Re: <ALL> Plug Wires (stock and high performance>

[Pete Read <pete.read@xxxxxxxxxx>]

A couple of years ago, Steve Castle asked me if I thought it was
worthwhile to change the plug wires on his well maintained, excellent
running, E34 M5.  I advised him to not waste his money (see my response
below for some ignition basics).  My main point is that newer cars
have very good electronic ignition systems.  If you find extra 
horsepower by changing to aftermarket wires, something is probably
wrong with your original wires.

Ignition Basics (original note) 
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My opinion on plug wires:   The original equipment wires are more than 
adequate and may last the life of your car.  The whole Bosch ignition
system (and most new car systems) can be considered high performance
compared to the old breaker point systems.  Those really did have a
problem at high rpms because there wasn't enough time between firings
to recharge the ignition coil.

A few facts:  It takes less than 15K Volts to fire a spark plug (nominal
voltage to jump the electrode gap).  The voltage needed is directly
related to the plug gap and any contaminants causing extra resistance.
The fuel air mixture also has some affect.  Optimum mixture takes the
least voltage, rich or lean takes a little more with lean being a
greater problem.  A system capable of 100K Volts wouldn't make the
car run any better.  It would still spark at 15K Volts -- the other 
85K Volts would just be unused reserve capacity for overcoming wear, 
extra deposits, and so forth.  

Your system should be able to produce at least 30K Volts or double 
the nominal voltage needed.  As the plugs wear, the electrode
gap increases and deposits form.  Even if voltage demand increases
30% as the plugs wear, less than 20K Volts will be needed.

The wear points in the ignition system are the distributor cap/rotor 
and the plugs (sparks across a gap).  Wires don't suffer much wear.
My guess is that most wire damage is from pulling them out when 
changing the distributor cap or possibly mechanical chafing.  I've 
also read that continued high voltage demands from lack of periodic
maintenance (plugs, cap and rotor) can eventually cause breakdown 
of the wire insulation.  

I'd only change wires if there is a misfire problem and the plugs,
cap, and rotor are checked first.  Misfiring is usually first
noticed at idle (non-optimum mixture because of valve overlap) 
and when accelerating (lean mixture when throttle first opened).
Resistance of the wires can be checked with a VOM.  That gives some 
basic information but won't help much with small cracks in the 
insulation.  Looking under the hood at night and spraying a small 
amount of water on the wires really does work in those cases.
 
  Pete Read
  '88 M5 
  Arlington, VA

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