In most cases you will be controlling timing with the Megasquirt, for this you need a trigger that does not vary angle with rpm or load. This is called a "locked distributor".

Early distributors such as points, HEI4, Duraspark etc, all have advance mechanisms built in. The HEI4 distributor shown above illustrates these mechanisms and is typical of pre-computer distributors. Similar distributors can be converted for use with Megasquirt -

Later engines may feature a distributor in conjuction with computer controlled timing - usually these distributor are "locked" from the factory and should already have good input and output phasing. (e.g. Ford TFI, GM HEI7/8, Bosch hall effect.) Align as per the factory manuals and determine how it is phased before you modify anything!

The crank angle at which the tach input triggers is of importance and needs to be configured in the Megasquirt. For best spark control there are some optimal and some disallowed crank angles.
A typical engine will have an operating advance range of say 10-50 BTDC timing (depending on engine type.) The trigger must not happen during this range of angles.
It can be really helpful to install timing tape on your crank pulley or temporarily mark on a range of angles.

The VR sensor input presents a simple pulse as the reluctor passes the sensor, this gives a timing position easily identified by eye. Use "Basic Trigger"

Allowed high angles Timing allowed in normal range (up to 5 degrees less than trigger angle)
Disallowed angles Timing will not work correctly.
Allowed low angles Timing allowed in normal range (greater than trigger angle)

Basic Trigger settings

Next you need to configure your coil control and set the output phasing.

The purpose of the "Trigger Return" mode is to have accurate cranking timing as well as accurate running timing. It achieves this by using the signal from both edges of a vane/slot. One edge is used for the timing calculations during running and will typically pass the sensor at 55BTDC or more. The other edge is used for cranking timing and must pass the sensor at the desired cranking advance angle e.g. 10BTDC

To help determine which edge is which and get your input capture edge settings correct, there is a code feature called "ignition trigger LED"

Enable this, Burn and power off, then power on.
With the ignition on you will have power to the hall/opto sensor and it will return a signal to the Megasquirt depending on whether there is a vane/slot in front of it.

A. Hall/opto distributor showing inactive trigger. Ignition trigger middle LED off.
B. Engine rotated forwards until edge at sensor. LED comes on. The crank angle here is the "Trigger Angle"
C. Engine turned forwards some more. LED stays on.
D. Engine turned forwards some more. LED turns off. This is the Return angle and will be used as cranking advance. This needs to be ~10BTDC. Rotate your distributor if needed, then repeat steps B,C,D.

If the LED lights when it should be off, then change the Ignition input capture from Rising to Falling or vice-versa. Then Burn and turn the Megasquirt off then back on again. Repeat steps A-D above until you are confident you have the correct angles within a few degrees. (You will finalise the angles later.)

Trigger return settings

Ford's TFI module was used throughout the 1980s and into the 1990s on many millions of vehicles in two main mounting positions - 'distributor mount' and 'remote mount'. There are also two electrical versions: "Push Start" and "Computer Controlled Dwell" : documentation claims that these can be distinguished by colour, but that appears unreliable. Checking the wiring on pin4 is likely best. The wiring of the modules is largely the same, just the distributor mount connects directly to a 3 wire hall sensor in the distributor. In most installations you do not need to concern yourself with that as only the 'PIP' and 'SPOUT' connections are of interest. The other connections should be left stock.

MS3X - wiring and settings

The original "High Energy Ignition" (HEI) distributors used the 4 pin module from the early 1970s (as shown in the photo at the top of the page.) That module is fine in the breakerless distributor as designed, but is not suitable for computer timing control. The later 7 and 8 pin modules and corresponding distributors are designed for computer control and should be an easy swap onto earlier engines - not only are those modules intended for computer control, but their distributors are already locked-out so no modifications are required. HEI7/8 uses three control wires to/from the Megasquirt.

Module photographs:

The 'Ref' signal from the module to the Megasquirt gives rpm and engine position information.
The 'Est' signal from Megasquirt to the module controls the advance when running.
The 'Bypass' signal from Megasquirt to the module allows the module to benefically control its own advance during cranking. Once the engine has been running for more than 5 seconds, the Megasquirt takes control of timing.

MS3X - wiring and settings

A dual-sync distributor is an aftermarket distributor that allows sequential to be supported.

There are three main options here:

Once the tach input is all setup it is important to confirm the output phasing is correct.