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post #20 of (permalink) Old 09-09-2005, 11:04 AM
Way Outta Control
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Re: TFI and HEI what\'s the difference?

TR - We've been through this before.

If you have access to an automotive ignition analyzer that can measure the spark in a time base - not all can, Sun cannot,- or lab scope, you can measure/see actual "burn time." That's the horizontal line right after the ionizing spike, before the oscillations.
That's the actual time the spark is jumping across the plug gap - hopefully starting a fire in the chamber (kinda like holding the match under the log.)
That's actual ignition time, not to be confused with the entire fire/intermediate, and recharge time.

Remember the ignition has to fire the plug, then "quick get ready" for the next plug's firing - it has alot to do in a short time span. The Kettering ignition types, including HEI, Dura, and points, the energy is built up in the magnetic field in the coil.

CD's like MSD, the energy is built up in the capacitor, the coil is used simply as a transformer. Still, "burn time" - the time spark is actually jumping across the gap is just as important. The fuel particles don't care where or how the spark got there, just that it's there.

Stock points type "burn time" is about .75 ms long at idle, stays that way until the coil saturation time is the limiting factor - around 3000, then it quickly rolls off.
Dura is about the same, as is Pertronics etc.
Stock complete TFI is about the same as HEI (but it's a POS!)

HEI is about 2.25 ms at idle, the limiting factor for burn on those isn't so much coil saturation time, but the available time between cylinder firings. At higher speeds it has to recharge the coil in order to be ready to fire the next.

If you look at the MSD with your scope, you'll see that the multiple firings stop around 2500, not way up there. Easy enough to see, try it.

Trying to fire the plug several times like MSD does certainly can help an old tired engine pass emissions (if that was the problem to begin with, as it's more likely to get a fire started. The HEI, although hotter than stock, still only tries once.

But, even above where it stops being an msd, the MSD still is a CD, so rise time - the time it takes to ionize all the gaps and "begin the spark" is faster, even faster than the HEI - much faster than a Ford's anything or points.

On your Jeep you said the Duraspark breaks down at 1700, I think you've mentioned you have the TFI coil on it? But you still have the resistor? Try something, see if it doesn't raise the breakdown point - try jumpering full 12 volts to the top of the coil. The module will run hot, so will the coil, but it should raise that breakdown point - unless the spark's jumping out where it shouldn't.

Once you get the MSD and the HEI as a switched circuit it should be interesting to do the comparison as far as actual driving it. I've done it, but not as an easily switchable system. Drive it, bring it back, covenrt it, drive it again - you lose something by not doing it right after the other.
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