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How Much Heat Can A Plug Wire Take

 
Old 02-10-2019, 11:58 PM
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6T2Vette
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Default How Much Heat Can A Plug Wire Take

I'm running a sbc 350 on something other than a Corvette, but the problem could be universal. I'm using 7mm cables with right angle plugs. No problem, they fit OK, but I'm using Sanderson hugger headers and they're running very close to the plug wires and the plugs themselves. I've wrapped the tubes with thermal wrap but they still give off a lot of heat, so the wrapping will keep the wire insulation from getting scorched but will the intense heat break down the conductivity on long trips?
Bob
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Old 02-11-2019, 07:09 AM
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Frankie the Fink
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The wires will harden and crack given sufficient heat - there is a reason big block plug wires are wrapped in insulation...
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Old 02-11-2019, 08:13 AM
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If by "tubes" you mean the rubber boots they cover the metal terminal and except for preventing shorting, they don't have a lot to do with the plug wires conductivity. They are usually a pretty highly temperature resistant rubber but as Frankie said can get hard and brittle after awhile. The thermal boot protectors are about the best you can probably do other than buy a different wire with a different boot configuration or angle. With the protectors - as long as they are not in direct contact with the headers - they should last OK if it's a good quality plug wire.
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Old 02-11-2019, 08:23 AM
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The wire will be fine at any temperature, but the wiring insulation will break down with heat.

If you stay below the insulation melting point, you'll be fine - for awhile.

The exact issue is how long will the insulation last for a given temperature. Or more properly, how many miles/years can one get out of a set of wires?

Note that Chevy used heat shields between the plug wire boots and ram's horn exhaust manifolds, and even then over time the insulation would harden and break down.

You can probably research your wire vendor's specs to check out their temperature rating, but expect to replace them with some frequency.

An easy check is the night time engine running check - watch for blue arcs running along the boots to ground.

There are plug wires with higher than average temperature ratings sold.

Last edited by Easy Rhino; 02-11-2019 at 08:24 AM.
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Old 02-11-2019, 08:56 AM
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Depends on the materials used in the wires. Silicone insulation resists breakdown at high temperature, a lot higher than factory neoprene and whatever stuff they used between the outer jacket and the conductor core. I wouldn't worry too much about conductor breakdown in any case. Most conductor cores are made from graphite impregnated glass fiber, either bare or covered with a winding of resistance wire like nichrome. These materials just don't break down at temps near an exhaust pipe as long as there's some air gap between the wire and the pipe. Using the wire wrap will more than likely keep you out of trouble.
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Old 02-11-2019, 09:46 AM
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Originally Posted by Frankie the Fink View Post
The wires will harden and crack given sufficient heat - there is a reason big block plug wires are wrapped in insulation...
I thought the main reason for BB wires "wrapped in insulation" is for radio interference. If I'm not mistaken BB cars with radio delete do not have the SS wraps.
Please let me know if I am off base here.
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Old 02-11-2019, 09:50 AM
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Originally Posted by woodsdesign View Post
I thought the main reason for BB wires "wrapped in insulation" is for radio interference. If I'm not mistaken BB cars with radio delete do not have the SS wraps.
Please let me know if I am off base here.
Maybe so. Ive never owned a big block Corvette.
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Old 02-11-2019, 10:09 AM
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Originally Posted by woodsdesign View Post
I thought the main reason for BB wires "wrapped in insulation" is for radio interference. If I'm not mistaken BB cars with radio delete do not have the SS wraps.
Please let me know if I am off base here.
You're correct. They are braided wraps for grounding.

Big block heads for C2s actually used heat shields that secured to the head with a hex-head screw. Seemed to be mostly unique to Corvettes - not sure how long they lasted or what other Chevy models they were used on - if any. I see lots of BB heads that don't have the drilling for the retainer screws including my GM open chamber aluminum heads.

Last edited by DansYellow66; 02-11-2019 at 10:13 AM.
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Old 02-11-2019, 10:35 AM
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Originally Posted by DansYellow66 View Post
You're correct. They are braided wraps for grounding.

Big block heads for C2s actually used heat shields that secured to the head with a hex-head screw. Seemed to be mostly unique to Corvettes - not sure how long they lasted or what other Chevy models they were used on - if any. I see lots of BB heads that don't have the drilling for the retainer screws including my GM open chamber aluminum heads.
That's prob what I'm remembering.. In which case, even the small blocks had heat shields...
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Old 02-11-2019, 11:33 AM
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Originally Posted by DansYellow66 View Post
You're correct. They are braided wraps for grounding.

