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Date:         Thu, 20 Feb 2003 23:38:29 -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"

Right, thanks Steve, point taken - I'm a bit hot off the mark, perhaps I should be more diplomatic .. whatever it is, I doubt if opening up the gap to extremes ever cause symptons such as backfiring through the exhaust (retarded) or pinking (which was being suggested!), provided the timing was pretty spot-on with new correctly set plugs.

The most sensitive engine I've ever come across was a 1293 cc Cooper 'S', with 12:1 compression ratio. These distributors had no vacuum advance and generally showed pretty accurate centrifugal advance curve, with a single kink, where the stiffer of two springs took over. 1 degree advanced on static setting and it would pink badly, one degree retarded and it would pop or even backfire when lifting off between gears - the later being preferable as it went like a scalded cat when spot on or slightly retarded, and real crap if even fractionally advanced. I find that many people run engines a bit too advanced for good pulling power when setting them up 'by ear', usually arriving at a setting ahead of the manufacturers, unwittingly.

The fact that the timing probably moves the right way under high cylinder pressures and rich mixtures, might mean that someone, sometime realised this (in the coil trade) and being inherently inductive devices, they never felt inductance per se was particularly a design goal, it just comes out in the design to resist heat and a duff environment (oil cooled) as quite high. Many would say these things are never considered or known about, but I generally find that the work has been done sometime way back and conclusions drawn, decisions made, long since forgotten about.

I thought there was a strong trend starting twenty or thirty years ago to make much higher powered ignition systems that would succesfully drive a spark across bigger gaps, some production engines even going so far as 0.035" plug gaps (maybe Audis?, can't remember, or maybe in BL's mad days!). The arguments here are obvious, mainly stemming from igniting weaker and weaker mixtures for economical cruise with the larger spark that should ensue when it does finally jump - though maybe that trend has diminished due to starting and poor running problems when a 0.032 or 0.035 gap, missing a service, becomes 0.037 or even 0.040" - and the consequent bad reputation gained.

Clive

----- Original Message ----- From: "Steve Delanty" <laurasdog@WEIRDSTUFFWEMAKE.COM> To: <vanagon@GERRY.VANAGON.COM> Sent: Thursday, February 20, 2003 10:24 PM Subject: Re: Correct Gap of Sparks

> Clive, > Honestly I don't know what difference in delay or required > voltage would be between .025 and .030". It would all depend > I'm sure on the particular coil, and also on the pressure in the > combustion chamber at the time of firing. > Ignition coils have quite a lot of inductance and are pretty > slow devices as far as transformers go. I've played with a few > high voltage projects using ign coils to generate HV and they > don't work well at switching frequencies over about 300-400 Hz. > It wouldn't surprise me if a change in plug gap of .010 could > change the firing delay by 10's of microseconds. > > I agree that the change in timing may be minimal, and that it > may be swamped by other factors (sloppy distributors), but > my point is it does make a measurable difference. > And every little bit helps, eh? > > If someone had some time with an O-scope and a high voltage > probe, it would be easy to put some real numbers to the whole thing. > > Steve > > > At 01:38 PM 2/20/2003, you wrote: > >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


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