Date:         Thu, 20 Feb 2003 21:38:08 -0000
Reply-To:     Clive Smith <clive.harman-smith@NTLWORLD.COM>
Sender:       Vanagon Mailing List <vanagon@gerry.vanagon.com>
From:         Clive Smith <clive.harman-smith@NTLWORLD.COM>
Subject:      Re: Correct Gap of Sparks
Content-Type: text/plain; charset="iso-8859-1"

Steve,
Great, and thanks for some serious input. A breath of fresh air.
Inductance has a lot to answer for...

>At 3600 RPM one degree of crank rotation takes about 46 microseconds...
Changes in spark gap can definately make small but easily measurable
changes in timing.

a) So what is the difference between a 0.025" and a 0.030" gap (typically,
assuming the ign. system will indeed fire across 0.030" gap
Unless its of the same order as 46 micro secs then the timing effect is
negligible as few can set timing to within 1 degree, even today and not long
ago that same oscilloscope would show the centrifugal and vacuum advance
mechanisms splattering the timing across a huge range, maybe +/- 5 degrees
of the mean (design) curve, in a bad dizzy.
b) So this effect effect would have no relevance to a capacitor discharge
system?
c)... AND, everyone, this effect is BENEFICIAL, as far as timing goes (not
spark quality). Because, as the charge volume and BMEP increase the spark is
retarded (guessing at 1/20th degree!) and as we lift off, the spark
advances - - exactly what the vacuum advance system does, 'cept it moves the
timing 100 or more tiomes as much.


Clive

----- Original Message -----
From: "Steve Delanty" <laurasdog@WEIRDSTUFFWEMAKE.COM>
To: <vanagon@GERRY.VANAGON.COM>
Sent: Thursday, February 20, 2003 7:11 PM
Subject: Re: Correct Gap of Sparks


> At 10:25 AM 2/20/2003, you wrote:
> > > If gap is too short, spark happens early; if too long, spark happens
late.
> >
> >Sorry to be negative... but WHAT ABSOLUTE TWADDLE -
> >the spark plug gap does not affect the timing one jot - weak spark,
splashy
> >spark, no spark, good spark, but as for when it comes, no.
>
>
> Clive, I disagree with the above.
> When the points open, the spark does not jump immediately.
> The magnetic field takes some time to collapse and the voltage
> across the secondary builds up over a brief time until it has
> enough voltage to jump the spark gap. The wider the gap, the higher
> the secondary voltage must rise and it takes a longer time to reach
> this voltage. It's easily observable on an oscilloscope.
> How long it takes the coil to reach a particular voltage is a function
> of the inductance of the coil.
>
> Find someone who's got an oscilloscope and play with an ignition coil
> and spark plug and you can watch the secondary rise time for yourself.
>
> At 3600 RPM one degree of crank rotation takes about 46 microseconds...
> Changes in spark gap can definately make small but easily measurable
> changes in timing.
>
> Just my .02
> Steve