Plug wire resistance on 63 L84

[Michael G.]

I'm having some issues with my dated repop plug wires so I thought I'd replace them. I'm looking at two different brands, one where the resistance measures ~7 K ohms per foot, the other measures ~ 2 K ohms per foot. I know less is better here, but is there a minimum resistance desired on these wires?

Thanks, Mike

[Jim L.]

Re: Plug wire resistance on 63 L84

I'm having some issues with my dated repop plug wires so I thought I'd replace them. I'm looking at two different brands, one where the resistance measures ~7 K ohms per foot, the other measures ~ 2 K ohms per foot. I know less is better here, but is there a minimum resistance desired on these wires?

Well, on my cars I run zero Ohms/foot.... solid core wires.

The purpose of the resistance is to limit the ignition current which thereby lessens radiated emissions .... ignition noise in the radio. If you are also running resistor plugs, and I bet you are, I don't see the need for you to add resistor wires also.

Now if you are running some finicky aftermarket ignition, like an MSD 6AL, or perhaps a rev limiter, there may be some reason the resistance of the plugs would be inadequate. Otherwise, solid core wires or whatever is available and inexpensive, ought to work fine.

Jim

[Michael G.]

Re: Plug wire resistance on 63 L84

Thanks Jim,

Now the other issue: when I measure the resistance of some of the dated wires, it is initially ~7 K per foot, then goes immediately to zero. I'm guessing that this is a defective wire, but I'd be interested in any comments.

Mike

[John H.]

Re: Plug wire resistance on 63 L84

Thanks Jim,

Now the other issue: when I measure the resistance of some of the dated wires, it is initially ~7 K per foot, then goes immediately to zero. I'm guessing that this is a defective wire, but I'd be interested in any comments.

Mike

Mike -

I recall going through three complete (new) sets of the dated repro wires several years ago the night before the Kissimmee Regional to come up with eight wires that were good, and they were fresh out of the box.

[William C.]

Re: Plug wire resistance on 63 L84

Actually, the resistance of the wires in used to suppress radio waves from the wires, resistor plugs will not do the trick for an AM radio. That said, the resistance of a particular wire will vary with then length of the wire, as the specification is stated in ohms per foot of cable length, so the longer wires running to cyls 1-2-3-4 will have a significantly higher resistance than the shorter ones at the rear of the engine. The design spec for the cable was 3000-5000 ohms per foot. Based on that, there is not a problem with a given wire at 7-8000 ohms (or as much as 25000 ohms) as long as it is consistant within a given lead. If the circuit goes open if the wires are disturbed (and you are CERTAIN that you have a constant GOOD connection to the wires with your meter) then there is a problem with the wire. The longest leads in a smallblock set at just under 4 feet long, and the shortest about 2 feet, so there is quite an expected variation in a "normal" set as designed.

[Jim L.]

Re: Plug wire resistance on 63 L84

Actually, the resistance of the wires in used to suppress radio waves from the wires, resistor plugs will not do the trick for an AM radio. .

Bill, I respectfully disagree. Resistor plugs and solid core wires are what I run in my '54 and I don't get ignition noise in the radio.

And it's not that I'm so close to the transmitting antenna that the radio signal swamps the noise signals. Quite the opposite. I'm about 50 miles away from the nearest transmitter.

Jim

[Michael G.]

Re: Plug wire resistance on 63 L84

Actually, its very odd. The resistance goes to zero without moving either the plug wire or the meter wire. This only happens with a few wires in the original set of dated wires. It does not happen with any of the wires in the new non-dated sets.

I'm not worried about radio reception, I'd prefer that the engine didn't miss, so I may get satellite

Mike

[Jim L.]

Re: Plug wire resistance on 63 L84

Actually, its very odd. The resistance goes to zero without moving either the plug wire or the meter wire.

I'm not doubting that the wires might be defective, but the symptom you report isn't consistent with the way resistance devices fail. They fail to an open-circuit condition, which would be infinite resistance. If you are seeing a true, zero Ohm condition, something else is going on.

Jim

[Michael G.]

Re: Plug wire resistance on 63 L84

Thanks Jim, I'm gonna try a different meter, but I think the reading will be the same because the meter I've been using is very consistent in finding the wires where resistance goes to zero.

I'm really asking because I'm looking for an excuse not to change the ones that go under the motor mount, They're a pain.

Mike

[Jim L.]

