13 replies to this topic

[Tom Ghan]

I've been driving my '92 donor for a few months, identifying and fixing issues, before the build get serious, and Ive been puzzled by spark plug wire to coil layout. The plugs are connected to the coil pack this way, when standing at the front of the car:
2 3 1 4

but all the documentation & service manuals says it should be :
4 1 2 3

Car is running pretty good, I have about 1000 miles on it, and I don't want to guess and break something. so any guesses as to why my engine is special, or is this a Florida thing?


thanks
Tom

[Jim Boemler]

The car will run nicely either way. The reason is that the plugs (1-4 and 2-3) are wired in series, and fire together.

[Tom Ghan]

Thanks Jim,
didn't know that about the coils. The Haynes book is not right about the order of connections on the coils, singling out the 92 as the same as 1.8L. I think all 1.6 engines have the same order. I don't like to second guessing the manual. bottom line, car runs just fine the way it is, so don't mess with it.

[DES4]

It is a "wasted spark" ignition system, it fires at the top of each cylinder stroke. Each coil is shared by two cylinders; one cylinder at the top of its compression stroke, the other at the top of its exhaust stroke.

[davew]

The clown is the only post with a correct answer.

1.6 firing order is 3214, a 1.8 is 4123 Tom you must have looked up the wrong year car. Everything below is assuming a 1.6 engine. Theroy is the same for a 1.8, just the numbers will change.

The Miata up to 2000, uses a waste spark system. Meaning it fires 2 plugs each revolution. So the passenger side coil, which fires cylinder 3 and 2, fires at TDC compression on one cylinder and TDC exhaust on the other. The TDC exhaust is "wasted". This is why you can reverse the plug wires on a single coil and still have the engine run properly.

When the left coil fires, the spark/electrons leave the coil and go through the plug wire, through the coil and jump the plug gap at TDC compression of cylinder #3. This creates combustion and off we go. But those electrons need to get back to the coil. So they travel through the cylinder head to spark plug #2, jumping the gap from the ground electrode to the center electrode while this cylinder is at TDC exhaust. Nothing happens. But the electrons go through the plug and wire back to the coil. The circle of life!!! On the next rotation cylinder #3 will be at TDC exhaust and the spark will be wasted until those electrons come around and jump the gap at spark plug #2 to create combustion on that cylinder.

This why when a single wire or plug goes bad, we loose 2 cylinders.

Dave

[exninja]

Dave, are you saying that the cylinder head is completely insulated? Sorry that this is going beyond the scope of the OP's question, but i'd like to understand this. I thought the whole reason the engine is grounded is for spark return to the negative terminal. The electrons would "want" to take the less-resistive path to ground via ground wire rather than jump the gap in the other spark plug. Am I wrong on the Miata?

[Tom Ghan]

I got it now!

And here is why I asked in the first place, darn manual.

[davew]

My electrical theory is little rusty, but I will try.

The secondary side of the ignition coil is not connected to ground. So the electrons have to find their way back to the coil by jumping the gap backwards and going up the wire to complete the path

[Jim Boemler]

There's no need for insulation, exninja. The coils are floating, meaning that there's no ground connection at all. When the coil fires, one end goes positive with respect to the other. Since the block is electrically in the center of two plugs, and the centers are grounded, one plug fires with a positive voltage, and the other fires with a negative voltage (as measured from ground).

[DES4]

Edit: I guess we weren't understanding each other...

To clarify all this, they are on a firing loop.The ground/positive end of the spark [secondary voltage] could easily be either the cylinder at the top of its exhaust stroke or its companion that is at the top of its compression stroke. Polarity would be irrelevant, hence crossing the two ignition leads would not affect performance.

Ignition coils create spark on the secondary-side windings through inductance as the current through the primary windings is broken and the electro-magnetic field collapses.

[Jim Boemler]

You're quite right, Dave, the Miata's system is indeed wasted spark. I don't think the other Dave was disagreeing with you on that point. I think he was merely noting that the extra spark isn't what makes the plug wires interchangeable, it's the fact that the plugs are wired in series. It's a fine point, since it's likely that every wasted spark system does wire the plugs that way.

[davew]

Back in the beginning days of waste spark (mid 1990's) GM actually had front spark plugs and rear spark plugs on their sideways V-6 engines. Cavalier, Lumina, Grand Prix etc. The front plugs had platinum center electrodes and the rear plugs had platinum ground electrodes. Must have saved 2 cents per plug. All the replacements where platinum on both electrodes.

More useless auto repair trivia.

Dave

[exninja]

Learned something new. Thanks!

[Bench Racer]

More useless auto repair trivia.
Dave

And then there is the rotary engine two spark plug trivia.

Most rotary engines have two spark plugs per rotor. The combustion chamber is long, so the flame would spread too slowly if there were only one plug.

Along with this ^ tivia fact, the two plugs didn't burn equal when visual looking at the plugs.