Could you do twin spark ?

[Patriq]

I've been concidering changing the top on my block to a top that came later in production with two sparkplugs per cylinder.
Could you connect two coils in parallell to the same EDIS unit and run twin spark ?

[brentp]

Patriq,

Yes- you should be able to run a second EDIS module and drive the PIP signal to both units. the SAW signal could come from either module.

I can't say with certainty that you can drive both EDIS modules with one VR sensor, but it should work.

[Patriq]

Oh, I was thinking only one EDIS module, but two coils. That the one EDIS controls two coils that work in parallell.
Since there are two sparkplugs that are supposed to spark at the same time in the same cylinder, couldn't one EDIS control two coils ?

[DannyP]

Brent's reasoning is sound. Use two EDIS, it is only designed to handle one coil which happens to fire two coils, don't forget this is a WASTED spark system. If I don't go full EFI, I will do two EDIS and coilpacks controlled by one Megajolt. This is a twin plug Porsche 911 six cylinder.

[Patriq]

Ok,
I'll probably go that route then if I go for twin spark.

Thanks for the input !

[Oliver Sedlacek]

Firstly, an EDIS (or nearly any other spark box) wont drive two coils in parallel. These boxes drive a fixed current into a coil, so if you wire up two coils in parallel, each coil gets half the current. As the spark energy stored in a coil is proportional to the square of the current, each plug will only get a quarter strength spark!

I've just fitted an Electromotive XDI to a Porsche flat 6, and the XDI instructions highlight an important point about twin plug, wasted spark systems. The instructions say that you must not connect both outputs of one coil pair to one cylinder, and the other the other coil pair to the opposite cylinder. You must wire one coil pair to plugs on both cylinders, and then wire the other coil pair to the other plugs on the cylinders. This is because a wasted spark system is specifically designed to fire one plug at compression pressures when the other plug is at atmospheric pressure. If both plugs on a coil pair are at compression pressures, they may not spark properly.

[DannyP]

You are right, Oliver. I should have said two EDIS and two SETS of coilpacks. And it is easy to forget that you must alternate the coilpacks to the cylinders. I will remember that, thanks.

[alexander]

Firstly, an EDIS (or nearly any other spark box) wont drive two coils in parallel. These boxes drive a fixed current into a coil, so if you wire up two coils in parallel, each coil gets half the current. .

hi oliver, i can see that the Electomotive documentation says their product limits the primary current to a fixed maximum, but do you have any reason to think that the EDIS does? it may well, but i dont recall reading anything on that point regarding EDIS. if, OTOH, EDIS simply grounds the primary and does not have any current limiting circuitry, then one could run an additional coil pack in parallel. that would then charge both coils, and double the current through the EDIS, leaving the question solely as to whether the EDIS could handle that. do we know if it can or cannot? perhaps it can. given the low cost of the EDIS modules, i would be trying two coil packs and one EDIS first off, for simplicity and neatness. if the EDIS died, then the question would be answered, and for inconsequential outlay.

alexander.

[Oliver Sedlacek]

The EDIS definitely limits coil current, primarily by dwell control. The time a coil takes to charge to its rated current is predictable, so the EDIS starts energising the coil a fixed time before the spark is needed.

[alexander]

hi oliver. if EDIS controls current flow by controlling the dwell time, that means it only controls the length of time current flows through the primary. that is the same as setting the points gap, right? if that is all it is, the EDIS is not limiting the maximum current. so if you put another, identical, coil in parallel, it will also have the same voltge applied to it for the same length of time, and will also charge to the same energy level. total current through the EDIS will double, in the absence of current regulating circuitry.

to me, it still sounds like the EDIS can operate two coils, subject to the current carrying ability of the EDIS.

alexander

[alexander]

i am curious about the idea that, when using two coil packs to drive twin spark plugs, that you must alternate the plug leads as oliver described. no doubt Electromotive have some reason for saying that, but i am struggling to come up with a theory as to why.

