One of the things I want to eliminate as a potential cause of my
high rpm  misfiring is EMI due to unshielded plug wires. I
 currently have Blue Max EMI  sheilded plug wires and just purchased
Magnecor shielded wires. What makes these  "shielded"? They have a
conductor with a very thin wire wrapped around the  insulation of
the core wire, and that is covered by another insulating layer,  and
that is covered with the outer sheath. But,my understanding is that the
  spiral wrapped wire is supposed to shield the inner wire, and the
way I have it  configured, with the inner conductor and spiral wrap
exposed and crimped in  contact with the teminal ends, doesn't the
spiral wrapped wire also conduct the  spark current and thereby
defeat the whole purpose? Isn't the shielding wire  supposed to be
grounded at one end to the engine and drain off any induced  current
from the high voltage secondary ignition lead, and be separate and
not  in contact with the    conductor? And if that is the case, how
am  I supposed to configure or  fabricate the terminals in such a
way as to ground the spiral wrapped shielding  wire?

  Alternatively, I do have some heavy braided wire that I could slip
over each  entire secondary lead and ground one end of or the other
to provide shielding.

  Comments and suggestions invited.

  Brian Trubee

  Yes the shield should be grounded on one end. It is insulated from
  the core. Some plug wires have a core that has some resistance
  winch is supposed to minimize radiation.

  By all means add the braid and ground on one end.

  I think somebody is selling spark plug shields.
  http://www.overnightautoparts.com/brand/msd.html
  http://www.innerauto.com/NGK_Brand/

  Paul Lamar

     
Be careful of VHF radio interference with only one end of the
shielding grounded. I had dreadful ignition static with my Mooney 20
J IO 360 after the mags had been off to the magneto man. The bonding
check was fine, but when the magneto man came back from holidays I
learned the new leads were only earthed at one end. All the experts
insisted that this could not happen, but it did, grounding the other
end cured the static  John Evans                   


Are you certain the "first" ground was good? If you ground both ends, the high
voltage can induce a current in the shield that can then cause other grief.

Dale Davies