[Miata] Spark Plug Wire Routing

Thu May 29 06:56:43 CDT 2008

I guess people that do this for a living are wrong and your right?  And BTW,
there was nothing SHORT about your answer.  

http://www.jasperengines.com/pdf/SparkPlugCross-FireTB.pdf

http://autorepair.about.com/cs/troubleshooting/a/aa090602a_2.htm

http://www.popularmechanics.com/how_to_central/automotive/1272351.html?page=
2

http://www.aa1car.com/library/ford_firing_orders.htm

Larry

White Knight      1991 Crystal White   #99 CSP
Silver Bullet        1992 Silverstone     #17 SM2  FM I+ Turbo
Honey B             1992 Sunburst Yellow #99 SM2L  JR Supercharger
Whooosh           2004 Titanium Mazdaspeed MX-5

LowCountry Miata  http://www.lowcountrymiataclub.net
Masters Miata
RAGS 074

-----Original Message-----
From: miata-bounces at realbig.com [mailto:miata-bounces at realbig.com] On Behalf
Of entropycat at verizon.net
Sent: Thursday, May 29, 2008 4:46 AM
To: aaMiata List
Subject: [Miata] Spark Plug Wire Routing

Greetings to the list!

   I seem to remember a Peanuts cartoon where Lucy was filling her kid 
brother Linus with all sorts of wrong information, e.g. that telephone poles

were specially bred trees that the phone company grew in place to hang its 
wires on, and other gems.   Every time she came up with one of these 
nuggets, Charlie Brown, in an aside, would complain "Ow... my stomach 
hurts".

   Maybe it's just my ulcer.... but I feel a need to stick my oar in. 
Following, please find a very short and incomplete course on transmission 
line theory.

   Transmission lines are used to move radio frequency energy from place to 
place.   They generally fall into three categories:  coaxial line, 
waveguide, and open wire line.   Of these three, only the latter need 
concern us in the matter of spark plug wires.

   Parallel or open wire transmission lines have the characteristic that if 
used to conduct alternating current, the currents in the two wires are equal

but opposite in direction.   This means that the magnetic fields around the 
two wires are also equal and in opposite directions, and therefore cancel. 
This has two benefits:  one, the line does not tend to radiate its energy 
away, and two, it does not exhibit large amounts of self inductance, which 
would be characteristic of non-parallel wires or loops or single conductors.

   Twisted pair wires are a subset of open wire or parallel transmission 
lines.   They have the advantage of keeping the distance constant between 
the conductors (necessary for any open wire line to work properly) while 
bending around corners.   They also do not require periodic spacers to 
maintain the correct distance as is the case with independent conductors 
forming a transmission line.   This is their ONLY advantage.   Therefore, if

you have the spacers and supports (as in the Miata) there is no advantage to

twisting the wires.

   Now here is another interesting thing:   Parallel wire transmission lines

need to have the circuit completed in such a way that the current returns in

the other wire of the pair.   Otherwise, there is no difference between a 
parallel line and a single line.   In the Miata ignition system, the coil is

double ended.   Each coil connects to a pair of cylinders (if memory serves,

they are 1 and 4, and 2 and 3).   Therefore, the Miata fires a pair of 
cylinders together... one on the compression stroke just before TDC, and one

on the exhaust stroke at the same point.   (Of course, only the compression 
stroke firing is significant.)   Therefore, in the Miata, the current path 
is from one end of the coil through the plug wire, across the gap in the 
plug, through the aluminum head, across the gap in the other plug, back down

the second plug wire, and back to the other end of the coil.   Neglecting 
what sort of impedance this pair of spark gaps represents with respect to 
the characteristic impedance of the open wire line formed by the pair of 
plug wires, this is a complete circuit in a pair of parallel wires and 
therefore exhibits some of the benefits of parallel wire transmission lines.

   This is not the case in a typical American V-8, for instance, or any 
engine with a distributor instead of double ended coils.   In these engines,

the current path is a big loop.   Current flows from the coil through the 
distributor rotor and cap to the plug wire, thence across the plug gap and 
into the engine block, which is grounded to the coil, forming the return 
path.   In this case, crossing the wires avails you very little.   Magnetic 
coupling between wires carrying unrelated signals (as in this case) is 
reduced if the wires cross each other at right angles, but repeated crossing

will just bring the wires closer to parallel with resultant coupling.   In 
this case, you have two remedies that actually work:  one, you can 
physically separate the wires as far as possible from each other, or two, 
you can run coaxial shielded wire as is done on aircraft.    Anything else 
is of the same order of usefulness as magnets on the fuel line.

   In reality, transmission lines of all sorts radiate if they are not 
terminated in a resistance which is equal to the characteristic impedance of

the transmission line.   Since the characteristic impedance of a spark plug 
and gap has to be a lot higher that the impedance of plug wires with any 
reasonable spacing, all of this is mostly academic.   In other words, run 
your plug wires any way you like.... it won't amount to a hill of beans 
worth of difference.

Regards,
Jim T.
entropycat at verizon.net
'74   914
'90   Miata  (x3)

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