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post #18 of (permalink) Old 09-08-2005, 11:48 PM
Junk Yard Genius
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Re: TFI and HEI what\'s the difference?


Since the HEI's spark duration is 2.25 MS - then 1000% longer would mean 2250 MS, or 2.25 seconds long. Hopefully the engine turns faster than that.
MSD has it's advantages yes, but not that much.

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The entire discharge EVENT lasts 2.25MS...
The ACTUAL USEABLE DISCHARGE is about 0.35 to 0.39ms, depending on coil design.
DISCHARGE EVENT includes ramp up time, discharge, and osculations cycling after the actual discharge...
Common misconception that the ENTIRE firing cycle produces useable spark energy.
Plus, the actual spark starts weak and 'thready', a stray trail of electrons ionizing gasses along the way, then useable spark energy and ends the same way as the discharge runs it's cycle and becomes a stray trail of electrons again.
ALL of that trail of electrons, no matter how weak, is considered 'DISCHARGE', even though it wouldn't make a hair on your arm stand up, and doesn't have a chance in hell of firing the thick atmosphere in a combustion chamber...

You also have to consider how long it takes surrounding fuel &amp; air to reach combustion temperature (thermodynamics: radiated thermal transfer to a <u>sustained</u> flash point)
Often times USEABLE SPARK ENERGY is long over before <u>sustained flash point</u> is ever reached.

The "multiple" spark stops being "multiple" at about 2000 RPM. It "runs out of breath" at about 6000 RPM, about the same as an HEI. Use a distributor machine to compare.

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There are two parts here...
1. My Jeep spends most of it's life between 600 RPM and 2,500 rpm, so my Jeep (and everyone elses) gets it most benefit from the MSD module.

Multiple fire doesn't stop at 2,000 or 2,500, just less of the multiple fires because as RPM increases, time for multiple fires decreases...
Not that the module can't supply the discharges, just that the engine isn't spending enough time at the correct window Before TDC for as many discharges to take place.

2. The reason for no multiple sparks after 5,500 to 6,000 RPM is simply that there isn't enough time for multiple fires at elevated RPM's.
The Single discharge is much longer and stronger than any inductive ignition can provide.

3. The Capacitive Discharge unit used in the 6 series module can charge (saturate) the ignition coil a BARE MINIMUM of 50 times faster than ANY HEI module, so there is NO POSSIBLE WAY that a HEI module could keep up with a MSD module at any RPM.
The HEI module can't even switch fast enough to keep up with MSD's saturation times, not to mention the coil can't DISCHARGE fast enough...

4. I have NEVER seen a stock style HEI module that could sustain acceptable spark energies above about 1,700 or 1,800 RPM, and total ignition breakdown occurs around 8,000 RPM on the best units.
HEI can fire a stock, low performance, next to no compression, overly rich beater engine for a while longer, but raise the compression, thicken the atmosphere, lean out the mixture and open up the plug gap, and the HEI sucks wind FAST!

Add water injection, a supercharger or turbo, nitrous, decent compression or lean out the mixture to make power, and the HEI is totally useless.

I have PERSONALLY tested, and raced MSD modules at up to and exceeding 15,000 (thats 15K RPM) with out failure, and that's with factory style ignition coils, over the counter plug wires, factory mag trigger distributors, and over the counter premium grade distributor caps &amp; rotors.
(AGAIN, totally off topic for Jeep owners, and no Jeep owner here is EVER going to turn over 7,500 RPM and mean it!)

5. There are NO distributor machines available to turn a distributor at 15,000+ RPM.
I use a digital triggering device (for accuracy), and fire the modules ON THE VEHICLE using a test plug(s) for visual verification, and an 'O' scope for patterns and firing voltages.
By testing on the vehicle, there are no questions about what losses or gains *MIGHT* be happening... Or what *MIGHT* be better for that particular vehicle...

Second part...
I have two ignitions on this little Jeep I just bought...
One is DuraSpark, one is HEI.
I'm also going to do comparative testing on a MSD 6 series when I'm done with the factory inductive discharge ignitions...

The factory DuraSpark module breaks down about 1,500 RPM on this particular Jeep.
The factory HEI module breaks down at about 1,700 RPM on this particular Jeep.
Via a laptop and interface, I've been tracking the trigger/fire delay, dwell, input voltages, appx. firing voltages, current draw, engine temp., exhaust temp and O2 content, ect. ect....

This was the perfect test bed, as it needed to be rescued from breaker points hell when I brought it home, and while I was installing the ignition, I just added the thermocouples, transducers, inductive pickups, ect...

If you are even in the slightest interested, I'll let you know how things turn out when I get further along...
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