Re: Plug wire resistance on 63 L84

Thanks Jim, I'm gonna try a different meter, but I think the reading will be the same because the meter I've been using is very consistent in finding the wires where resistance goes to zero.

How are you measuring this resistance? i.e. to which points are you connecting your meter leads? Let's be sure your technique is appropriate for the measurement you want to make.

Also, what kind of meter are you using?

I'm really asking because I'm looking for an excuse not to change the ones that go under the motor mount, They're a pain.

I understand that. I really do.

I have a neighbor who has owned a couple of C4s. He bought a '71. It needed plug wires. He sputtered and quacked for days about his realization that they ran the plug wires between the motor mount and the block. Dumbest thing he'd ever seen. On and on....

Jim

[Jack H.]

Re: Plug wire resistance on 63 L84

Adding on to what Clup said, the notion of designing resistance into the spark plug wires IS to thwart radiated noise associated with the spark plugs firing. From an electrical engineering view, a square wave (spark plugs turn ON, stay on, then turn OFF) is the mathematical equivalent of an infinite series of the sum of the square wave's fundamental sinusoidal frequency and ALL of its sinusoidal odd harmonics, appropriately attenuated, and added together.

So, the plug wires can/do act as active radio transmission antennae covering the RF range. That poses an interference potential BOTH to the radio in your car as well as the radio in car next to you on the road.

That's one of the reasons ALL Corvettes had their radio antennas located at the REAR of the car (get the antenna AWAY from the engine compartment). It's also the reason why RF shielding was used in the engine compartment.

Using resistive based plug wires is a method of reducing the radiation profile of the spark plug wires. The resistance is created by making the wire's inner conductor out of carbon doped stranded nylon vs. raw copper.

As such, these resistive wires age with time (carbon burns out here/there creating 'gaps' in the conductive path) and they eventually need to be replaced. Measuring them to determine good/bad, can be tricky...

Their resistance profile can change with temperature as well as with physical routing. Physically flex a suspected 'bad' wire and it might change in terms of measured resistance!

[William C.]

Re: Plug wire resistance on 63 L84

Jim, the issue is not with the radio in the owners car, the federal standard is to deal with radiation passed to other cars, TV sets (remember the 1950's?) and the like. Kind of arcane, but ALL the vehicle manufacturers used some form of resistance wiring in the 50's-60's to meet the federal regs. The outfit I worked for made the wiring for GM and Chrysler, Ford had another supplier for a similar product.

[Jim L.]

Re: Plug wire resistance on 63 L84

Jim, the issue is not with the radio in the owners car, the federal standard is to deal with radiation passed to other cars, TV sets (remember the 1950's?) and the like.

This is a new one on me, Bill. I've never heard of there being limits on radiated emissions on autos produced in the '50s and '60s. Is there an SAE spec or an FCC standard that described these limits? By intuition, it seems that if the ignition system doesn't radiate enough energy to disrupt the closest radio... the one in the same vehicle.... then radiation to an adjacent vehicle would automagically be a non-issue.

In my industry we didn't have to worry about radiated emissions from electronics until, oh, the early '80s as best as I recall.

Jim

[William C.]

Re: Plug wire resistance on 63 L84

Yes, since I retired, I don't have access to the specs anymore, but I was told by the original developers of the TVRS wire (in the early 1960's when I was involved in the manufacturing operation) that it was specifically designed to meet an FCC spec in the early-mid 50's. Remember that at least by 1955, all gm including corvettes was using the resistance wire, originally the wires were marked "GM Radio" then TVRS was trademarked "Television-Radio Suppression" The FCC got involved specifically in response to complaints about TV interference due to autos in the early 1950's. As an aside, in the mid 80's when HEI had become the industry standard, some of our engineers had learned that out move from 7mm to 8mm diameter cable was counterproductive due to the way high voltage was actually moved along the surface of the cable, I met a VERY drunk ford ignition engineer in the bar at a hotel one night. In tho course of a casual conversation I asked him why they had gone from 7mm to 8mm cable about when we did, and his response was "we did it because you guys did!" I didn't mention that we were moving in the other direction!

[Ken A.]

Re: Plug wire resistance on 63 L84

I'm having some issues with my dated repop plug wires so I thought I'd replace them. I'm looking at two different brands, one where the resistance measures ~7 K ohms per foot, the other measures ~ 2 K ohms per foot. I know less is better here, but is there a minimum resistance desired on these wires?

Thanks, Mike

Delco/GM spec's for TVRS were 3500 ohms/foot.