Wednesday, November 30, 2005
Megasquirt Ignition Control - a quick overview
Separate Spark Control Options
Stock distributor control
This is how Megasquirt was originally designed to run. Simply connect the tach signal wire (Pin 24) to the negative terminal of the coil. This will work with any main board, processor, and firmware.
Other ECU for spark, Megasquirt for fuel
In some cases, you may wish to let the stock computer control the timing. This is usually useful for cases where the stock timing control is quite complicated, such as engines with ion sensing ignitions or the Mazda Renesis. Or on other occasions, you may already have a different, tuneable ECU controlling the timing.. In this case, the Megasquirt simply needs a tach signal input from the computer. While any board and firmware can handle this, many of these applications are rather complex and demand MSnS-E.
Original equipment distributors come in many varieties. The Megasquirt community has developed specific, plug and play solutions for some of the more common. Others call for a little bit of custom work, but in most cases this is still relatively straightforward. When piecing together a custom solution, the two factors you will need to consider are the sensor used to determine timing and the module you will use to fire the coil.
One thing that all distributor control methods have in common is that you need to disable any built-in advance mechanisms in the distributor if it has any. Sometimes this may call for partially disassembling the advance mechanism or brazing parts together. In other cases, a more elegant solution is to install a distributor from a later engine with computer controlled timing. For example, Chevy smallblocks have TPI-era distributors, while Chrysler fans can use the distributor from the much-maligned Lean Burn system. Megasquirt can also work with a crank trigger.
One other note if you are considering a distributor-based system. If you have an odd-fire engine like an early Buick V6 or a motorcycle with a V-twin, the only spark control option currently available is to use a crank triggered wheel decoder and the MSnS-E firmware. This is still in the experimental phase. If you are not sure if you have an odd-fire engine, the usual giveaway is that it will have unequally spaced terminals inside the distributor cap. You currently cannot use a distributor mounted trigger with an odd-fire engine.
Virtually any distributor mounted pickup that generates one pulse for every cylinder each time the distributor makes a complete revolution can work with either MS-II or MSnS-E. The same is true for a crank trigger with half as many pulses as the number of cylinders.
For more information on distributor pickups, follow this link for MS-II or this one for MSnS-E.
Variable Reluctor (VR) Sensor
A VR sensor actually works as an AC dynamo. One sure tip-off that you have a VR sensor is that it will use a two-wire connection (A few have a third wire, but it is a ground for the housing). Spinning the distributor will generate an AC signal across these wires, with the voltage rising the faster it spins. This type of sensor turns up on '70s era Chrysler and General Motors electronic ignitions, among others.
The V3 main board can read the output of a VR sensor directly, so if you are buying a new board and want to use a VR sensor, the V3 is going to be your best choice. Using a VR sensor with a V2.2 board will require an additional circuit or device to convert it into a signal the board can read. A GM four-pin HEI module or the Bowling and Grippo EasyVR circuit will work here, although there are other possibilities.
Alternatively, you can use a seven pin GM HEI module for both the input and output (see below). Follow this link for more information on how to connect a VR sensor to a V2.2 board.
Hall Effect Sensors
A Hall Effect sensor detects the motion of moving magnets on a trigger wheel, or uses a single magnet hidden behind a moving metal shutter. Hall effect sensors turn up in the Ford TFI (see below), Chrysler K-cars, and many Japanese and European imports. Any main board can read one of these if you connect it with the appropriate input circuit. See this page for more details.
Optical sensors come in many varieties, but they usually all produce a simple square wave output that can serve as a trigger for either firmware option and any main board.
Breaker points are a simple mechanical switch. There are also several aftermarket ignitions, like the Pertronix Ignitor, that behave just like points. Using one as a trigger for spark control does require a few minor wiring changes since these ground the coil to trigger it, but it is otherwise straightforward.
Crank Angle Sensors
Some systems use a distributor but control the timing with a crank angle sensor. This usually takes the form of a toothed wheel mounted on the crankshaft with a VR sensor pickup. Common wheels may have 36 teeth with one missing (a 36-1 wheel) or 60 teeth with two consecutive teeth removed (a 60-2 wheel). Currently, MSnS-E is the only firmware that can support this setup, and it allows for a wide variety of wheel designs. Here is more information on wheel decoders.
Nippondenso dual wheel trigger
Some Nippondenso ignitions use two sets of toothed wheels, one with a continuous set of teeth and the other to indicate camshaft position. MSnS-E has a specific option for this setup.
Ignition Modules and Systems
Megasquirt as an Ignition Module
The V3 main board has a built-in ignition circuit, also known as the high current driver. Either MSnS-E or the MS-II can control this, allowing the Megasquirt V3 to both read common distributor pickups and control the coil on its own.
The Microsquirt has two such circuits, allowing it to operate the ignition coils on a four cylinder bike directly with no external modules.
