spark plugs?

[tim_haynes]

guys is there a specific spark plug to use for track use? i have bosch fitted now, but im thinking of fitting a new ignition system, msd or something, my motor has a c30-94 charger and is 350bhp at the minute spark is struggling.

[GeoffBob]

Tim, a bit of techno-info first (apologies if I am teaching you to suck eggs):

The dielectric breakdown strength of any gas (or mix of gasses) goes up with pressure, meaning that the higher your CR or greater the intake pressure, the higher the voltage at which the plug-gap will break down into conduction (ie: arc over). One way to get around this problem (and help the plug to break-down under pressure) is to increase the electric field strength between the electrodes by closing the gap up and/or grinding the centre tungsten electrode to a point. Neither is really an ideal fix though since the first results in a weaker spark and the second in increased rate of erosion of the centre electrode.

If you simply want to improve the plug I would suggest looking at the iridium tipped Bosch plugs. I would however suggest that you consider a capacitor discharge ignition (CDI) unit as the best solution to this problem. I have one on my own car, where I run up to 1.5 bar boost . The CDI runs at 450V and discharges into a quad coil pack reclaimed from an E36. Throws a spark about four inches long on the work bench and will happily run up to 12,000 rpm (not that my engine would ever do that). At 7500rpm (my limit) the unit pulls a goodly 10A of current.

PM me if you want the details on the unit, I might have one spare that you can have (for free, I don’t trade on the zone) so long as you pay postage. I'm in Pretoria.

[tim_haynes]

You obviously know what your talking about unkike me!!

sounds great the cdi unit,

what does it involve, would be very interested if i can fit it myself.

[GeoffBob]

Yes, you would be able to fit it yourself, but there might be some issues with running it from your Go-Tech Pro ECU. You'll need to decide if you want to keep or do away with your distributor. I've PM'd you the details.

Regards
Geoff

[MillRat]

Good topic.

Geoff (or anyone else out there who is in the know), I use the Bosch iridium plugs with high voltage leads and a standard coil in my E30 race car (M20B29 engine, motronic 1.3 ecu with cam mounted distributor), and "as far as I am aware" I am not having any problems.

I have often wondered if a heavier duty coil would make any difference (my car makes about 200 hp at the wheels). Thoughts?

[GeoffBob]

Hi Michael,

The voltage required to "spark" a spark plug (that is to say, ionise the gas between the two electrodes and bring it into conduction) is a function of four things:

1) The electrode separation (gap size),
2) The shape of the electrodes,
3) The gas pressure, and
4) The Electrode temperature

Adjusting the gap is something that I think we are all familiar with, and it is commonly known that a smaller gap will "spark" at a lower voltage. As a rough guide it takes about 30,000 volts to spontaneously ionise (arc over) a 1 cm gap (made from two parallel copper plates) at 1 standard atmosphere of air. In other words, a 2 cm gap (in these same conditions) would take about 60,000 volts, and so on so forth.

What is less well understood is that the shape of the electrodes has a major effect upon what is known as the "electric field strength" between two electrodes, and that the tendency for a gap to "arc over" is a direct function of the "electric field strength", which is itself defined by both the voltage and the shape of the electrodes. As everyone knows, nothing sets off a nice electric discharge like a sharp pointy metal bit and high voltage. problem is, a sparkplug with a sharp pointy centre electrode will tend to erode very quickly, which is where the element Iridium comes in. Iridium is even more resistant to electrical erosion than Tungsten is, and so Iridium electrodes can be manufactured sharper (more pointy) than tungsten electrodes, and so you have a spark-plug that doesn't need as high voltage to make it spark ”“ Nifty.

The one that usually has everyone stumped, however, is gas pressure. As a general rule (at least at the sort of pressures we are working at) the electrical breakdown strength of a gas goes up with pressure. This means that if you operate a very high compression ratio (CR) or a forced induction (FI) engine, it will take a higher voltage to "spark" the spark plugs. This is, off course, only a problem on a FI engine when the engine is ”amaking boost”a, since it is then that the combustion chamber pressure is at its highest, just prior to the ignition of the air/fuel mixture. On a high CR normally aspirated engine it is a problem all the time, and accounts for why it can be so hard to start a high CR engine during cold weather.

The most effective solution to the issue of pressure and temperature is to fit a higher voltage ignition system (typically a capacitive discharge ignition) or to fit a coil with a higher turns ratio. Please note that, in accordance with the laws of physics, the voltage at which your spark-plugs "spark" or "arc" is defined by the four points I listed above, and NOT by your coil or ignition system. The only time fitting a higher voltage ignition system is of any use is if your existing ignition system is not generating enough voltage to "spark" your plugs under certain conditions (such as while making boost above a certain pressure, or in sub zero temperatures)

For example, lets say your existing electronic ignition system with existing coil will happily deliver up to 35kV, and your plugs are happily sparking at 25kV under all conditions. The output of your coil will thus never exceed 25kV, even though it has the "potential" to make it to 35kV if say, you widened up your plug gap or "boosted" the engine. Fitting a 50kV coil, say, will therefore do nothing for you since your plugs will continue to breakdown at 25kV. However, if you fitted a supercharger and suddenly found yourself in the position where, above 1bar boost, you need more than 35kV to spark your plugs, you would definitely need to look at fitting a higher voltage ignition unit or coil.

HTH
Regards
Geoff