electronics can work out that this gap is bigger than the rest and synchronise the timing relative to this position. Since the toothed wheel is rigidly mounted to the rotating parts of the engine there is no slop or backlash in the system and timing precision is maintained. Crank triggered systems inevitably use a ‘wasted spark’ ignition system in which the spark plug fires every revolution as opposed to every other revolution. They use twin coil packs that generate strong sparks even at high revs since each coil fires only half as often as a standard coil. These features result in rock solid ignition timing and a strong spark throughout the rev range.

The advance curve is held electronically within the software and is capable of being easily changed by re-programming.

Which System?

So now we know that a programmable ignition is the way to go, but which system? There are a number of options on the market including simplified versions of full blown ECUs. The ones that are commonly used are Megajolt/E, Megasquirt and Polevolt. I have gone down the Megajolt/E route for cost reasons and have not regretted it. I’m not qualified to comment on the others but I understand that they operate in a similar way. I’ll describe the Megajolt/E system based on my experiences.

The Megajolt/E system comes out of America and was offered in kit form and as assembled units. It is a programmable controller that uses the ignition system from Ford cars to create the spark. Thus you can get a box of electronics for a reasonable price and the rest from a scrap Escort or Mondeo for next to nothing. The set up is as in the diagram. I bought my kit for about £50 and soldered it up myself, but the latest units are ‘surface mounting’ technology which is not suitable for DIY so the built-up price is just under £100; still very competitive. The ignition system is to be found in Mondeos and CVH Escorts of 1990’s vintage. They provide all that you need from the EDIS module (wing bulkhead), position sensor (back plate, pointing at flywheel), twin coils (end of cam box) and plug leads, wire connectors and wires.

The final piece in the puzzle is the toothed wheel. This needs to be configured to suit your particular engine. It has to have 35 teeth and a space where the 36th tooth was. I had mine plasma cut from 8mm thick steel plate (not aluminum or stainless) but I believe that they are available form the Internet. Here is my wheel installed on the crank of the FIAT engine.

Installation within the car will take some wiring skills and mechanical fixing of toothed wheel and sensor. Full details are available from the excellent Autosport Labs web site (www.autosportlabs.net).
I strongly recommend protecting the controller against heat, vibration and moisture as with all electronics since the box is not itself sealed or protected.

Now it’s in the car, what do I do with it?

You’ve got to program it of course, and you do this with a laptop. All the software for the laptop is a free download from the web site. You’ll find all the information you need there and an excellent forum where lots of problems have been discussed and solved. I found the ‘Megajolt community’ very helpful and supportive. There are a lot of people out there who have had the same problems and concerns associated with fitting and programming the Megakolt/E so you don’t have to feel alone if you are not confident tinkering with electronics or software.

The main feature of the software is the ignition map in which the ignition advance values are detailed. It is shown as a spreadsheet with load against revs as below.

Load is a term to describe either inlet manifold vacuum or throttle position. I’m not allowed to use this feature in the 750 car since the rules ban more than one ‘input’ into the ignition. However, I have the system on my Frogeye Sprite and use throttle position to greatly improve its drivability on part throttle. Perhaps this feature is less important on a race car where full throttle is used more often, but I’ll let you ponder that one. You must select either manifold or throttle input for the load, not both.

Values of ignition advance have to be determined on a case by case basis. A good starting point is to mimic the distributor values. The Autosport labs web site has a library of maps that can be studied. Ultimately the values should be honed on dyno or rolling road. Luckily, the software allows quick and easy adjustment of a live system. In addition, two maps can be stored and there is provision to switch between them when running. This might be useful if you have wet and dry settings for example. Unfortunately 750 formula rules do not allow me this facility in a race.

Other features of the system include provision for shift lights and for a rev limiter. There is an output to drive a rev counter or logger. Take care to select a rev counter that is compatible with the output signal.

Is it all worth it?

Well I think so! I can’t think of any disadvantages if you are prepared to put in the effort to install and program the device. I can list a few advantages.

• Very competitive price even if compared to a replacement distributor.
• Precise advance curve that does not have limitations.
• Quick and easy adjustment to advance curve when tuning the engine.
• Simple change between two ignition maps using remote switch.
• Output to drive shift lights that are fully programmable.
• Soft and hard rev limiter function.
• Output to drive rev counter or for logger input.
• Limited logging capability.
• Use of rugged Ford ignition components that feature a 10 degree advance ‘get you home’ mode if the ignition controller fails.

The big plus for me on the race car, apart from the precision of the timing, was to be able to set the tick-over advance to get a smooth tick-over without compromising the maximum advance. On the road car there was a noticeable improvement in drivability, especially on pick up using large throttle openings from low revs.

The big challenge is selecting the advance curve. Again the race set up was quite straight forward since it is a simple 1 dimensional curve. The road car is far more of a challenge since it is a 2 dimensional curve. This is where lots of trial and error come in or better still, a good dyno / rolling road operator.

I recommend a visit to the web site. Download the software (free) and play with it. Sift through the forum and see how others have got on with it. There’s lot more information there and you can get to know all the details before you commit to buy. If you decide to go down this route I’m sure you’ll get a lot of satisfaction from the journey and reap the benefits when you get it up and running.

Dave Robson

Originally published in the 750 Motor Club bulletin