Part 6 – Ignition Control and Tuning
For us mustang guys there is a whole host of different ways you can get the megasquirt to do ignition control. All you need to do is feed megasquirt a signal so it knows when piston #1 is at TDC (Top Dead Center) and program megasquirt to do the rest.

The options that were available to me were:
- Use stock points/pertronix to trigger megasquirt.
- Use a crank wheel with magnetic pickup.
- Use a TFI distributor from a late model mustang.
- Swap to magnetic pickup distributor and feed signal directly to megasquirt
- Use a magnetic pickup distributor along with a 7pin GM HEI (computer controlled timing) module

There is good and bad to all of these solutions. The TFI and HEI are nice because they take care of the dwell settings for you (easy to install/tune) and have a “limp home mode” if the megasquirt suddenly stops talking to them. The biggest downfall to these is that you can’t adjust the dwell. Megasquirt has some really advanced and useful dwell control features that it would be a shame not to take advantage of. The crank wheel is probably the best choice as it’s the most accurate and you can take advantage of all of the megasquirt’s abilities, the downfall is that it’s a lot more difficult to install. Also keep in mind that with the crank trigger, you still need a distributor to “distribute” the spark to the different plugs. Using a plain magnetic pickup distributor (such as what you would use with a MSD box) is a good blend between difficulty and control, however it’s not as accurate as the crank trigger. And the final option is to modify your stock distributor. You can use the stock points to send megasquirt a signal, or you can use a stock distributor with a pertronix unit installed. This is what I chose to do because I wanted the best accuracy possible, while still keeping the stock distributor.

So why do you need to modify your distributor? Well it’s simple if you think about it… you want the megasquirt to control timing now.. so you need to remove the mechanical and vacuum advance so that no mater what the engine RPM or vacuum, the megasquirt always gets the signal at TDC. Disabling the vacuum advance is easy..just remove the vacuum hose, but the mechanical advance is a little more involved.

Here’s a picture of what the stock distributor looks like with the cap and rotor removed. You’ll notice the pertronix unit was already installed…picture a set of points in it’s place if you would like. From here you’ll want to remove the vacuum advance diaphragm, the points/pertronix, the screw holding the grounding strap, and the snap ring holding the pivot point in place. From there you can lift off the vacuum advance plate.

Here’s what it looks like with the vacuum advance plate removed. Next step is to remove the 2 screws holding down this next plate and to remove the plate.

Now we can finally get at the mechanical advance assembly. 1st step is to remove the weights by removing the snap rings holding them down and then lifting the weights out.

Here’s what it looks like with the weights removed. This should be all you need to do, but I wanted to do a little extra just to make sure nothing moved around at 6000rpm.

So for extra insurance I added a couple more return springs. These springs pull the mechanical advance closed with much more tension than the stock springs so I’m confident everything will stay in place at high RPM. Spot welding the 2 plates together would have been best, but I wanted to be able to return this to stock if need be. And that was it for distributor modification, it turned out to be easier than I expected.

I will note that my original plan was to leave the vacuum diaphragm off, but noticed the vacuum advance plate wasn’t very stable without it, so I changed my mind and put it back on (just left the vacuum advance hose off).

There are some minor changes needed to the factory wiring. The signal from the points/pertronix no-longer goes to the coil -, instead it is fed into the megasquirt. However, you need to install a 1Kohm pullup resistor to get this to work. Usually the power flowing through the coil would do this, but since this wire isn’t connected to the coil anymore, you must use a pullup resistor. And finally, the coil ignition output from the megasquirt needs to be connected to the – terminal of your coil(instead of the black pertronix of points wire).

That’s it for the hardware and wiring, now it’s on to tuning.

The 1st thing to do is to set your trigger offset. There’s a good chance that your distributor isn’t sending a signal at exactly TDC. So what you want to do is tell the megasquirt exactly how much off the timing signal is. Once megasquirt knows this it will automatically compensate without you needing to think about it. The megasquirt has a built in utility for this… what you do is change the offset value until the timing on the display matches what the timing light is showing you on the timing pointer.

I purposely chose to use a 10* BTDC offset so if my megasquirt ever stops working I can simply connect the pertronix back up to the distributor and be able to get the car home (timing would be locked at 10* so it won’t run well, but it would get me home). This is the same idea as the “limp home” feature in newer cars except instead of it happening automatically, I’ll need to jumper a wire to get this to work.

Next you’ll want to check out the cold ignition advance settings. The settings in this example seem to work great for me. Personally I think this is a great feature… makes cold starts/running much easier and smoother.

Dwell control is another great megasquirt feature. You can set the dwell time, add more dwell for a hotter spark when acceleration enrichment (equivalent to accelerator pump in a carb) is activated, and also change the dwell time for high/low battery voltage situations.

It allows you to get the absolute hottest spark out of your coil as possible, while also allowing you to compensate for low battery voltage such as in cranking. You can literally toss you ballast resistor away…

And finally it’s time to setup your ignition advance table. Here’s an example spark advance table. The height of the 3D graph shows how much spark advance there is. The horizontal axis is RPM, the vertical axis is vacuum. In this example you can see that the least spark advance happens at 500rpm (idle) with there is no vacuum. However, there appears to be considerable advance at idle as the vacuum rises.. You can also see in this example that advance climbs pretty quickly up to about 3500RPM and doesn’t seem to increase much after that.

To tune your ignition advance you’ll have to start with an educated “guess”. Most ford V8’s like about 36* of total timing and like to have it “all in” by about 3500rpm. From there you’ll have to guess at the effect of the vacuum advance (can add as much as 15* at high vacuum). This should get you started, from there you’ll have to drive the car around. 1 great thing about the megasquirt is that you can change the timing in real time while driving the car… no need to shut the engine off, etc. This is when it’s really helpful to have someone drive while you can tune.