Although longish, a good, authoritative read:

Short on time,skip to page 49, coil vs transformer.

http://www.worldphaco.net/uploads/CAPACITIVE_DISCHARGE_IGNITION_vs_MAGNETIC_DISCHARG E_IGNITION..pdf

I said I wouldn't respond but have to now as you are slandering me on a public forum which is completely unprofessional! I have not once insulted you, yet you are hurling insults at me on every post, why?

You know nothing about me and my company. If you'd like to know more, please turn to page 106 of December's issue of 911 & Porsche World where there is a article about us.

Interestingly, the document you pointed to is one of the reference documents for our design. Now that guy really knows his stuff. Also interesting then, that in a 73 page document, the word 'pressure' (as in cylinder pressure) does not appear once.

We have just taken time out of our busy day to conduct a real world experiment.

On our rig test rig, I ran a plug with a 0.7mm plug gap and recorded the primary trace and the average current draw at 5000 RPM which was 3.05 Amps.

I then widened the plug gap to a be well over 2mm, three times the original gap.

By your reckoning then, the CDI would draw more current but guess what? It doesn't.

in both cases, the CDI draws 3.05 amps and the primary winding trace is exactly the same.

I'd post up the traces but they are identical so there isn't much point.

Gentlemen,

can we close this tread now?

I have got an answer to my question. The theory around the CDI box is interesting but beyond the scope of my question. However, it is interesting to see that there is a modern option to a 45 year old design.

Thanks

jonny,
dont let west get to you.
EVERYONE here knows how he is and everyone here reading this can tell you know what you are talking about and that west just likes to argue.

the good thing that came out of this is it made you go back and do a test that definitively proved you were right.

As has been said about the Permatune and the MSD systems, but yet no real
demonstrable benefit over the Bosch CDI has ever been provided over the years.

Not sure, either way, widening the spark gap would be a close equivalent to a combustion chamber maximum pressure, and with an air/fuel mixture.

But the truth of the matter should be easy to discern.

Adjust the O-scope timing displaying the rise-time of the secondary voltage such that the firing point, HV level, of the plug gap can be seen in both cases.

http://forums.pelicanparts.com/uploa...1449685186.jpg

Science of Performance Spark Plugs - Page 2 of 4 - DSPORT Magazine

west,
the question was about current draw of a CD unit, he proved it.
not how much voltage is required to fire a plug.

you have probably scared a great new guy from posting again. 103 posts, he has not been around long enough to know what kind of A$$ you are. personally i think it is funny when you make yourself look bad but maybe he does not, not to mention the insults were not called for.
thats usually the last resort for someone that can not admit they were wrong.

Yes, and my contention was that CDI current draw goes up with RPM (to which no one disagreed), and engine load, effective compression level. The dispute arose over the latter.

The chart I posted shows quite clearly that the initial spark voltage for firing a plug has a 2:1 difference, pressure-no-pressure.

You may wish to discredit the chart but common sense indicates that there will be a difference.

The chart shows that you need more voltage to fire the plug under higher compression. I don't dispute that. It is true, all day long.

To remind you, your argument was that if the CDI didn't 'need' the extra voltage to jump the gap, that energy was 'saved' and returned to the capacitor, hence you summised that as the cylinder pressure increased the CDI boxes used more current. This is false.

If this were true I invite you to place your finger (or other precious appendage) on a spark plug electrode. By your reasoning, there would be no gap, thus no voltage required to jump it and the energy would be 'returned to the capacitor'. *

You are more than welcome to borrow our test kit to do this experiment as long as you post your findings here, preferably with pictures.


* Seriously, don't even think of trying this folks!!!

Again the base subject matter was CDI current draw/load...

And I stated that RPM and compression was/is part of that equation.

Are you now conceding that point...?

Just to cover the bases: If it takes a higher voltage(***) to fire a plug under compression vs not then more power is required as compression increases.


