[Healeys] Spark plug gap

Thu May 4 08:56:15 MDT 2023

I'll chime in here (as is my wont). I'm not an EE--my electrical 
knowledge is self-taught--so if I'm incorrect feel free to correct.

The theoretical output of a coil--which is essentially half of an AC 
transformer--is determined by the ratio of its secondary winding to its 
primary. For example, a coil with 10 primary windings and 20,000 
secondary windings will boost the primary 'signal' 2,000 times, so it 
would boost a primary signal of 12V to 24,000 volts. When the primary 
pulse to the coil is discontinued--e.g. points open--both the primary 
and secondary induced electrical fields collapse, and the secondary 
field is directed to the plugs*. The secondary field collapses until it 
produces sufficient voltage to overcome the resistance of the plug gap; 
hence the width of the gap determines the strength of the spark. For 
example--pulling numbers out of my exhaust pipe--if a 0.025" gap 
requires 10K volts to produce a spark, then a 0.030" gap might require 
12K volts to produce a spark. So, a larger gap produces a 'hotter' 
spark, up to the theoretical limit of the coil, and a hotter spark is 
better, no? Well, yes and no; theoretically, as long as the spark is hot 
enough to jump the gap it will ignite a proper mixture. A very lean 
mixture is harder to ignite, and a very rich mixture could 'drown' the 
spark ('stratified charge' engines use a richer mixture to ignite a 
leaner mixture). A hotter spark may more reliably ignite a non-ideal 
mixture, and may help a bit to ignite a proper one (higher compression 
engines require a hotter spark). But, as Josef noted, since every spark 
transmits some of the non-ground electrode--probably billions of 
atoms--to the ground electrode a hotter spark will erode the electrode 
faster (since erosion starts with the first spark and with a wider gap 
you should re-gap more often). Note condensers (capacitors) are used in 
points systems and SU fuel pumps to suppress spark arcing and subsequent 
points erosion. Which is why ...

Exotic metal--platinum or iridium--plugs are more resistant to erosion, 
which is why newer cars can go 80K miles or more without a 'tuneup,' 
which, for all intents-and-purposes just means swapping plugs (note they 
come in 'single' (default) or 'dual' types, meaning one or both 
electrodes are the exotic metal, respectively) . Old-style plugs with 
copper and steel electrodes can reliably go 15K miles or more, with one 
or two re-gaps, so exotic metal plugs aren't particularly useful for our 
cars which might do a couple thousand miles a year but, hey, knock 
yourself out. Note that platinum and iridium plugs come pre-gapped, and 
you'd best not mess with them. When I swapped plugs on my '08 Mustang I 
couldn't find a gap spec anywhere, but IIRC I futzed with them anyway 
(of course) and think they were all about 0.025". Fortunately, the car 
still ran well after my meddling and I traded it in in a few years 
anyway. Curiously, I watched a David Vizard video where he claimed he 
saw a /slight/ mileage increase using E3 'DiamondFire' plugs, so there 
may be something to the plug voodoo. Fun fact: All the iridium on planet 
Earth came from a single asteroid, and is found in a layer of said earth 
deposited around the same time the dinosaurs went kaput. Coincidence? I 
think not.

Anybody who's worked on 'modern' cars, say, newer than 2000MY or so, has 
noted 'Coil On Plug' ('COP') technology, where each plug get its own 
coil. Now, the coils on these are small, about the size of an old 35mm 
film canister--remember those?--and you might say to yourself 'Self, how 
do they stuff all that wiring into that little film canister' and the 
answer is, the wires gotta be real tiny and, yep, as you might expect 
they are fragile compared to our Bud Light can-sized (sorry) old school 
Lucas coils. Early COP systems--looking at you, VW--were prone to 
gremlins and I still hear about COP failures on newer cars and other 
makes, esp. with DOHC engines which always, eventually, seem to get 
leaky valve cover gaskets which dump oil into the plug tubes (as opposed 
to leaky Healey valve cover gaskets which just dump oil, well, everywhere).

OK, caffeine rush is fading. Ciao.

Bob

* Note coils for our Healeys come with either 'SW' ('switch;' i.e. 
ignition switch)  and 'CB' ('contact breaker;' i.e. points/distributor)  
or '+' and '-' labels on the primary terminals. They'll work either way, 
but some voodoo is employed whereby, when properly connected the 
secondary field is routed in series with the primary circuit to give a 
slight boost to produced spark.