If you have a V2.2 and want to build your own ignition module, you can wire up an external circuit built around a VB921. This is a category of transistor called an IGBT, or in plain English, a big honkin' transistor that can control your ignition. You'll find a description of the circuit to build here, about halfway down the page. As with the V3 board's high current driver, this works with both MSnS-E and MS-II. These are also useful if you have a V3 board but want to drive several coils.
Bosch ignition modules
Bosch built a series of ignition modules with smart dwell control, used in many '80s era European cars. Wiring one directly to Megasquirt is very straightforward. This works with any main board and can use either MSnS-E firmware or MS-II. Ordinarily, these were paired with Hall effect sensors, but when controlled by Megasquirt the actual distributor pickup is irrelevant.
GM 7-pin HEI
Used in many fuel injected GM products, this module contains both a VR sensor driver and an output to the coil. It requires one extra input wire, but can work with any main board and either MSnS-E or MS-II. Here is a brief explanation of how to set up Megasquirt to work with a 7-pin HEI module.
Ford's TFI system uses a Hall effect sensor and can be adapted to any Megasquirt board with minimal wiring changes. Both the MSnS-E and MS-II support Ford TFI. Here's the basics of using a TFI ignition with MSnS-E. The MS-II has its own instructions for use with TFI.
Aftermarket ignition modules
Boxes like an MSD-6 or Mallory HyFire can run off a simple tach signal from the Megasquirt. This works with both MSnS-E and MS-II, and you can use any main board version.
If you don't have a distributor, or wish you didn't have one, Megasquirt offers several solutions. Most of the options currently available use what is called a wasted spark system, triggering coils in pairs. This eliminates the need for a cam sensor and can greatly simplify installation.
The Ford EDIS system used a separate control module with its timing controlled by a digital signal from the main computer. This makes it a popular choice for home-brewed distributorless ignition, since it handles much of the timing control needs with minimal input from Megasquirt. Putting EDIS on an engine that never used it is simply a matter of finding a way to put a 36-1 wheel (see Wheel Decoders, above) onto your crankshaft and mount a VR sensor that can read it. EDIS works with 4, 6, or 8 cylinders. You can use either the V2.2 or V3 main board for this. Both MSnS-E and MS-II can control this. Here's what you will need for this option. The MS-II page provides a very in-depth explanation of how EDIS works.
General Motors's distributorless ignition works in much the same way as EDIS, except that it has a different trigger wheel assembly. You can control it with either MSnS-E or MS-II. Here is more information on how to interface it with MSnS-E and MS-II. Some Buicks used a different DIS setup called C3I, which is presently only supported by MSnS-E.
Neon / Eclipse Wheel Decoders
MSnS-E has a special mode written especially to operate the distributorless ignition on many Chrysler and Mitsubishi four cylinder models. This is not available with MS-II.
General Purpose Wheel Decoders
MSnS-E has a wheel decoding program that can control coils directly, using a single crank-mounted wheel with one or more missing teeth and a VR sensor for the input trigger. Most installations control the coils with VB921 circuits, although Bosch modules are also a viable option. This can be a bit of work with the electronics, but allows for building custom distributorless ignitions. The wheel decoder can control up to six coil packs, allowing for as many as twelve cylinders. Here's what you need for a wheel decoder.
The production Microsquirt will have two coil drivers built-in, allowing it to opperate a wasted spark ignition on a four cylinder engine directly with no mods and no external circuits.
Bleeding Edge Ignition Solutions
The gurus behind the firmware are constantly working to develop support for new devices. The MS-II team is working to develop a router board that will allow direct coil-on-plug control as well as a general wheel decoder to allow waste spark ignition on MS-II controlled engines. Meanwhile, the MSnS-E developers have released code that can control six coils for coil-on-plug applications and even the double distributor setup found on some Lexus V8s. There are more options that are still having their bugs ironed out, so if you want to try this, take a look at the Megasquirt Forum.
Here's a chart to recap which options work with which firmware and main boards.
|GM 7-pin HEI||Yes||Yes||Yes||Yes||Yes|
|Neon / Eclipse||Yes||Yes||No||Yes||No|
|Hall effect sensor||Yes||Yes||Yes||Yes||Yes|
|Missing tooth wheel (VR)||No*||Yes||Yes||Yes||Yes|
|Nippondenso dual wheel||Yes**||Yes**||Yes||Yes||No***|
*Can be used with an external VR sensor input conditioner.
**Dual wheel setups often require an additional VR sensor conditioner even with the V3 board.
*** Currently in beta testing.
|Built-in ignition module||No||Yes||Yes||Yes||Yes|
|External VB921 circuit||Yes||Yes||No||Yes||Yes|
|MSD or similar aftermarket device||Yes||Yes||Yes||Yes||Yes|
|No. of coils*||-||-||2||6||1|
* Maximum number of coils or coil packs that can be controlled directly by ECU without any external timing controls such as EDIS or the upcoming GPIO Board.
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