Spark Plug Gap and Voltage

[Rock10]

We have a 1936 40. It was converted to 12 volts by previous owners.

I finally got around to changing plugs a few weeks ago. When I looked up the gap, I noticed that Buick changed from .025 to .032 at the same time the changed to 12 volt systems. That makes sense as the 12 volt coils  produce more high voltage than the 6 volt. So I increased the gap to .030 on the new plugs and the engine runs smoother.

Just wondering what everyone thought. I am thinking of going to .032.

[edinmass]

Define converted……….primary and secondary coil voltage really doesn’t make much difference. Compression ratio and resistance across the gap is probably the reason for the change….as well as a hotter plug. Usually 6 to 12 conversions are poorly done, and you probably have a 6 volt coil still in the car. That’s the problem with unnecessary modifications. You really don’t know what you are working with. Did they jam an alternator in it? 

[Rock10]

Don't want to get into a 6 vs 12 argument.

The car had a new harness, 12v battery, starter, generator, most bulbs, and COIL. I recently replaced the coil with one that has an internal resistor.

I went with a standard Champion 516 (D16) plug.

The bigger gap will allow for more complete fuel burning. I would imagine a 6V coil can't jump a .032 gap. That's why they are set at .025.

Again, in 1953, Buick changed the gap and I think that's when they changed to 12V.

[EmTee]

If I recall correctly, my '38 shop manual recommends a gap of 0.25~0.28", (I forget which).  It does say, however, that the gap can be opened up to something like 0.30~0.35"  (again, I don't have it in front of me).  My point is the reason that is given in the book for opening the gap is to address a 'rough idle' complaint.  I have mine set to the minimum recommended gap and have no issue with idle quality.  It seems somewhat counterintuitive that the wider gap would give smoother idle, but maybe a somewhat 'fatter' spark helps in that regard?

 

Edited March 26, 2023 by EmTee
typo (see edit history)

[Bloo]

It has nothing to do with 6v vs 12v really. The "higher voltage" thing is just marketing hype for aftermarket ignitions. What a higher voltage coil gives you, assuming low enough RPM and enough dwell (time) to charge it properly, is a higher "ceiling", so to speak. In other words, if the coil wire is disconnected, how high will the voltage rise before it stops rising and peters out.

 

All else being equal, If you put a 50,000 volt coil on a car, almost nothing will change. If the coil draws more current it will burn up the points faster. In practice, it is the spark plug gap that raises the voltage, nothing else. Well, nothing except bad ignition parts that are effectively also raising the gap the spark must jump. A old car probably runs around 8000-15000 volts, depending on the gap and to a lesser extent compression ratio, load, rpm, etc. The voltage can be measured on a scope. It is approximate. If the voltage rises to the "ceiling" the plug will not fire. This will happen under heavy load (lugging) or high rpm or both. A higher "ceiling", as given by a fancy coil, does nothing unless you were hitting the "ceiling" and the engine missed under load.

 

1 hour ago, Rock10 said:

The bigger gap will allow for more complete fuel burning.

Not really. Maybe a little bit. Exhaust analyzers don't lie about that.

 

1 hour ago, EmTee said:

My point is the reason that is given in the book for opening the gap is to address a 'rough idle' complaint.  I have mine set to the minimum recommended gap and have no issue with idle quality.  It seems somewhat counterintuitive that the wider gap would give smoother idle,

At the risk of opening a can of worms, that's a timing issue (burn time). The difference between .035 and .045 gaps in systems of the 1970s-1980s era can be noticed running in the shop.

 

1 hour ago, Rock10 said:

The bigger gap will allow for more complete fuel burning. I would imagine a 6V coil can't jump a .032 gap. That's why they are set at .025.

I would imagine that too, at least under heavy load, and did imagine it until I found out a bunch of guys on the VCCA forums were running .040 on stock early 30s stovebolt sixes with stock coils. There are a bunch of caveats in those VCCA threads I won't go into here because it may not apply. I'm running .040 on the Pontiac and have been for a couple years now. I am not suggesting you do that on a Buick. Running the system at a higher than stock firing voltage means the insulation in the ignition parts will have to hold that voltage back. The parts were not designed for it and the insulation may not last. That goes for the coil too. The terminals inside the cap and rotor may be too close together to prevent crossfiring, especially on humid days. The short spark plugs I use might not have enough porcelain to use with bare terminals at the higher voltage. So far, none of that has bit me, even in heavy rain, but it might.