AFAIK, wasted spark coils are in fact one coil with a plug lead on either end. to the current path is out one lead, through the first spark plug, through the metal of the head, back through the other spark plug, up the other lead, and through the coil. it is a closed loop. further, the energy level to which the coil charges is, as noted, determined by the coil and the EDIS. point is that the when the coil discharges, the total current has to pass all points in that circuit, equally. that means that the same spark must jump across each spark plug, and it must be the same amount of energy whether it is discharging into air or highly compressed fuel/air mixture.

so why would it make any difference which way the leads are connected?

any ideas?

regards
alexander.

[brentp]

Alexander,

The air gap on the compression stroke has a higher electrical resistance than one at atmosphere. If both plugs in a wasted spark pair fire into a compression stroke, the overall resistance across both plugs is higher, correspondingly reduces current and therefore spark energy... possibly to the point of mis-firing.

I was just using my test bench for other purposes and remembered I had a high-current (100A) shunt I could use to monitor current consumption. I'll hang two coils in parallel on an EDIS-4 module and take some measurements with the scope, and post back here.

[Oliver Sedlacek]

Alexander,
On a contact breaker system, the dwell is set at a constant angle, not time. For example, with at 600 rpm the firing interval is 50 milliseconds (4 cylinder conventional engine). Setting the dwell to 45 degrees (50% duty cycle) means that the coil charges for 25 milliseconds. At 6000 rpm all the numbers scale by a factor of ten, so the coil charges for 2.5 msec. This fixed dwell angle leads to some serious compromises, as the spark energy drops off as the coil doesn't get fully charged, whilst at low rpm the excess current heats the coil up.

A fixed dwell time system is far superior, and may be designed around, for example, a coil that charges in 4 milliseconds. At 6000 rpm the duty cycle is 80%, and at 600 rpm the duty cycle falls to 8%. These coils would burn out if they were charged for long periods like 25 milliseconds, but the EDIS takes care of this problem.

[alexander]

oliver: yes, fair point and i agree with you wholly on that. what i am getting at regarding the EDIS running two coils in parallel, however, is this: that part of the EDIS is a switch, which applies battery voltage to the primary coil for the dwell time, whatever that may be, which charges the coil. then it switches off, causing the discharge in the secondary. if you connect two coils in parallel, the EDIS will charge both of them to the same extent as if there was only one. it will, however, double the current through the EDIS. likewise if you had 3 coils in parallel. i think that would work, save only that the EDIS might not be able to handle the heat caused by the higher current passing through it.

brent: good! some data would be interesting. re the spark resistance. now i am being rather speculative here, based on what i have garnered on the less than perfect Net, but... i agree that highly compressed fuel/air mix has a higher resistance, but i think the effect of that is that a higher threshold voltage across the plug gap is required to ionise the gas in between and allow an arc. after the arc has formed, however, the resistance is negligible. the energy built up in the coil is a function of the coil, the applied voltage, and the dwell. as oliver has pointed out, that is fixed. so, however you look at it, the same amount of energy has to dissipate every spark event, and i dont see how cylinder conditions can change that. a certain voltage is induced in the secondary; once the arc has formed resistance across the plug is very low (dont know what value. anyone?); initial current flow is then V/R, which is initially very high due to high voltage and low total resistance in the loop.

the crux of the issue is that i believe the arc, once formed, to be of very low resistance in all cases. which would mean cylinder conditions (ie which stroke) makes no difference. and, dissipated energy should be the same no matter how the plugs on a twin coil setup, are arranged. OTOH, the documentation of the Electrotive product says otherwise, and i have learnt in life that rank amateurs generally should not dismiss the words of professionals too lightly!

so contrary thoughts and opinions welcome....

regards
alexander

[Oliver Sedlacek]

It doesn't work like that. The EDIS adjusts the dwell time so that the coils charge up to a pre-determined current. If you put two coils on in parallel, the inductance is halved and the target current is reached in half the time. The EDIS will set the dwell to this shorter time, and it will not provide more current.