Plug wire resistance on 63 L84

[Jack H.]

Re: Plug wire resistance on 63 L84

You're recalling the advent of the FCC's Part 15 rule making that was rifle shot at both commercial and consumer appliances that were deemed to be 'computer' equipment (makes/generates digital pulses in excess of 10 KHz). Yep, that rule making came along in the 80's and it drove electrical designers NUTS because few had experience with 'hushing up' the conducted and radiated noise profile of their products...

If you've had the 'pleasure' of cracking open a copy of FCC Part 15 and studying it, you'll find automobiles were EXEMPT. That's because Detroit lobbied HARD and there was already a set of standards in place. That's what Clup is refering to when he said resistive plug wires had been around since the late 50's...

Gosh, I can remember the advent of Part 15 clearly! We had to send our electrical designers back to 'school'... The digital guys had no earthly idea of the consequences of putting ad hoc digital waveforms on a bus and routing the bus every which way through the product's maize of circuit boards. One of the things taught in the 'back to school' class rooms was transmission line theory + impedance matching to thwart reflected echos.

If I remember correctly the 'trigger' event for this FCC rule making was the introduction of digital games. I seem to remember a police episode of hot pursuit in Las Vegas where inter-cop radio communication broke down and the bad guys got away. After the FCC sent out RF vans to track/trap the source(s) of radio interference, it was the new electronic slot machines in the gambling casinos that caused the lion's share of unintended/unregulated RF 'trash'...

[John H.]

Re: Plug wire resistance on 63 L84

Back in the mid-70's when CB radios were a big thing, I had a 100-watt linear under the dash in my Nova for occasional use (it would light a fluorescent tube held within an inch of the antenna on the deck lid when switched on and the mike was keyed). Back then, the first production car with Bosch electronic fuel injection was the VW 411 "squareback"; if I was next to one at a light, all I had to do was flip on the linear and key the mike and the VW would stall. Yup - big-time RFI.

[Jim L.]

Re: Plug wire resistance on 63 L84

Mike's original question was:

....is there a minimum resistance desired on these wires?

And so far that question hasn't been answered. A practical answer is something in the 4000 Ohm range for the total resistance between the coil center terminal and the sparking plug.

A simple thought experiment involving two data points is all that's needed to see how I came up with this number.

Data point #1 is Bill's mention that the GM spec for TVRS cable calls for a minimum of 3000 Ohms/foot.

Data point #2 is an observation that Chevrolet built a lot of 6 cylinder Chebbies with AM radios that worked just fine. These used TVRS cable and non-resistor sparking plugs. For cylinders numbered 3&4 on an old 235, the plug wire can be as little as about 8" long. The coil was located very close to the distributor on many applications and an 8" coil wire, also made of TVRS, would almost certainly reach.

So, 16" of 3000 Ohm/foot TVRS wire is adequate to suppress ignition noise. This length of wire would have an end to end resistance of:

3000*16/12 = 4000 Ohms

But what about my earlier suggestion that solid core wires and resistor plugs is also adequate?

I couldn't find a spec for the resistive element of any sparking plug in any of my catalogs (Delco, NGK, Champion, Autolite, etc....) so I picked a couple of AC plugs at random and just measured them. One measured 4700 Ohms and one was 5600 Ohms.

While a sample size of two is obviously not statistically significant, it does reveal what resistance ballpark we're dealing with. And, it shows why, if a good minimum of 4000 Ohms overall is adequate, that metallic core wires and resistor plugs is also adequate to control ignition noise.

Jim

(No Fourier series was harmed in the creation of this posting.)

[William C.]

Re: Plug wire resistance on 63 L84

Frankly the answer was given fairly early, the GM spec for the resistance cable was 3000-5000 ohms per foot of cable length, and the lengths vary by location from approximately 2 feet to just short of four feet in lenght.

[Jim L.]

Re: Plug wire resistance on 63 L84

Bill,

That depends on whether Mike's trying to duplicate the GM spec for the plug wires or to simply achieve enough end-to-end resistance to suppress ignition noise. If he's running resistor plugs, there is no practical minimum resistance for the spark plug wires.

Whichever answer he actually wanted, he now has both.

Jim