8mm or 7mm spark plug wires? Does it matter?

[Eric Falk]

I have a 56 Sedan Deville with a 365 engine.  Stock spark plug wires are 7mm but I want to add some color to the engine bay using colored spark plug wires.  The only ones I can find are 8mm and contain silicone.  Does anyone know if the 8mm wires will perform as well as the 7mm wires?

Thanks in advance for your thoughts and information.

[Brandon]

If you are not concerned with authenticity, they'll be better than the 7mm ones as I think they are much better insulated from the heat and are superior in conduction.
I have them on my Continental Mark 111 (460ci)-lots of heat trapped in the engine bay-and they perform well.

[TJ Hopland]

Most wires dont give you enough info to truly compare them.  Most of the time the MM size is the OD of the insulation.  In theory as long as 1/2 the insulation is greater than your spark plug gap you should be fine because the spark is going to go to the shortest distance to ground.  0.0394" = 1mm so if you are running a gap of 0.035"  a 3mm wire may be too thin (assuming a 1mm core) but a 4mm should be safe in a perfect world.  There are other dynamics at work like mixture and compression and factors in the insulation materials but if it would work in free air as long as the insulation is something that wont conduct or absorb moisture that should make it even better.   

I have read that the thicker insulation does help for heat issues but the issues they are talking about are headers which tend to be much hotter than what we are going to find under the hoods of our cars.  If you are not going for a show car I dont see any downside to going to the larger wires, now days they dont even cost more. I have used modern wires on tractors and boats that came with solid wires and it didnt seem to hurt anything.   Best thing I have found for older stuff is to buy a set that only has one end on and you cut the wire to length and crimp on the other end.  Some sets even come with a crimp tool you put in a vise.  They also sell fancy crimp tools.  If you are carefull you can do it with a pliers.  I have always got the neatest results doing it that way, even the direct fit sets dont seem to fit quite right.