Ignition testing

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Originally Posted by bradrs

It could be a series resistor as you describe, but I am thinking it is more along the lines of the gain of the transistor.

Have you looked at the coil - signal? Make sure the scope is able to handle those 400+V flyback voltages, I've seen too many people not carefully check out the scope parameters and blow a channel doing that.

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I thought about that too. It could be done either way (resistor or transistor gain). The gain on the one you reference does drop quickly. Good point on the scope - hate to damage test equipment.

Jim,

In the photo, is that the power supply on the right hand side, with the 2 DMM leads attached? It looks like some sort of eval board? Or is that something custom? The only reason I ask is that the eval boards I have used for testing power supplies, did not have very large capacitors on the output. You might be able to clean up those traces on your scope by using larger capacitors.

And if they had one of those in 1969 I probably would have used it. As it was, dual point distributors were the best I could get until MSD came along.

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Originally Posted by bradrs

Basically by adding another 1-2V, you are probably adding another 10-30% spark energy, and a bit more voltage to start the spark. It sounds like that has been enough for most people here. No add on boxes either. Seems like a no brainer to try this first. That, and the slight reprogram of the ECU both are good, simple fixes.






The DLI is supposed to work not by increasing dwell, but by laying a CDI style spark on top of the standard inductive spark that is being generated by the stock ignition.

The inductive spark builds up the current(dwell time) in the coil primary, and that is where the energy is stored. By shutting off dwell, the magnetic field in the coil primary collapses, and that makes the secondary voltage for the spark.

The CDI stores the energy in a large capacitor in the module. When it fires, it dumps all the energy at once into the coil primary which immediately creates the voltage in the secondary.

The DLI is supposed to work by sensing the stock signal, and firing its CDI style spark at the same time, laying it on top of the inductive spark.



On your Roadrunner, instead of just trying to increase dwell, you'd be better off by using something that regulates the dwell time to get a fixed current. When you increase dwell angle with points to get more current at high RPM, now you have a really long dwell at low RPM. A module that adjusts the dwell time to give a consistent dwell current is going to give a better result. And it will allow you to use a low resistance coil that charges even quicker, for better high RPM performance. Of course it isn't OE, so there is that downside.

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Originally Posted by Jimvr4

It's definitely more than 10-30%. At idle it's possible to increase the current from 5A to nearly 8A, while at 7500 rpm nearly 5A can be maintained where the stock system drops to 3.6A. That's nearly 40% more current at 7500 rpm.





This is also what I thought but deeper reading seems to disagree. The way the DLI hooks up across the coils means that it can actually provide an independent transistor control from what the ECU is doing with the PTU. Think of it as two separate switches that are controlling the coils and imagine that before the ECU turns off the PTU that the DLI transistor in parallel is also shorting the coil to ground. Then when the ECU turns off the PTU the dwell continues because the DLI is controlling it now instead of the ECU. The DLI uses the increased dwell time to store charge in the capacitor and then discharge it to get the ~400V flyback voltage in the primary side.

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Making it look a lot like "dual points" in operation.

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Originally Posted by anyonebutme

They make like 18V batteries with a 12V tap. Run the 18V tap to the coils and starter and everything else 12v. Don't know how long the coils will like running on 18 volts, but hey, as Clarkson would say: POWWWWWWAAAAAAAAAAAAAAA.

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Great concept. Only issues come in how you charge that 18V battery while maintaining the 12V side separate from the higher voltage. It can certainly be done, but how complex can the system be and still qualify as "street" or DD?

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Originally Posted by bradrs

That is the exact same sort of thing I was seeing on the PTU coil - signal. It would climb, and climb quickly once you hit current limits. It could be a series resistor as you describe, but I am thinking it is more along the lines of the gain of the transistor. Here is a datasheet of a darlington transistor that is used for ignitions. This is probably not exactly the same as was used, but is typical for something from that time frame:

http://www.jetav8r.com/Vision/TCIReb...E5740Specs.pdf

Looking at the specs, you see the gain drop WAY off as it approaches 8A, by a factor of 10 or more. Which means you'd need to drive it with that much stronger of a signal to keep it saturated.

And by reading the voltage where we were reading it, that makes it clear it is NOT the same as saturated coils.

Jim, it looks like you are at a 10mV/A scaling? If so, if you average out the noise, it looks like about a 7A peak at idle. That looks promising. Have you looked at the coil - signal? Make sure the scope is able to handle those 400+V flyback voltages, I've seen too many people not carefully check out the scope parameters and blow a channel doing that.

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Unfortunately I don't have access to a high voltage probe so I can't risk exposure to the flyback voltage. The gain of the part you reference does drop off however from the other curves it looks like plenty of gain is still available. And that particular part still has a very low Vce deep into saturation.

@ jimvr4 my testing was using the 12vto20v box as I linked.
A DC-DC box, a good diode and the second fuel pump relay would make BAS that would get you home if the DC/DC converter failed.

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Originally Posted by Jimvr4

Unfortunately I don't have access to a high voltage probe so I can't risk exposure to the flyback voltage.

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Could you set up a small resistor divider network of some high impedance resistors? A 10:1 divider should do the trick.

I wonder about increased stress on the PTU & coils if running 18 - 20V and higher current all the time. A DD probably doesn't need that a lot of the time. I like the idea of switching it on via ecu or some other control only when needed.

I was worried about running that voltage all the time. Hence the bsa.
Btw i recommended increaseing the limit to 90% and the dwell time by 25%