***Higher secondary voltage results from more current flowing in the primary vs lower HV firing point. More power into the coil.

Look at page 45 figure 30 of the linked document.

The bottom diagram indicates that a diode is used to "syphon" off the energy remaining in the coil once the initial arc extinguishes. The diode also serves to "clamp" the coil primary at ~12 volts, ~1200 volts at the secondary, no second arc.

Now, remove the "clamp" diode and add one, fast switching, in parallel with the SCR, anode to ground. This new diode is often referred to as an "energy recovery diode". That may result in a second arc, but if not, ALL of that +1/2 cycle unused energy will partially recharge the capacitor

But, let's disregard both diodes for a moment, pretend they don't exist.

Now we have a circuit most representative of the OEM Bosch CDI.

Now once a spark occurs, and is extinguished, the magnetic field begins to collapse, the coil polarity reverses, and now our current flow circuit encompasses the highly inductive, low resistance, inverter transformer's secondary. So the level of reverse current will be slowed enough that no second spark occurs and the unused energy will be used to return the capacitor to a partial charge.

Once the current flowing in the inverter transformer's secondary decays enough the inverter will restart.

Now sure how or why you think this bares on the base question other than a higher firing voltage will result in more energy being used per cycle thus the partial charge will be lower.

The CDI has no idea what the engine is doing or reacts in anyway, it's just a ~400 VOLT ONE SHOT. The only thing that could possibly happen is there would be less voltage delivered to the COIL, because of High REVS, and that's it. Instead of the points taking the brunt from the coil, Porsche decided to intergrade this device. It's very efficient and my cars have never started or run smoother. I have a setup in my 72 Montreal which has 2 CDI's, but after repairing a few for some fellow Alfistis, I decided to install them in both my 75's and this is what I learned. The Coil wire must be ROBUST, I burn't out 4 of them, but figured that out. The COIL as well must be ROBUST as well. Busted 1 of them as well, what a light show. The max current is 5 Amps, but for a short period at very high RPM. Measured this with a differential scope probe. You ask for the max! YES the average is ~3 Amps. I had to do this before I installed it in my Race Car as well as my Sedan. Here's my prototype setup in my OLD Winta Beata. She's in ALFA Heaven. I also tested the all CDI's I repaired with this setup. Figured I better road test them.

http://forums.pelicanparts.com/uploa...1449700569.jpg

Wwest, you continue to miss the point again and again. RPM causes current increase yes. I pointed that out in post #9. Loading, no.

I'm sorry, I can't make it any simpler for you. We will have to 'agree to disagree'

But you have admitted that the HV level needed to fire the plug increases with combustion chamber pressure....

Any junior engineer would therefore conclude, absent other factors that might be in play, that as the firing voltages increase so does the power consumption. The chart indicates that as pressure increases the firing voltage went from 10KV to 20KV.

That would seemingly require a doubling of the coil primary current flow, therefore doubling the level of power stored in the coil's magnetic field just prior to spark initiation.

No, the current in addition to the base, 2A, will double.

Keep up the good work and don't bail.

Something is stuck in his craw. Rising voltage thing?

He like to wrestle.

We need folks like you here.

Also, the most violent push-back from Willy is "gifted" to those who know something. For real delivery and consistently produce performance in the real world environment. No charge. It's a gratuitous service he provides. We are lucky to have him. :D

You therefore, have made the grade. Look at the bright side.

Awesome, bro (and so true)! :D

he has sucked you in like a black hole in outer space.
he is like being on crack, you cant get off it and the more you use it the worse it is for you. he kinda brings you down to a new low.
you are now one of us.

I have offered up an explanation for my position that CDI power input increases with a rise in pre-combustion pressure. "Explanation" (see chart), to which Jonny agreed to be factual...

Still, Jonny states that CDI input power does not increase with pre-combustion pressure, without offering any explanation for that conclusion.

Can anyone of you offer up an explanation for Jonny's belief..?