• From: Austin, Texas
  • Registered: 2004-09-18
  • Posts: 161

Re: Why use a larger spark plug gap?

Scott Novak wrote:

After the spark has formed and the voltage across the gap has dropped to a few thousand volts, most of the Bosch coil's output is wasted by the high internal loses of the coil.  A Jacobs coil, which has much lower losses, has the capability to put out over 3 times the voltage and current than the Bosch coil does into a normal spark.

Unless I missed something, why is it beneficial to have the super-high voltage anyway? Hotter spark, good, I understand that. But if the voltage to jump the plug gap is 5k, or 8k, or 12k or whatever, then any excess voltage over that is not even utilized. If the stock ignition was designed to work within these parameters stated above, then why would I benefit from the Super-Duper-Mega-Blaster-2 coil with 20,000 volts output?Am I mistaken?

Last edited by John N (2006-01-20 16:27:19)

'68 Beetle, now off the road, awaiting resto.

Everything you need to ask is already answered here
  • From: Los Angeles, CA
  • Registered: 2005-08-21
  • Posts: 49

Re: Why use a larger spark plug gap?

John,

From what I have learned through research and real-time experimentation, the coil output, ignition type and distributor all must be matched to take advantage of the extra coil output, those items all being matched....high output coil, good electronic ignition, good quality cap on the proper distributor...then you can open up the gap on the plugs and take advantage of the hotter spark, more complete combustion and therefore better performance and fuel economy...the devil is in the details, as you can see by reading the many posts on this subject...which coil, with which ignition and distributor gives you the best peroformance...its an ongoing discussion, but with the ideas presented here, I've been able to experiment with my 71 SB and am getting closer to the right combination for me...probably you can try some different combos and see what works best for you too.

Tom

  • From: Minneapolis, Minnesota
  • Registered: 2005-10-15
  • Posts: 288

Re: Why use a larger spark plug gap?

  • From: Austin, Texas
  • Registered: 2004-09-18
  • Posts: 161

Re: Why use a larger spark plug gap?

Scott Novak wrote:

Ideally, you should choose an ignition coil with a turns ratio with just enough voltage to jump the spark plug gap under the worst conditions.  It will give you more output current than a higher voltage ignition coil and you will develop higher spark power.

Wouldn't the Bosch coil have been designed with that in mind?

'68 Beetle, now off the road, awaiting resto.

Everything you need to ask is already answered here
  • From: Minneapolis, Minnesota
  • Registered: 2005-10-15
  • Posts: 288

Re: Why use a larger spark plug gap?

John,

The Bosch coil ignition system was designed with economy of manufacture in mind.  Electronic ignitions hadn't even been invented.  Low loss magnetic suppression ignition wire with a spiral conductor wound over a permeable coil didn't come into the marketplace until the 1970's.  Detroit had more powerful Kettering ignition systems than Bosch ever made for the Beetle.

I'm not sure what the turns ratio is with the Bosch coils.  The Bosch coil has too many internal losses to work great.  It wasn't designed with high performance in mind.  You can walk into any auto parts store and buy a generic ignition coil with a separate ballast resistor that will out perform any stock Bosch ignition coil with the internal ballast resistor.  Coil size DOES matter.  Larger gauge lower resistance wire takes more space and costs more.  You need a bigger package and better magnetic core material than Basch uses.

A Jacobs Ultra coil can put out 3 times more current than a stock Bosch coil with the same ignition system into the same load.  I've measured this.  You can buy a Jacobs used Ultra coil typically for between $25 and $30 bucks on e-bay.  For a 4-cylinder engine operating at less than 8,800 RPM, the Jacobs Ultra Torquer coil puts out even more current.  It has a lower turns ratio.

It's not really an apples to oranges comparison between the normal Bosch coil and a Jacobs variable magnetic coil.  The Jacobs approach is vastly superior.

Scott Novak