 

Buick is a special case. While their good looking enclosed wiring was not unusual in the mid 30s when voltages were really low, it breaks ALL the rules of how you should route ignition wires. Running them parallel risks electrical coupling. Running them close together risks tests the limits of the insulation. Running the wires inside grounded metal REALLY tests the insulation. Bare ignition terminals running that close to the cover are like hanging a "Kick Me" sign on your back. Electricity always takes the easiest path. Taller spark plugs move the terminal even closer to the metal, and depending on heat range you may not be able to get any plugs as short as the AC plugs of the 30s. It just barely worked then. I've seen those covers spaced away from the engine to stop the arcing. In the 50s with high compression engines and wider gaps Buick was BEGGING for trouble, and trouble there was and is. There are scads of old threads on this forum about ignition problems related to these issues.

 

So @Rock10, you and I are both asking for trouble from unwanted ignition coupling between cylinders and from insulation failure. Since my wires are held far away from anything grounded on an insulator, I have only the coil, cap, and rotor (especially the rotor) to worry about. You have literally all of it.

 

Edited March 26, 2023 by Bloo (see edit history)

[EmTee]

14 minutes ago, Bloo said:

Buick is a special case. While their good looking enclosed wiring was not unusual in the mid 30s when voltages were really low, it breaks ALL the rules of how you should route ignition wires. Running them parallel risks electrical coupling. Running them close together risks tests the limits of the insulation. Running the wires inside grounded metal REALLY tests the insulation. Bare ignition terminals running that close to the cover are like hanging a "Kick Me" sign on your back.

I agree 100%!  I thought I detected random misfire on my '38, so I insulated the cover studs with pieces of 5/16" fuel line and added 90° boots to the spark plug wires.  That seemed to help.  Like I said above, I'm running the plugs at 0.28" as the book suggests, so that should lower the 'demand voltage' to something the system can handle, while taking into account the long, parallel routing and tight quarters under the spark plug wire cover.

[Rock10]

49 minutes ago, Bloo said:

So @Rock10, you and I are both asking for trouble from unwanted ignition coupling between cylinders and from insulation failure. Since my wires are held far away from anything grounded on an insulator, I have only the coil, cap, and rotor (especially the rotor) to worry about. You have literally all of it.

 

Modern cars run high voltage ignitions with no problems.

Our wires are new with boots on the ends. The coil is new as is the wiring in the distributor. My main point was that Buick saw fit to increase the gap when they changed to 12 volt systems. And I noticed an improvement in the smoothness of the idle and when you rev the engine.

I'm not suggesting everybody regap their plugs on vintage equipment. Just if you have converted to a 12V coil, you may benefit from a larger gap.

Really didn't want to start a war.

 

[Ben Bruce aka First Born]

6 hours ago, edinmass said:

Define converted……….primary and secondary coil voltage really doesn’t make much difference. Compression ratio and resistance across the gap is probably the reason for the change….as well as a hotter plug. Usually 6 to 12 conversions are poorly done, and you probably have a 6 volt coil still in the car. That’s the problem with unnecessary modifications. You really don’t know what you are working with. Did they jam an alternator in it? 

 

 Sad! Assumptions.

 

  Ben

[Ben Bruce aka First Born]

46 minutes ago, Rock10 said:

Modern cars run high voltage ignitions with no problems.

Our wires are new with boots on the ends. The coil is new as is the wiring in the distributor. My main point was that Buick saw fit to increase the gap when they changed to 12 volt systems. And I noticed an improvement in the smoothness of the idle and when you rev the engine.

I'm not suggesting everybody regap their plugs on vintage equipment. Just if you have converted to a 12V coil, you may benefit from a larger gap.

Really didn't want to start a war.

 

 

 I am with you, Rock.

 

   I went to 12V, internal resisted coil 10 years back.  20,000+ miles. No problems. I run a gap of .040.  

  My engine is not much different from yours.     Enjoy your BUICK!

 Thank you for taking the time to  {try to, anyway} educate us.

 

  Ben

[Rock10]

1 hour ago, Ben Bruce aka First Born said:

 

 I am with you, Rock.

 

   I went to 12V, internal resisted coil 10 years back.  20,000+ miles. No problems. I run a gap of .040.  

  My engine is not much different from yours.     Enjoy your BUICK!

 Thank you for taking the time to  {try to, anyway} educate us.

 

  Ben

Thank you.

[Bloo]

2 hours ago, Rock10 said:

Modern cars run high voltage ignitions with no problems.

Our wires are new with boots on the ends. The coil is new as is the wiring in the distributor. My main point was that Buick saw fit to increase the gap when they changed to 12 volt systems. And I noticed an improvement in the smoothness of the idle and when you rev the engine.

I'm not suggesting everybody regap their plugs on vintage equipment. Just if you have converted to a 12V coil, you may benefit from a larger gap.

Really didn't want to start a war.

 

No war here. As I mentioned earlier, I'm doing it too. If you are getting away with it more power to you. It does feel smoother. You made a small change. I'll bet at .040 you could have trouble with Buick's wiring setup. Maybe not.

