CDI ignition - gross vs nett spark energy

[JohnP]

For HEI or other high energy inductive ignitions, use a coil with primary resistance of about 0.5 ohms. The Ford TFI coil is very good.

A BOSCH coil from a late '80's BMW six cylinder is very good. It's a black cylindrical, oil filled, coil with 0.5 ohm primary. Avoid the BOSCH Blue coil. It has a built in ballast resistor for use with points.

Mallory UniLIte, Pertronix. etc use the recommended coil and ballast resistor if specified, since they are small modules with almost no heat sink.

[barnym17]

I along with lot's of fella's ... really don't know shit from chewed dates when it comes to electrics,
! - does a huge big fat spark actually make more power ?
2 - If i'm using an inductive type of dizzy, what are the best things I can do to increase it's performance eg : large wires, some sort of trick coil ??

ta robbie

It's not so much that a big fat spark itself improves power, what it will do is help prevent misfires that occur in every engine due to many factors such as mixture quality ,slightly fouled plugs etc.Many times it may allow running a wider plug gap which will enable firing a leaner mixture.Which is exactly what the oems did in the 70's and early 80's for emissions purposes. An anology could think of lighting a piece of paper with a match and then one with a torch. both work well in this case,but now imagine trying to light a 2x4 the match won't do it but the torch will.
If you have your engine well tuned the difference in power between a points setup and any other ignition will be slight to non existant,But in the real world when mixtures are almost never perfect plugs aren't changed after every run etc. A good electronic ignition blows them away.Also when making power the goal is to be sure all available oxygen is used up so richer mixtures are used so a fat spark or multi sparks get it lit when sometimes a point style wouldn't allowing for possible power gains.

[lada ok]

....Ta ..

[JohnP]

Just to put things in perspective, the coil on plug ignition on a modular Ford engine makes 26mj. Some guys are making 900hp with it.

A guy asked me to test a "hot coil" for his Ford GT. It was advertised as making 44mj. I first tested the stock coil and came up with 26mj. Then the hot coil, and confirmed it made 44mj, validating my test setup.

photos here:
https://picasaweb.google.com/1001267426 ... 3521055890

[clshore]

Thanks for sharing the test results.
I repurposed a TV High Voltage probe (from pre-prehistoric time when TV's used picture tubes rather than flat screens) that was useful
for checking high voltage sections.
It's a nice large plastic piece with insulated handle and a spike on the end that you can insert under rubber boots to directly read
signals while the motor is running.

Getting current reading in-situ is a bit more challenging, I've had good luck with .01 Ohm resistors between ground and the coil Neg side.

Floating the scope power supply with a separate battery and inverter (or battery powered scope) let's you put a current sense resistor nearly anywhere,
but you must take care with wire routing since scope 'ground' may wind up being several KV with respect to the car 'ground'.

I suspect that you have a more than passing acquaintance with MegaJolt Light Jr, etc.

[JohnP]

Sorry, no I haven't seen a megajolt.

I've been making ignitions since 1980. They were analog until I designed the SafeGuard in 1990.

[clshore]

Yah,
My first foray into digital ignition stuff was 1975, I was a tech working on a digital DME aircraft navigation product.
I saw the data sheet for 74188 32x8 bit field programmable PROM memory.
It occurred to me that I could digitally split engine RPM into 32 segments, and read the 8 bit word to set advance for each segment,
instead of using centrifugal weights on my racecar.
Built a prototype that worked OK, but didn't have a reliable trigger, so spark jitter was still an issue.

[JohnP]

Good job. I avoided digital about as long as I could. In '88 I got a surplus AIM 65 and started playing with the 6502.

[bmcdaniel]

I suspect that you have a more than passing acquaintance with MegaJolt Light Jr, etc.

Megajolt is what I'm using with my EDIS setup. Now it's called "Megajolt E":
http://www.autosportlabs.com/product/megajolte/

[Circlotron]

My house electrical panel is rated at 200 Amps.
When I use my stove, it consumes only 20 Amps.
Doesn't mean my panel is defective, that's just what the stove uses.

Once the spark ignites and current starts to flow, it uses what it uses, determined by the gap and mixture being burned.
You can't somehow cram more energy into it.

The AC power provided by your electric company is a voltage source.
As you correctly point out, the stove uses what it wants because it is a resistive load connected across a specified voltage.
How much power goes in is determined by a fixed voltage and a current that depends on the load.
Remove the load (switch off the stove) and the voltage stays the same but the current goes to zero.

The power provided by an inductive ignition (and to a lesser extent a CDI) is a current source.
You can either short circuit or (within limits) open circuit a coil secondary and the current will stay the same. The voltage will vary wildly, from basically zero to the max it is capable of in an effort to maintain the current at what it wants to be.
How much power goes into the plug is determined by a fixed current and a voltage that depends on the load (gap, pressure, electrode temperature, shape etc).
Remove the load (pull off plug wire) and the current tries to stay the same but the voltage goes to maximum.

Therefore, if you have an ignition with a greater current capacity, it WILL cram more energy into the plug.

[Circlotron]

^^ Further to that, with an arc welder you can dial the amps up and down as you like. When the amps is at a high setting the arc is more intense, as you would expect. Somewhat similar to a spark plug but on a larger scale.

[user-23911]

I would expect at least a 75% loss of energy.
Why?
It's called the maximum power transfer theorem. Part of Electrical Engineering.

http://en.wikipedia.org/wiki/Maximum_po ... er_theorem.

There's a 50% loss going from storage capacitor to ignition coil primary, then another 50% loss going from primary to secondary.

It's no different from inductive ignitions where the output is usually overstated by at least 50%.

The advantage of a bigger spark is being able to have a bigger plug gap without misfire. A bigger gap means a quicker burn which means less timing advance needed so less chance of detonation.

You can't get reliable voltage readings on the secondary side either, the reliable way is to read primary voltages the multiply by turns ratio to get secondary voltage.
The reason for that is when you hook up the scope to the secondary side you are adding capacitance to the circuit, maybe by as much as 100%. Not only that but you're loading it with a bit of inductance and resistance.

You want your spark to last as long as possible which means you DON'T want low resistance in the secondary circuit.
Volts times amps times seconds = Joules.

Someone asked where the lost energy has gone?
It gets reflected back to the source ....that "other " capacitor soaks some of it up, the circuit becomes an oscillator where the energy gradually gets used up, turns to heat in the resistances.

[Circlotron]

I would expect at least a 75% loss of energy.
Why?
It's called the maximum power transfer theorem. Part of Electrical Engineering.

http://en.wikipedia.org/wiki/Maximum_po ... er_theorem.

There's a 50% loss going from storage capacitor to ignition coil primary, then another 50% loss going from primary to secondary.

It's no different from inductive ignitions where the output is usually overstated by at least 50%.

There would indeed be 50% loss if the source and load impedances were equal. Sometimes they are purposely mismatched though. But yeah, it's an interesting thought.

[maxc]

Most new mosfet driven 12 volt dc to 120 volt ac inverters are 90% efficient. So efficient high voltage is possible.

[JohnP]

Joe is talking about the power loss when black boxes are impedance matched.

http://en.wikipedia.org/wiki/Maximum_po ... er_theorem