My main goal is to test/set advance curve.
Halifax is on the money. One thing to note is the fact that ALL electronic ignition systems will retard with RPM. All of them. Some more than others, but they all do it. This is because an electronic ignition system has electronic switches instead of mechanical ones (points are mechanical).
Just as it takes time to open and close points, it takes time for the electronics to switch on and off. It’s called “slew rate” and the amount of retard changes with components. It sounds complicated and it kinda is, but it isn’t.
It takes time for the electronics to switch on and off (or whatever other function they may have) so as RPM goes up, the slew rate becomes retard. That happens once you’ve reached full mechanical advance.
That’s why I like to test the complete ignition system once I have the curve I want. You can see the amount of retard, where it starts and at what RPM it retards more.
This is important because most guys never check for it, and in the car with a timing light it’s near impossible to see.
So, let’s say you get full advance at 2500, and you set total timing at lets say 2800. But the ignition box (or the trigger in the distributor if it doesn’t need a box) starts to retard at 3000, and it’s 2 degrees. And at 4100 it’s another 2 degrees retarded. And at 6000 it’s another degree retarded. That’s a total of 5 degrees of retard at 6000 RPM and that’s AFTER full adavance, but you’ll never see it with a timing light in the car, because no one verified the timing that high and because it’s damn hard to see the tach, look at the marks and all that.
So instead of having full timing through the RPM, range, you lose advance. And in this example (using arbitrary numbers for the sake of discussion) if you set the total at 35 degrees at 2800 RPM you only have 30 total at 6000 RPM.
You can’t see that if you don’t test the entire ignition system, or try to see what happens with the ignition in the car.
That’s why locking out the timing is such a bad idea. Most guys who lock the timing out set the timing at idle. So they never realize they don’t have full timing with RPM.
Also, remember that timing lights have a slew rate as well!
I hope this didn’t confuse you.
