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Old 11-27-2017, 01:15 AM   #1
55superbeetle
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Default Spark plug wire question

So on my latest lsx swap project I plan on running the coils under the dash for a few reasons so I obviously need to make new wires. Because I like to make everything harder I plan on using ngk screw on wire ends for the plug side and the normal crimp on deals for the coil side. The ngk ends only come with a built in 5k resistor as they are normally used with a solid core wire. The solid core stuff I am used to on german cars either had a resistor rotor or another resistor built into the coil side of the wire along with the resistor on the plug end for RFI suppression. Will I be ok with a solid core wire with only the 5k resistor on one end and if not will adding 5k to normal suppressive wire effect spark output. The coils will be within a foot or two of the ecu so I'm sure rfi suppression will be important. Just not sure how all that stuff plays out. I will also be running a resistor ngk plug.
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Old 11-27-2017, 01:42 AM   #2
gsellstr
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Running a resistor plug AND resistor ends should easily suffice, even with solid core wire.

Might hit up Kingsborne, they may have some other options for ends if you are still planning. Might be able to get non-solid core resistance wire with non-resistor ends as well.
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Old 11-27-2017, 04:41 PM   #3
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More is not necessarily better. Additional resistance does not alter the ability to fire the gap on the plug; but, it does alter the spark current wave shape by extending the discharge time and slightly reducing the available spark energy. Suppressing RFI is a good thing; but, I would be inclined to start with one of; resistor plugs or resistor plug caps or suppression style wires. I would treat the suppression style wires as a last choice because the amount of suppression is affected by the length. Short wires will be less effective than long wires. At least with resistor plugs or caps you are getting a known quantity with equal values on all cylinders.

I am guessing that these are logic switched coils that you are using? Since you are locating them in close proximity to the ECU, make sure that the primary current supply to the coils is routed well away from the ECU sensor wiring and power supply wiring. The primary current gets switched at the same frequency as the high voltage side of the coil and while it may lack the high electrical field associated with the secondary spark plug wiring; it more than makes up for it with equally problematic high frequency magnetic fields in the coil primary supply. They don't call it EMI (Electro Magnetic Interference) without reason.
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Old 11-27-2017, 05:02 PM   #4
Dirty Rick
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Mounting the coils under the dash is a recipe for fail.

Your dash electronics will love the increased EMF radiation!
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Old 11-28-2017, 03:10 PM   #5
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Quote:
Originally Posted by Dirty Rick View Post
Mounting the coils under the dash is a recipe for fail.

Your dash electronics will love the increased EMF radiation!
I was thinking the same thing; but, the OP did say "Because I like to make everything harder", so that fits.

The engine compartment being surrounded by metal does a modestly effective job of shielding stuff that is outside of the engine compartment from EMI. Moving the coils and a portion of the spark plug leads to the cold side of the firewall circumvents some of that shielding. Resistor plugs, caps, plug wires can reduce EMI; but, not eliminate it. Locating the coils inside the passenger compartment may work; but, does seem to be an invitation for problems.

Another thing to consider. I don't know the construction details; but, I would expect that you might end up with some fairly long spark plug wires. Longer spark plug wires increase the inductance of the wires which can alter the spark discharge current waveform. This might actually help reduce the radio component of EMI; but, might have some performance issues if you push the boundaries of what is considered normal practise.
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Old 12-02-2017, 04:46 PM   #6
55superbeetle
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Thanks for the input. I think I will reconsider and find another place to put them. I have just seen a few other builds that have them under the dash with no ill effects. I'm not planning on a radio or anything but I am going to use a raspberry pi computer as a digital dash so I don't want to end up with issues.
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