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Ive tried both sizes of wires and seen no advantages with an 8.5MM.
In fact the 8.5 MM was a pain to stay on the distributor cap.I had to take a razor blade and skin the sides of the boots thinner.
It was too fat at the boots and with a hypertech coil or performance coil pack on top,it will be even harder to fit the wires on there.
(May vary on brands,I used Magnecores for 8.5)
8.5 also wont fit wire connectors and youll need to get larger ones.A few times the boots popped off the dist. cap from being too fat there.
Id stay with 8mm, and thats what Im using now and it fits great and no problems.