 

Modern cars don't enter into it. No manufacturer would do today what Buick did, It worked fine the way they sold it, so I'm not really casting any shade on them. In my opinion (you know how that works, everybody has one) Buick should have got rid of the shields and parallel wire routing decades before they did. Yes it's traditional and yes it looks good, but by the 40s and probably quite a bit earlier the engineers at Buick HAD to know better.

 

I doubt 12 volts had anything to do with it either. There is a limit to how much current you can switch with points, and that is what limits the "ceiling", at least if you want the points to last at all. In theory, doubling the voltage should have doubled the power available (Watts = Volts x Amps), but in practice almost nobody did that. They used a ballast resistor and ran the system on about 7.5 volts, about the same coil voltage as a 6 volt car that is running and charging. If you can run wider gaps on 12 volts you can do it on 6 volts too, because there is no difference in the supply voltage and very little difference in the coil. That point would not have been lost on Buick's engineers.

 

The first real limit is the insulation, or rather how high can you take the spark voltage before things start coupling and firing 2 plugs at once, arcing over to ground, crossfiring in the distributor cap, burning a hole in the rotor to the grounded center shaft, and so on.

 

The second limit (at about .035-.040" gap) is how much current the points can handle. When you design a coil to charge with more energy during the dwell period so the "ceiling" can be higher, it draws more current. I recall Smokey Yunick described in his book "Power Secrets" the lengths he and his team went to in order to continue using points ignitions (rather than magnetos) in high-RPM V8s in professional racing back in the late 60s and 70s. In the end, with 8 cylinders and 7000+ RPM, it just doesn't work. The dwell times are too short to charge the coil. It gets exponentially worse as RPM increases, and trying to up the current enough to compensate for that makes slag out of the points. You just need an electronic switch to even attempt it.

 

Fortunately, few if any of our antiques or street cars spin fast enough or have enough cylinders that points can't do the job fine on either 6 or 12 volts.

 

A third limit is at about .060, when it becomes nearly impossible to hold the spark in even when you really work at it. GM tried .080 on some Oldsmobiles(?) in the 80s. They were using 8mm silicone wires, an electronic (HEI) ignition module with current limiting, a coil fed with 12 volts via heavy wire and a distributor cap the size of a lard tub. It just couldn't keep the energy contained, and I believe they eventually recanted. Mechanics learned right away to set those at .060 or less.

 

Edited March 27, 2023 by Bloo (see edit history)

[Morgan Wright]

8 hours ago, Rock10 said:

We have a 1936 40. It was converted to 12 volts by previous owners.

I finally got around to changing plugs a few weeks ago. When I looked up the gap, I noticed that Buick changed from .025 to .032 at the same time the changed to 12 volt systems. That makes sense as the 12 volt coils  produce more high voltage than the 6 volt. So I increased the gap to .030 on the new plugs and the engine runs smoother.

Just wondering what everyone thought. I am thinking of going to .032.

 

What matters is the voltage at the spark plug, not the voltage of the battery. The number of turns on the secondary coil determines the voltage at the spark plug. The whole 6 V versus 12 V makes no difference, you can make a million volts with a 6V battery if you want.

[Rock10]

T

3 minutes ago, Morgan Wright said:

 

What matters is the voltage at the spark plug, not the voltage of the battery. The number of turns on the secondary coil determines the voltage at the spark plug. The whole 6 V versus 12 V makes no difference, you can make a million volts with a 6V battery if you want.

Buick changed the gap size when they changed to 12 volts.

The larger gap made our car run smoother.

[Morgan Wright]

3 minutes ago, Rock10 said:

T

Buick changed the gap size when they changed to 12 volts.

The larger gap made our car run smoother.

 Totally new engine, they switch to 12 V for the nailhead V8 and the 6 V was for the Fireball straight 8.

 

Totally different engines. 

[edinmass]

1 hour ago, Ben Bruce aka First Born said:

 

 Sad! Assumptions.

 

  Ben

Nope……..I have seen hundreds of them done poorly, and few done well. Like the Packard I converted back last week. And as Bloo and I pointed out……..the coil being 6 or 12 volts doesn’t make a difference. Bloo is certainly the best go to guy here on most electrical issues, and absolutely the best at describing from a scientific standpoint on how most systems work. The car should have never been converted in the first place. Obviously if he bought it that way, you work with what you have. I don’t want to beat a dead horse, or dump on someone’s car. Tuning the car with a five gas machine would be the best way to play around with plug gap…….but the performance envelope of the engine probably would dictate that the gap only has little influence on its performance overall. 

[Ben Bruce aka First Born]

Rock, you can't win with some folks.  But you asked what they thought, lol.  Now don't EVEN mention the alternate to carburetor!!  Stroke time for some.  Personally, I like mine. 

 

  I am glad we can all go our own way, doing what we want. 

 

  Ben

[Rock10]

You all have a great day.