Big block heads for C2s actually used heat shields that secured to the head with a hex-head screw. Seemed to be mostly unique to Corvettes - not sure how long they lasted or what other Chevy models they were used on - if any. I see lots of BB heads that don't have the drilling for the retainer screws including my GM open chamber aluminum heads.
Yeah, that's exactly what the braided sleeves were for and yeah the shields are bolted to the heads with a 1/4-20 hex bolt in about the 2 o-clock position about the spark plug hole on the exhaust port deck. I wish someone made stainless steel repops of the braid. Heck I wish someone sold just the braids. I've already reused mine with aftermarket magnetic suppression plug wires. The repops (not sure of the originals) are made from tinned copper. The tin tarnishes just like tinned bundy weld brake lines - fully functional but they get dull with age. The heat shields AFAIK were used on all 65/66 BBs. They were dropped for 67 and later models. 65 and 66 BB Corvette plug wires have clips at the plug end to connect the braid end to the shield. 67 and later wires had a lug at the end to put under valve cover bolts. The braids perform the same function as all that lower shielding on SBs.
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Old 02-11-2019, 11:54 AM
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I remembered the 67 BB cars have the plug end of the braid grounded to the valve cover bolts (as I had to do on my 66 due to aluminum heads) instead of to plug shields but couldn't recall if 67 heads had the bolt holes for the shields or not. Thanks for refreshing my memory.

I also wish you could just buy the braided shields. I put 8 mm Taylor wires in mine which are bullet proof but the braided covers don't look as fresh as they did 20 years ago.
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Old 02-11-2019, 11:33 PM
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It's awful tough to beat these Design Engineering 010512 Protect-A-Boots Spark Plug Boot Protector Sleeves, 6" - Black (Pack of 8) for $33
  • Helps prevent burn-through, hardening, and cracking of boots from under-hood heat
  • Withstands 1200F direct continuous heat
  • Non-flammable, protects against chemicals, and resists water
  • Double-wall construction for added durability
  • Universal-fits most straight or angled boots; installs in seconds
Amazon Amazon



Good luck... GUSTO
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Old 02-12-2019, 01:35 AM
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Originally Posted by 6T2Vette View Post
I'm running a sbc 350 on something other than a Corvette, but the problem could be universal. I'm using 7mm cables with right angle plugs. No problem, they fit OK, but I'm using Sanderson hugger headers and they're running very close to the plug wires and the plugs themselves. I've wrapped the tubes with thermal wrap but they still give off a lot of heat, so the wrapping will keep the wire insulation from getting scorched but will the intense heat break down the conductivity on long trips?
Bob


I built a 421 small block for a friends 63 Impala SS. They chose the block hugger headers. There are several brands some have plenty of plug clearance and then there one set that they apparently never gave any thought to anyone ever putting spark plugs in. I wound up shortening the center 4 spark plugs to get any clearance between the header and the boot. Otherwise they were just going to touch period. It does have Brodix straight plug heads but no header should be that close to the engine. Another friend has 2 different brands and they both have adequate room.
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Old 02-12-2019, 05:22 PM
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Originally Posted by GUSTO14 View Post
It's awful tough to beat these Design Engineering 010512 Protect-A-Boots Spark Plug Boot Protector Sleeves, 6" - Black (Pack of 8) for $33
  • Helps prevent burn-through, hardening, and cracking of boots from under-hood heat
  • Withstands 1200F direct continuous heat
  • Non-flammable, protects against chemicals, and resists water
  • Double-wall construction for added durability
  • Universal-fits most straight or angled boots; installs in seconds
https://www.amazon.com/Design-Engine...52211089&psc=1



Good luck... GUSTO
Given that those are so inexpensive, I might just put them on a normal set of '65 L76 spark plug boots under the radio shielding, since I don't want to even have to replace those wires and have to pull them through the lower V-shielding.
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Old 02-12-2019, 10:50 PM
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Originally Posted by woodsdesign View Post
I thought the main reason for BB wires "wrapped in insulation" is for radio interference. If I'm not mistaken BB cars with radio delete do not have the SS wraps.
Please let me know if I am off base here.
I agree - the braided steel wires and steel tubes on BB's are for radio interference shielding
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Old 02-13-2019, 12:02 AM
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I use Header Insulators and a fiber woven sleeve over the insulation! I have wire numbers to hold the slip on sleeve! I use Porsche loops to guide the looms!
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Old 02-13-2019, 10:53 AM
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Those covers look good, I used Moroso on my 85- not cheap but work
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