Sparking the MGB

• Using MSNS-Extra 26h4
• Crane XR700 module providing sparkl.
• Crane opto-trigger and shutter in the distributor
• Standard MGB static ignition timing set to 10 BTDC.
• Locked mechanical advance
• Disconnected vacuum advance.
• Stock plug type N9YC @ 0.025” gap

MGB 18GH Advance requirements:

Eng # Yr	Distributor Lucas #	Vacuum Unit Lucas #	Max Vacuum Adv. Dist. Degrees	Max Centrifugal Adv. Crank Degrees	Ignition Timing Crank Degrees	Dwell Deg.	Gap
18GH 1970	41288 25D4	54411985 Ported	Starts 5 hg in Finishes 13 hg in Advance 10	18 - 22 @2200 rpm	20 BTDC @ 1000 rpm Disconnect Vac	57 - 63	014° to 016°

From: British Automotive.

Mechanical Advance

Max advance at 2200 crank RPM of 10 + 18|22 = 28|32 degrees BTDC.

MGB 18GH from above:

RPM (crank)	Advance (crank)	Total @10 static
500	0	10
800	8	18
2200	18-22	28-32

Vacuum Advance

The throttle plate opens to expose the advance port to manifold vacuum. At idle the port is covered and no vacuum advance is provided.

5 inHg = 16kPa from atmospheric

13 inHg = 44kPa from atmospheric – 10 degrees advance

Assuming 95kPa as atmospheric:

95 - 16 = 79kPa à 0 degrees advance

95 - 44 = 51kPa à 10 degrees advance

I used Werner’s spreadsheet to work out my ignition table: IgnitionMapMGB70.xls.

Design

The Crane optical trigger will pull the output (grey wire) low when the rotor shutter slit passes the LED.

Megasquirt is looking a for a low signal on IRQ1 to trigger the spark. The grey wire from the Crane reader needs to be connected to XG1 on the megasquirt board.

Board Modification

1. Remove the jumper from XG1 to XG2 (see photo)
   
   
   
2. Cut/remove D5.
3. Jumper XG1 to D5 / pin 24 (COIL).
4. Jumper the U4 side of D8 to +12V – RHS of D9 using a 1.3k to 2k resistor.
5. For the spark output use FIDLE. Jumper the bypass relay as shown.
   

Interfacing to the XR700

When running the leading edge of the signal will trigger the megasquirt during cranking mode and running mode. Set to an advance of 10 degrees BTDC, megasquirt when running will be in “next cylinder” mode and fire the spark at the first available opportunity when it detects that edge.

When triggering the spark MS will drive Idle-1 which will turn on Q5. This will pull IDL low. IDL (or FIDLE) can be used to drive the XR700 input directly.

1. Cut the grey wire from the XR700. Run the distributor side of that wire to COIL of the relay board.
2. Run the FIDLE back into the grey wire to the XR700 ignition module.
3. Use megatune to program spark output into the FIDLE output.
4. Set dwell using the “dwell settings”.

My home page is Mike's 250lb Locost Site.