On 5/4/2023 4:43 AM, josef-eckert--- via Healeys wrote:
>
> Sorry, I am out here. This guy wants to play till the engine is shot.
>
> -----Original-Nachricht-----
>
> Betreff: RE: [Healeys] Spark plug gap
>
> Datum: 2023-05-04T12:45:37+0200
>
> Von: "simon.lachlan at alexarevel.plus.com" 
> <simon.lachlan at alexarevel.plus.com>
>
> An: "josef-eckert at t-online.de" <josef-eckert at t-online.de>
>
> Thanks for that. I’ll certainly bear it in mind. Likewise, I’ll keep a 
> close eye on my engine’s temperature. The latter is quite well 
> protected with a modern rad, all the correct baffles, 5 blade fan plus 
> electronic fan. But, as I said, I’ll keep an eye on it.
>
> Someone mentioned that he used “Iridium” plugs from NGK. I googled 
> them….horribly expensive. Can they be worth it in our old cars?
>
> Simon
>
> *From:*josef-eckert at t-online.de <josef-eckert at t-online.de>
> *Sent:* 04 May 2023 07:06
> *To:* simon.lachlan at alexarevel.plus.com; Healey, Forum 
> <healeys at autox.team.net>
> *Subject:* AW: [Healeys] Spark plug gap
>
> For me, i prefer Champion plugs plugs to NGK, but that´s a matter of 
> taste. I think both, Champion and NGK, are of similar quality. Matter 
> of taste, as I wrote. But NOT the car manufacturer determines the plug 
> gap, its the plug manufacturer. Definitely the car manufacturer takes 
> this for his specs in the workshop manual.
>
> There is to much hype about plugs anyway. I am working as a 
> professional, Elecrtonics and Elecrtrcian engineer, at a big classic 
> car parts supplier here in Europe and I never go away from the given 
> plug gap as set by the plug manufacturer. Austin-Healey engines  are 
> not high tech, they are tractor engines and can take a lot of misfit, 
> but anyway BMC knew at the time what´s best to do with them, even its 
> printed 60 or 70 years ago.
>
> Some people try to adjust the fuel mixture with a CO-tester. You can 
> do that, but it doesn´t give best results for road use, especially in 
> hotter climates and high above sea level.
>
> When you are very good in engine engineering and have all the tools 
> you can try to find some more horsepower in your engine, but you 
> always pay that with some more heat produced, which is not at all good 
> for our old BMC engines.
>
> Josef
>
> -----Original-Nachricht-----
>
> Betreff: RE: [Healeys] Spark plug gap
>
> Datum: 2023-05-04T00:06:58+0200
>
> Von: "simon.lachlan at alexarevel.plus.com" 
> <simon.lachlan at alexarevel.plus.com>
>
> An: "josef-eckert at t-online.de" <josef-eckert at t-online.de>
>
> You may be right. Not sure.
>
> Actually I thought that the gap was determined by the car’s 
> manufacturer ie one could expect to find the same plug in two 
> different cars and to find that the recommended gap was not the same 
> for each car.
>
> Likewise, the BMC manual suggests Champion plugs at 0.024” to 0.026” 
> and that was some 60+ years ago. I’ve always been told that Champion 
> plugs aren’t the best in the world and I’m sure that a modern NGK can 
> stand a little more heat than the Champions that were around 60 years ago.
>
> Anyhow, I’ll keep an eye on them. I’m a pretty tame driver so there 
> won’t be undue stress under the bonnet.
>
> Simon
>
> *From:*josef-eckert at t-online.de <josef-eckert at t-online.de>
> *Sent:* 03 May 2023 20:56
> *To:* simon.lachlan at alexarevel.plus.com; healeys at autox.team.net
> *Subject:* AW: [Healeys] Spark plug gap
>
> Spark plugs are designed and tested to be used with the gap given by 
> the producer of the spark plug.
>
> When you widen the gap the plug gets hotter as it is designed for and 
> the electrode can melt or wear very fast.
>
> Its a very short win you gain modifying them.
>
> I never understand why people think they are better engineers and know 
> better than the designers of the system they use.
>
> Josef
>
> -----Original-Nachricht-----
>
> Betreff: [Healeys] Spark plug gap
>
> Datum: 2023-05-03T21:39:57+0200
>
> Von: "Simon Lachlan via Healeys" <healeys at autox.team.net>
>
> An: "'Healeys'" <healeys at autox.team.net>
>
> I run my MkII BT7 on NGK BP6ES which were recommended by some of the 
> wise ones on this list.
>
> The car has a “Sports” coil and a 123Ignition distributor. DW fast 
> road cam…etc
>
> Now, talking of “recommended”, the rolling road guys told me to set 
> the plugs at 0.028” and pooh-poohed anything bigger when I suggested 
> that I’d heard of much greater gaps with my approximate setup.
>
> So, I went with 0.028.
>
> The other day, I decided to experiment and set my spare set at 0.032”. 
> Same NGKs, no mileage on them..decent plugs. Anyhow, it made a very 
> surprising difference. All round performance is much enhanced.
>
> Despite it’s 3:54 diff and 28% OD, it was never sluggish on our local 
> hills. But now it fairly zooms up them.
>
> To be frank, all a bit of a surprise.
>
> Not boasting…just putting that out there in case anyone else is 
> thinking of going up a few thou…..
>
> When I asked about gaps a few years back, some of the suggestions were 
> near 0.040”. Surely that a bit much?
>
> Simon
>
> 
>
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