Why use a larger spark plug gap? (Page 1 of 2)

Still trying to convince yourself I see.  So, how long did it take to copy that out of the Jacobs book?  --Ryan

Hiya Tom,

I too have a 30/31 sitting atop a 1600 dp.   Runs great!  And, like you, my mileage leaves something to be desired.  Last time I figured it it was coming in at just under 19 mpg.  I'm ready to go to a 34PICT3 (which oughta be on there in the first place) but I've been looking for some input (again, like you) on running a 30/31 versus a 34PICT3 and how the two carbies might compare on fuel economy.

To be honest, another reason I have not switched over is that with the 30/31 in place, this engine runs like the proverbial Swiss watch.  But the mileage issue is nagging at me, so the more info I have, the better decision I make.

So................Tom and I are wondering:  30/31 or 34PICT3?  How will they each affect mileage on a 1600 dp (assuming it's in otherwise stock condition)?

Mike

1970 AS Bug

You've already admitted that your info is from the Jacobs book, or at least you alluded to it.  What more is there to say?  As far back as 2nd grade I can remember it being drilled into me that unless what you wrote can be backed by none other than your own research you must cite your references, or risk being accused.  Would you care to share your method for studying flame front propegation within an ACVW combustion chamber?

God, I feel sorry for anyone that has to work with you in a professional sense.  I mean if anyone argues with your point of view do act like this in public?  --Ryan

I would like to work with Scott before RCB. Rcb always has a tude. The dude with the tude and quick to argue. My point. Name has to be hidden or my car will get keyed for I might not agree with RCB. I would "man up" and show a name, but my testicles are in my wifes purse (of course).

Tom,

The Compufire module will not put out anymore power, than the stock points, using the same coil.  You are switching the same 12 V to the coil without changing the dwell.  If you want more power output, you need to use a coil with lower impedance, and, or you can use an adaptive spark ignition system that will adjust the dwell time for the optimum spark, and, or use a capacitive discharge ignition that sends a higher voltage to the coil.

Once the spark has formed, the voltage across the gap drops considerably.  The voltage across the gap will be relatively constant at a given crankshaft angle.  However, the resistance across the gap will change with the current through the spark.  Also, the voltage across the gap will vary with changes in cylinder pressure as the crankshaft rotates.  Once the fuel mixture begins to ignite, the voltage across the spark plug gap drops even lower.

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. (I've measured this.)

Also, the low loss Jacobs coil can fire severely fouled spark plugs when the Bosch coil can't.  A Jacobs coil doesn't require perfect engine conditions to fire the spark plug, and keep it free of fouling deposits.

While the Bosch blue coil may have the voltage to fire a wider spark plug gap, with a spark plug in good condition, the current through the spark is much lower than with a Jacobs coil.  The Bosch coil puts far less power into the spark plug gap.

Platinum plugs will arc-over at a lower voltage than normal spark plugs.  As a result, there is less power in the spark plug gap, and less power to ignite the fuel mixture.  Fine wire electrodes will have the same effect.

If you use an improved coil and switch the coil with the Compufire, you will still need to use a ballast resistor.

If you use an adaptive spark ignition unit, such as a Jacobs, you can eliminate the ballast resistor for more output.  The Compufire module will trigger the Jacobs ignition (and others) quite nicely.

I tested a Jacobs Ultra Coil with resistance added to the primary to equal the 2.8 ohms resistance of the stock Bosch coil.  The ballast resitance caused the output to drop 16 % with 100,000 ohm load, which is in the range of a normal spark resistance before the fuel ignites.  The common Bosch Blue coil, has an internal ballast resistor which cannot be removed.

As cheap as you can find Jacobs ignition systems and ignition coils on ebay, I'd suggest that you try one.  If you don't like it you can resell it for what you paid for it or more.  I bought one Jacobs ignition system on e-bay for for $35 and sold it $132.50 two weeks later.  You just have to be a smart shopper.

Of course, once you've tried an adaptive spark ignition system like a Jacobs, it's not likely that you will want to go back to a stock ignition system.  At present, I've got two Jacobs ignition systems.  One is capacitive discharge and the other is an inductive switcher.  I haven't decided which one I like best.

Ask your well established mechanic why there should be any performance loss with a wider spark plug gap, if you have an ignition system that is capable of delivering a high current spark into a wider gap.  I have my doubts that you receive a logical answer.  I'd like to hear what ignition system he used to make his conclusions.

Scott Novak

After about 20 seconds of looking I found this.  I've already asked you point blank if you were quoting the book, here's your reply.

"What specifically is it that Jacobs says that you have a problem with?"

As I said, alluded to.

As for the gaps thing, you might want to check the other board you CC'd this to.  Two very respected and well seasoned VW guys just agreed with me.  Actually one went on to say that he's run up to .100 gaps with success, but he doesn't care if he burns the tips off the plugs because it a pure race engine.  However for his and his wifes drivers, he sticks to .035"-.040".  The other individual said .040" for a CDI.  Both agreeing with what I've already said, it's a waste.  --Ryan

Ryan,

You don't appear to read very carefully.  What Lawless said was that with a "STOCK STYLE" ignition, it doesn't make enough difference to worry about.  Even so, at .035" with a (Bosch) Blue coil, he still is running a larger than stock gap.  He also did NOT say that he used a .040" gap in his daily drivers.

He also stated that he likes to run the gaps as wide as possible, and that the gap is only limited by the secondary wiring and the spacings of the distributor cap.

Burning the outer electrodes off of the spark plugs has a lot more to do with running nitrous in the old days without good controls, than because he was running a .100 spark plug gap.

With an MSD ignition, wires and distributor, he said he's running around a .050" gap.  But he didn't mention which coil he was using.  Basically it's an unqualified answer, as we don't know if the ignition coil, or spark plugs are limiting his optimum spark plug gap.  I don't know what the internal spacings of the MSD distributor cap are and if it may also be limiting the maximum spark plug gap.

John Connolly said use a .040" gap with a CDI ignition.  Again the answer is unqualified as he didn't make any references to which ignition system, ignition coil, distributor cap, ignition wiring, and spark plugs were used to draw his conclusions.

It's a little premature to be patting yourself on the back.

Scott Novak