funny u should mention that

since im restoreing a model A i love anything article wise dealing with em. a guy found a ford stamped 2 plug per cylinder head for it

also when i removed the head off my engine there was one plug that had two grounding prongs towards the electrode... goes to show you that has been done repeated times over;)

I had a '75 Chevy sportvan with a 4bbl 350, and in my quest for better mileage, I got Taylor wires, and an MSD coil. I opened the gap (at night) in increments until I could see the spark arcing to the heads, then I closed them again until the arcing stopped. I also indexed them. I got 21-23 mpg in that thing with all of it's huge windows.
My friend just got rid of an old Firetruck that had a V-12 with 2 plugs/ cyl. 24 plugs!

My Father-in-law had a Datsun or a Nissan pu truck, must be about 15 or so years ago, that had two plugs per cylinder.

Dave

The Civic Hybrid uses 2 plugs per cylinder as does a few Mercedes and all new HEMI engines.

Not all engines had both of them in the cylinder though. Some of them just had a plug in the exhaust that was there to fire in the middle of the exhaust stroke for emissions.

I was messing around over the summer, testing my battery. I disconnected all my wires to see what the bat's voltage dropped to over a long crank. my coil pack was throwing 3 in. arcs that were visible in the day (shade). At the same time i checked my wires resistance, and found that one was bad. there is a break in the conductor somewhere, but it's arcing inside the wire somewhere and works fine (It's on my list of things i should buy, still).

I was thinking, if it could through a arc that long, why even have a gap. if the plug didn't have the ground thing the ark would jump much further (it would be sweet if it jumped to the piston) This might be really bad for my coil pack, which i can't afford to replace right now, so i haven't played with this yet.

I really can't see why fancy wires would do anything. It's just a wire, as long as it has a good insulator and a conductor. And at 10kV or so, a damp piece of twine would be a fine conductor.

The plug gap has to be smaller because it's harder for the spark to jump the gap in the cylinder when there is air/fuel mist present. It's harder for a spark to jump the gap when the air/fuel charge is more dense, which is why turbocharged cars tend to specify smaller plug gaps, such as .028" on my car or .024" on the Mitsubishi EVO. A general rule is to decrease the gap if you're engine is misfiring.

Plug wires need to conduct electricity and they also need to make sure the electrical charge goes down the wire and not into the surrounding metal parts. So it's a great conductor surrounded by a great insulator, which makes it slightly more expensive than damp twine.

As I was thinking about this topic last night when I should have been sleeping. I thought about how much harder it is on the disrtibutor cap & rotor, when you have a hotter coil, and increased gap. This is kind of troublesome for me, since they don't make my cap anymore. I don't know if I can back fill the Aluminum points.
About the wires, it was explained to me that the wires are designed to move the energy, by having the electrons in them already, or somesuch. I didn't understand it, fully, but it made sense at the time.

Really good insolation is key, but if you've ever taken a wire apart (or ripped one i half, as i have), the conductor is wimpy. Most have less wire than a standard 22gage wire.

I would think that compressed air (with out fuel) would be a better conductor, as there are more atoms therefore more electrons in the given gap. A vacuum is the greatest insulator, as there is nothing for electricity to flow through.

The gap width would change the temp of the arc. Greater resistance equals fewer amps, with the same voltage this means less power. Power(watts)=V*i(amps) and i(amps)=V/R
so a short gap would be hotter, if all else remained the same.

That's crazy that the EVO uses a .024" gap. My car has a very similar engine (with out the bells and wissles, oh or power) and it has a recommended .044 gap

Ok a few points you guys are missing . . . first the xB has coils on top of each plug with no wires between it and the plug so it is burried in the head pretty well and only fires when the ECU tells it to on the power-compression stroke. You should have had some free power if there was propane in the air - it would add to the fuel and result in some fuel trimming if it was there long enough.

As far as larger spark gap it works like this. The bigger the gap the higher the voltage needed to jump the gap. If you make the gap twice as big you can spark with twice the energy making it ignite the A/F mixture better in the case of lean mixtures. This assumes that the voltage doubles and the current stays the same which may or may not happen since the energy of the spark is determined by the coil and the magnetic field that is collapsing to make the spark. The coil only holds so much energy but put in a high output coil and you have it. Caps and rotors have to pass the energy to the correct plug and usually that is more a problem with higher output coils and larger plug gaps since the current and voltage increases but usually they can handle it easily until the damp day comes along or the inside of your cap gets some carbon buildup in it. Spark gap is where the performance happens more than any where else - bigger gap helps to ignite the fuel under load or at idle as long as the coil can provide the voltage to jump the gap. Iridium plugs with the thinner center electrode help make the spark jump at lower voltage because of the e-field is higher on a smaller tipped electrode (a sharp point is ideal) and it is made out of platium or iridium to withstand the higher more intense spark temperatures so they will last longer. MSD Multiple Spark Discharge helps because it will spark multiple times to better ignite the fuel and can be more intense (more current) and remember the A/F mixture is swirling around in the cylinder head.
Higher pressure makes it harder for the plug to spark and high vacuum makes it easier up to a point then as there is less and less molecules of gas to ionize it gets harder to spark - neon lights have a partial vacuum so they light up so if they leak a little air back in they will fail to light up.
Dual plugs were an option and an easy modification on BMW Boxer engines and were done to cause a double flame front to ignite the fuel faster. This allowed less timing advance in high compression engines to reduce spark knock but gave a faster burn to get more power. There really is no need in todays car engines.
Spark plug wires, for those out you who still have them, have to insulate the high voltage and carry the high currents that the coil puts out. The high current is only short in duration but is still present. The wire also tends to generate some e-fields making radio and TV noise so strands of fiberglass (I think) coated with carbon dust were used to reduce the radiation that it generates. Fiberglass was used probably to prevent the wires from stretching.

UPDATE -

New coil pack finally arrived. Installed a regapped the now one week old copper plugs to .060in (from .044in). Although the idle is smoother, no FE or power improvements jumped out at me. Ill monitor the averages the next few days to know for sure, however.